PHYSICO-CHEMICAL STUDIES
Transcript of PHYSICO-CHEMICAL STUDIES
PHYSICO-CHEMICAL STUDIES
OF MIXED LIGAND ZINC (II)
COMPLEXES
By
Dr. Dinesh Vasant Bhagat
M. Phil., Ph.D. University of Mumbai (SET Qualified)
Associate Professor, HOD, Department of Chemistry
K.E.S. Anandibai Pradhan Science College, Nagothane.
2021
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Title: PHYSICO-CHEMICAL STUDIES OF MIXED LIGAND ZINC (II)
COMPLEXES
Author(s): Dr. Dinesh Vasant Bhagat
Edition: First
Volume: I
© Copyright Reserved
2021
All rights reserved. No part of this publication may be reproduced, stored, in a
retrieval system or transmitted, in any form or by any means, electronic,
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permission of the publisher.
ISBN: 978-93-89817-48-5
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This book is dedicated to my Parents and
“All My Respected Teachers”
Who taught me at different level.
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ACKNOWLEDGEMENT
I take this opportunity to express my sincere gratitude to my Guru Dr. Vikas V.
Vaidya, Associate Professor, Department of Chemistry, Dr. Sunil Patil, Director, Department
of Student Development University of Mumbai who always inspired and encouraged me to
write books on my subject of specialisation.
I also extended my sincere thanks to all my teachers who taught me at
different levels and make me able to write my first book on, “Physico-Chemical Studies of
Mixed Ligand Zinc (II) Complexes”. I am very much gratified by my research supervisors
who always encouraged me towards the synthesis element detection, spectral interpretation
and biological study indeed in this book throughout for all the synthesis. I have given some
reactions, preparations in same style and it will be very easily to understand the procedure of
making mixed ligand complexes using primary and secondary ligands.
I express my sincere thanks to the publisher Ideal International e-Publication Pvt..
Ltd. for publishing this book and taking keen interest in it. Finally, I owe a sense of gratitude
to my mother Prema Bhagat, wife Mrs. Vidya Dinesh Bhagat, my son Tanuj and daughter
Prachiti for their pleasant cooperation and moral support during writing this book.
Dr. Dinesh Vasant Bhagat
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Preface
This book entitled, “Physico-Chemical Studies of Mixed Ligand Zinc (II)
Complexes” has been particularly addressed to the graduate and postgraduate students who
have opted for the Inorganic and Analytical Chemistry study course as per the UGC syllabus.
This book is equally useful for those students who are preparing for the NET-JRF-CSIR,
SET, SLET, GATE, NET-ICAR and other competitive examinations like MPSC and UPSC.
This book includes six chapters and covers basic theory of metal complexes, analytical
techniques, spectral analysis, biological studies and structures of metal complexes. Oxidation
is discussed in details with different sets of examples. The number of metal complexes is
studied with their spectra’s and biological activities.
Although almost precautions is taken to make this book error free but few
errors may creep in, for which I apologies in advance. Suggestions are welcome for
improvement of work and publication.
Place: - Nagothane, Dist: - Raigad (M.S.) Dr. Dinesh Vasant Bhagat Date: - 04/04/2021
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INDEX
Chapter Title Page no.
1 Introduction
2 Theoreticals
3 Experimental
4 Results and discussion
5 Biological activity
6 Summary
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Chapter No. 1
Introduction
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INTRODUCTION
The chapter deals with the several aspects that are associated with the formation of
coordinated complexes. The fundamentals that govern the formation of such complexes
continue to grow as new compounds are synthesized and new principles are formulated.
Coordination chemistry is an important branch of science and well known for its association
with the life. The relation between coordination chemistry and life can be illustrated by an
example of vitamin B12 which is one of the naturally occurring coordination compound in
biology. Some other important complexes are chlorophyll, haemoglobin, myoglobin,
cytochrome, etc.
Numbers of attempts have been done on the study of biological importance of
complexes containing more than one ligand. This chapter deals with several facets of
polydentate ligands, factors affecting stability of chelates and mixed ligands and the literature
survey of amino acid and 8-hydroxyquinoline as ligand.
The coordination chemistry has also an industrial application. The complex of
aluminium and titanium is used as a catalyst for low pressure polymerization of ethylene.
Fiber and fiber reinforced plastic materials are made by using coordination compounds of
silica and copper. Coordination chemistry has also left its foot prints in the field of medicine,
analytical chemistry and physics. The coordination chemistry is an amalgamation of organic,
inorganic and biochemistry. It is an interdisciplinary science and extended vastly from
defined to unlimited research field because of its significant industrial application and
relevance with life. The interest in coordination chemistry has undergone rapid development,
which has been reported in a large number of publications, reviews, conferences and
symposia.
Historical Background:-
In 1704, Purssian Blue was discovered by Diesback and this discovery was a key
for the development of coordination compounds. Many compounds were discovered
thereafter and many theories were put forth to explain the electronic structure and chemical
bonding of these compounds. The ‘Werner’s Coordination Theory was then proposed by
Alfred Werner in 1893. Today’s inorganic chemistry research is centred on Werener’s
Coordination Theory.
He introduced the concept of secondary or auxiliary valence and synthesized
many compounds based on the concepts of primary and secondary valencies. By means of the
numbers and properties of the isomers obtained, Werner was able to assign the correct
geometric structures to many coordination compounds long before any direct experimental
method was available for structure determination. He put forth that the factor determining a
structure of a coordination compound was the number of groups or atoms directly attached to
it, i.e. its coordination number. His views on coordination compounds are still the foundation
of this subject and fundamental postulates that he proposed are still found to be valid.
Lewis proposed the ‘Electronic Theory of Valence’by referencing the ideas of
Werner. His theory was extensively applied to coordination compounds by Sidgwick who
made significant contributions to the theory of valency and chemical bonding. Sidgwick
easily incorporated most of the structures given by Werner into electron pair bonds proposed
by Lewis. The concept of coordinated bond was developed by him. He proposes that the
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ligand species share a pair of electrons with the central atom to form bond of the same
general nature which was found in classical covalent compound. The research in coordination
chemistry has progressed leaps and bounds in the last couple of decades.
Metal Chelates And Chelating Agents:-
When a central metal ion (M) unites with a group of neutral molecules or ions, a
coordination compound is formed. The ligand is a neutral molecule or an ion capable of
functioning as the donor partner to the central metal ion. A bond thus formed between metal
and ligand is called metal-ligand (M-L) bond. A chelate or a metal chelate is formed by the
process called chelation, if the coordinating ions or molecules are attached to the central
metal atom in such a way so as to produce a closed ring. The molecule forming chelate with
the ions is called chelating agent. The metal chelate thus formed is studied by considering
central metal atom, chelating molecules or ligands and the nature of bonding between metal
and the ligand.
a) Central Metal Atom:-
Nature, oxidation state and the coordination number of the central metal atom
influence the properties of metal chelate to a considerable extent. This can be studied by
comparing the compounds formed by different metal atoms with a particular chelating agent.
The variation in the structures and properties of complex formed by a metal depends on its
oxidation state and coordination number. The coordination number is a number of donor or
ligand atoms that are directly bound to the central metal atom. The coordination number
varies from metal to metal.
b) Chelating Molecules:-
As discussed above, the chelate molecule forms a closed ring of coordinating
molecules and central atom. These molecules are mostly organic in nature and form covalent,
coordinate or both types of bonds with donor atom. The chelating agent can undergo
chelation only if -
i) It has two appropriate functional groups that can combine with a metal atom by donating a
pair of electrons. Some functional groups are acidic and unite either by replacing hydrogen or
without replacing it. Some acidic groups that unite with metal ions by replacing hydrogen are
–COOH, -OH, -SO3H and =N-OH. The functional groups that form coordination linkage but
do not replace hydrogen donate a pair of electrons. Such type of linkages are seen in most
functional groups such as primary, secondary and tertiary amines,=N-OH (oxime), -OH
(alcoholic hydroxyl), >C=O (carbonyl) and –SCN (thiocyanate), etc.
ii) The molecule has its functional group situated so as to allow the formation of a closed ring
with the metal atom.
Generally, when there are one or more ring structures and there are 5 or 6 atoms in
the ring, the complex so formed is stable as the strain is reduced. The rings get closed by the
formation of covalent linkages or coordinate bonds or by combination of the two. One of the
examples for such chelate compound used for determination of nickel is formed by the
reaction between Ni2+
and dimethylglyoxime.
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The classification of chelate compounds follows the number and the kind of
attachment involved. A polydentate molecule may be attached to the central metal atom
through two kinds of functional groups which may be acidic or coordinating by means of
covalent and coordinate linkages.
According to the classification given by Diehl, the chelate compounds are classified
as follows –
a) Bidentate:- It has two donor groups. Both the groups may be either acidic or coordinating.
Or there may be one acidic and one coordinating group.
b)Tridentate:- It has three donor groups. All the three groups may be either acidic or
coordinating. Or there may be two acidic and one coordinating group or one acidic and two
coordinating groups.
c) And so on, for quadridentate, quindentate, hexadentate and polydentate ligands.
Several organic molecules have six groups (two acidic and four coordinating). They
are capable of attaching themselves in six octahedral positions of the coordination sphere of a
central metal atom One of the examples of such organic molecule is 1,8-bis-
salicylideneamino-3,6-dithiaoctane that reacts with Zn, Ni, Cu or Co to form a chelate
compound.
The maximum coordination position can be attained by a polydentate molecule
which is wrapped itself around the central metal atom. Martell and Calvin have summarized
the chelating agents and their uses.
The stability of chelates is affected by-
a) Size of the chelate ring:-
Chelates having more membered rings including central metal atom are more stable
than those having less membered ring. For example, six membered ring chelates are more
stable than five membered ring chelates, which in turn are still more stable than four
membered ring chelates.
b) Number of chelate rings:-
More the number of the chelate rings, greater is the stability of the chelate.
c) Resonance effect:-
Resonance enhances the stability of chelate.
d) Chelate effect:-
The chelated complexes have more stability than the non-chelated complexes.
e) Steric effect:-
If the bulky group is attached to the donor atom of the ligand or it is present near the
donor atom then the metal ligand bond is weakened resulting in the lowering of the stability
of the complex.
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Classification of Chelating Agents:-
The chelating agents establish linkages with the metal ion by more than two positions
through covalent or coordinate bond. Such chelating agent is referred as bi, tri, quadri, penta,
hexadentate, etc. depending on the number of donor sites two, three, four, five, six, etc.
respectively.
The classification of the chelating agents and their metal complexes is also done on
the basis of donor atoms present in the ligand. There are number of organic and inorganic
ligands containing donor atoms like O, N, halogens, P, As, Sb, S, Se, Te, etc. Some classes of
chelating agents are
a) Metal Chelates of Oxygen Ligands:-
The chelating agents like oxyanions, alkanoates, dicarboxylates, β-diketones, o-
hydroxy carbonyl compounds, etc. coordinate through the oxygen donor atoms. The rare four
membered chelates are observed with the chelating agents containing oxyanions like SO4-2
,
SO3-2
, SeO4-2
, MoO4-2
and PO4-3
. A five membered ring structures is suggested for metal
complexes of dicarboxylic acids by electronic and infrared spectral studies. For higher
homologues of dicarboxylic acids, an eight membered ring structure is also reported. The six
membered ring structure has been reported for the complexes with acetylacetone (β-
diketones).
The same type of ring structure is also observed in uranium complexes of β-diketones
of the type [U(R.CO.CH.CO.R/)4] and [UO2(R.CO.CH.CO.R/)2]which is confirmed by
infrared spectral studies.Vanadyl complexes of some β-diketones have also been studied for
their magnetic spectra and ESR spectra. Stable octahedral complexes are readily formed by
acetyl acetonatesof certain bivalent metals by taking up two extra ligands such as water,
alcohol or ammonia and organic amines. Spectral studies of metal complexes of β-diketones
have been reviewed. The literature shows extensive studies on metal complexes of various
oxygen containing ligands.
b) Metal Chelates of Nitrogen Ligands:-
Alkyl and aryl diamines, substituted 1,10-phenanthroline and 2.2-bipyridyl
derivatives, biguanide, guanylurea and their derivatives are categorized as metal chelates of
nitrogen containing ligands. There are two main groups of complexes of this class viz. alkyl
and aryl amine metal complexes and aromatic heterocyclic base complexes. The extensive
study has been carried out on metal chelates of ethylenediamine and its derivatives. Sahu and
Mohopatra have studied the Mn (II) and Cd(II) complexes of dicyanodiamide. The aromatic
heterocyclic bases such as 1,10-phenanthroline and 2, 2-bipyridyl were first used as
coordinating agentsand Brandt and others have worked on metal chelates of these and related
ligands. Ahuja and Singh synthesized uranyl complexes and their structures8were confirmed
through physical methods. Metal complexes of biguanide, guanylurea and their derivatives
were studied by Ray and they are found to be highly stable. Birdar and Gaudar developed the
synthetic procedure that affords a series of six coordinated Ni(II) complexes distinguished by
a bicyclic ligand frame work which encapsulates the metal ion and imposes a trigonal
prismatic or near trigonal prismatic stereochemistry.
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c) Metal Chelates of Nitrogen and Oxygen Ligands:-
Several polydentate ligands containing nitrogen and oxygen are reportedwhich
functions mainly as bidentate and multidentate chelating agents. The complexes of
polydentate ligands containing nitrogen and oxygen donor atoms are interested because of the
mode of coordination. Following are few of the examples
i) Amino Carboxylic Acids:-
The amino acids like anthranilic acid, glycine and other α and β-amino carboxylate
ions have been used as chelating agents to form complexes with various metal ions.
The complexes of neodymium with glycine have been studied by polarographic
technique. Cu (II) complexes of α and β-amino butyric acids show that the chelating agents
behave as bidentate ligands. Cotton studied the infrared spectra of amino acid complexes.
The formation and stability of mixed ligand complexes of Cu (II) with malonic acid as
primary ligand and some amino acids as secondary ligands have been reported.In most cases,
the amino acids act as bidentate ligands, coordinating through the amino and carboxylate
group resulting into thermodynamically stable five membered rings for the α-amino acids and
six membered ring for the β-amino acids. The amino acid such as methionine may offer
additional coordination site through sulphur atom. The anions of amino acid form stable
complexes with a wide variety of metal ions. Tridentate bonding in solids is seen in aspartic
acid complexes of Co(II), Ni(II), Zn(II) and Cu (II) and is illustrated by X-ray spectral
studies. In contrast, similar studies show that glutamic acid coordinates to one metal ion via –
NH2 and one –COO – while second –COO –binds to a second metal ion.
Binary complexes of amino acids are known from the time of Werner, e.g.
[Pt(glycinato)2]. Some other binary complexes of amino acids known are [Cu(alaninato)2],
[Co(histidinato)2], etc.
The advanced research concentrates on ternary complexes than the binary metal-
amino acid complexes, particularly the complexes of the type (aa)–M–L, Where (aa) is an
amino acid, L is other ligand or may be different amino acid and M is a metal ion, e.g.
Co(asp)(ala) and Co(glu)(ala).
ii) Amino phenols:-
As o-aminophenols have an ability to coordinate with metal ions in body they are
found to be carcinogenic. The metal chelates of o-aminophenols are more stable than those of
diketones and substituted salicylaldehydes. The stability constants of several metal chelates
of o-amino phenols have also been studied.
iii) Nitro and Nitroso Compounds:-
These compounds have weak donor properties. Even though, the basicity of nitroso
group in aromatic compounds can be enhanced by the presence of strong electron donating
group at para position. When p-nitrosoaniline (and its N-methyl and N,N/-dimethyl
derivatives) form complex with Co(II), Cu (II) and Ni(II), there is decrease in N-O stretching
frequency. It indicates that nitroso-oxygen is involved in the coordination. Several transition
metal complexes of simple nitro compounds are known though they are commonly used as
solvents. A compound of Cu(NO3)2.CH3NO2 has nitromethane molecule coordinated by one
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oxygen atom but is bidentate as in TiCl4.CH3NO2. The spectral data revealed the structure of
the complexes of Ln (II), Ce (III), Th (IV) and U (VI) with 7-nitroso-8-hydroxyquinoline-5-
sulphonic acid.
iv) Oximes:-
Oxy-imine is abbreviated as oximes. The structure of oxime is denoted as C=N-
OH. They coordinate with metals to form metal complexes. The nitrogen of oxime has very
strong donor property because of which it can form chelates through oxime nitrogen.
Dimethyl glyoxime is most commonly used oxime in inorganic analysis. Many workers have
reported metal complexes of oximes. The successful work has also been carried on the
structural information of metal complexes of oximes by Chakravorty. Spectral data has
revealed the structural information of complexes of dioxouranium(VI) with aldoximes. The
complexes of U(VI) and Mo(VI) with oximes are studied for their stability constant using
colorimetric techniques.
v) 8-Hydroxyquinoline (Oxine):-
It is N- and O- donor bidentate ligand and also an earliest analytical reagents
used for metal ion estimation. It forms neutral complexes with bivalent, trivalent and
tetravalent metal ions by loss of proton. Philip has studied the coordination chemistry of
oxines. Schulman and Dwyershowed the in vivo use of oxines in microbial system. The
detailed study has been carried out on wide useof oxine and its closely related bidentatesfor
analytical solvent extraction and colorimetric determination. The stability constant and X-ray
diffraction studies of some bivalent metal oxinates have been reported. Spectral properties of
platinum and palladium chelates of 8-quinolinols have been reviewed. The Co(II)bis/-chelate
of 2-methyl-8-hydroxyquinoline has property to catalyze the disproportionation of nitric
oxide. The extensive work has been carriedout by Y. Yamamoto and E. Toyota on the
preparation and properties of some ternary Co (II) complexes of this ligand. Recent work
indicates the antimicrobial studies of some mixed ligand transition metal complexes of 8-
hydroxyquinoline.
Binary Complexes:-
Binary complexes are the one in which metal atom or ion is bound to two or more
ligands of the same type. They are well known since the time of Werner. They are well
studied for their structural and stereo chemical properties.
Stability of Binary Complexes:-
The stability of binary complexes is affected by following factors
a) Nature of the metal ion
For the formation of stable complex a high charge on metal and a small radial
distance are important factors. A hard metal ion (Li+, Na
+, Ca
2+, Co
3+, etc.) would most
strongly bind with the donor atoms (N, O or F) that possess high electronegativity, low
polarisability, small radii and are difficult to oxidize.
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b) Nature of the ligand
The stability of binary complexes depends upon basicity of the ligand. This is due
to the fact that H+
and metal ion both act as Lewis acids towards Lewis base ligands.
c) Chelation
Chelation is also one of the factors that affect stability of complexes. Non-
chelating ligands are less stable than the complexes of chelating ligands. The stability of
chelate complexes decreases as the chelate ring size increases from 5 to 7 where as a chelate
with more chelate rings form even more stable complex.
Mixed Ligand Complexes:-
Mixed ligand complexes are the one in which metal atom or ion is bound to two
or more different ligands. A mixed ligand is known as ternary ligand if a metal is bound to
two different ligands. For example, MAB, where MAB is a complex species and A and B are
two different ligands. This is 1:1:1 ternary complex. Similarly the quaternary complexes have
a metal and three different ligands.
Stability of Ternary Complexes:-
Equilibrium constant is the key factor to study the stability of ternary
complexes. The equilibrium constant for binding a second ligand is usually lower than that
for the first, except in some special cases where K2 is greater than K1 for the same ligand.
When the metal binds to two different ligands A and B, there are many instances of increased
affinity of the metal for ligand B, due to binding of ligand A and vice versa. The complex
formation can be represented as follows,
M + B MB + KMB = [MB]/[M] [B]
MA + B MAB + KMAB = [MAB]/[MA] [B]
For a binary system A = B, the difference in stability, log k, usually varies from -0.5
to -0.8 for the monodentate ligands and from -1 to -2 for bidentate ligands. For the above
equations, it follows that the influence of both ligands in a ternary complex is mutual and of
the same extent both the ligands are either stabilized or destabilized in their coordination to a
particular metal ion. Actually log k corresponds to log Keq for the process,
MA + MB MAB + M Keq = [MAB] [M]/[MA] [MB]
Hence, a positive value for log k would indicate that the equilibrium is in favour of
formation of the ternary complex. It therefore indicates, enhanced binding of B as a result of
the effect of A on the properties of central metal and vice versa.
An examination of the results of the studies leads to several interesting conclusions.
For example, in M-(bipy)-L complex, where L=en, glycine, pyrocatechol, the ligand L
enhances the stability of the complexes of oxyanion ligands such as acetate, oxalate,
phenolate, etc. but reduces the affinity of the metal for amine to amino acid ligands. The very
large difference in K1 and K2 for bipyridyls results from steric interactions of the ligands as
well as from the antagonistic or competitive bonding effects associated with two trans -
acceptor ligands. Exactly opposite trend is observed for Cu2+
-en system, where the strong -
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donor effect of the saturated amine reduces the Lewis acidity of the metal. Amino acid
ligands have similar effect of destabilization of -donor ligands, especially oxyanions and
stabilization of -acceptor ligands.
Applications of Mixed Ligand Complexes:-
Mixed ligand complexes are widely used in industries. On account of their
catalytic property, they are well known as catalyst in the industrial processes such as
hydrogenation, hydroformylation, oxidative hydrolysis of olefins and carboxylation of
methanol. The alkenes are activated through catalytic property of platinum complexes. The
catalytic property of ternary transition metal complexes is useful in various oxidation
reactions of industrial and environmental importance. They are also known for catalysing
decomposition activity of hydrogen peroxide and their participation in biological activity.
The antibacterial and antifungal activity of 8-hydroxyquinoline and some of its complexes
has been reported. Bis-(3,5-diisopropylsalicylato) Copper (II) complex is known for its anti-
convulgant and antitumor activities. The role of ternary complexes in activation of enzymes
and in the storage and transport of active substances is well sighted in literature.
Aim of the Present study:-
The literature survey and a brief account of the former work justify the task
undertaken for the investigation.
Extensive research has been carried out for the study of mixed ligand complexes
and their importance in various biological processes. It has been found that many ternary
complexes of some metals are important for activation of enzymes and they are used for
storage as well as for transport of active materials. The correlation between the stability of the
metal-ligand complexes with their anti-microbial activity has been studied. Antitumor
activity of some mixed ligand complexes also has been reported.
Complexes of many metals with 8-hydroxyquinoline have been studied for their
biological activity. Metabolic enzymatic activities for many metal complexes of amino acids
have been reported. Many researchers have studied characterization, antimicrobial and
toxicological activity of mixed ligand complexes of transition metals and actinide metal ions.
Synthesis and characterization of some transition metal complexes derived from amino acids
have been reported.
It is well known that the copper complexes play important role in various biological
processes. The antibacterial and anti-fungal properties of copper (II) complexes have been
reported. Recently synthesis, structural characterization and antibacterial studies of some
biosensitive mixed ligand copper (II) complexes have been reported. Many complexes of
copper (II) metal ion have been investigated for their chelation and biological properties.
Antioxidative and anti-tumour properties of copper (II) metal complexes have also been
reported. The spectral, magnetic and biological properties of ternary complexes of copper(II)
metal ion with amino acid as secondary ligand have been studied.
The present work was therefore undertaken to study the mixed ligands of copper(II)
a with 8-hydroxyquinoline (HQ) as a primary ligand and different amino acids (HL) such as
L-valine, L-asparagine, L-glutamine, L-arginine and L-methionine as secondary ligand. The
metal complexes have been characterized by elemental analysis and various physico-
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chemical techniques such as molar conductance, room temperature magnetic susceptibility,
electronic spectra, IR spectra, thermal studies and XRD.
Microbial techniques were applied for preliminary screening of antibacterial activities
of these complexes and it is discussed in the Chapter 5 of this book.
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CHAPTER 2
THEORETICALS
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THEORETICALS
Elemental Analysis:-
Elemental analysis is a process where a sample is analysed for its elemental and
sometimes isotopic composition. Elemental analysis can be qualitative and it can be
quantitative.
Methods:-
The most common form of elemental analysis is CHN analysis and is accomplished
by combustion analysis. In this technique, a sample is burned in an excess of oxygen and
various traps collect the combustion products such as carbon dioxide, water and nitric oxide.
The masses of these combustion products can be used to calculate the composition of the
unknown sample. This information is important to determine the structure of an unknown
compound as well as to ascertain the structure and purity of a synthesized compound.
Carbon is converted to carbon dioxide, hydrogen is converted to water, nitrogen is
converted to nitrogen gas / oxides of nitrogen and sulphur is converted to sulphur dioxide
during combustion process. If in the sample other elements such as chlorine are present, they
will also be converted to hydrogen chloride. If determination of additional element is not
required then a variety of absorbents are used to remove these additional combustion
products.
The inert gas such as helium is used to remove the combustion products from
combustion chamber and then combustion products are passed over heated (about 600oC)
high purity copper. This copper is situated at the base of the combustion chamber or in a
separate furnace. The main role of copper is to remove any oxygen not consumed in the
initial combustion and to convert oxides of nitrogen to nitrogen gas.
The combustion products are then passed through the absorbent traps in order to
remove only carbon dioxide, water, nitrogen and sulphur dioxide. The combustion gases can
be detected by (i) GC followed by thermal conductivity (ii) partial separation by GC followed
by thermal conductivity and (iii) infra-red technique and thermal conductivity. Quantification
of the combustion gases requires calibration. The calibration is performed by using high
purity ‘micro-analytical standard’ compounds such as acetanilide and benzoic acid.
Analysis of Results:-
The analysis of results is performed by determining the ratio of elements within the
sample and working out a chemical formula that fits with results. The method for
determination of ratio of elements from the results is shown below
1) Take the percentage of each element found and divide by the elements mass.
Perform the same procedure for all elements.
2) Find the smallest value from step 1 and divide every value obtained in step 1 by
this smallest value.
3) Multiply the results in step 2 by a factor to obtain reasonable values for either
carbon or nitrogen and then compare it with standard.
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Automated tools have been released to simplify this process. Each of the tools is
different in its working.
Conductometry:-
Electrolyte is substance which produces free ions when it is placed into a solvent
such as water. The molecules of electrolyte split up into individual atomic components which
form ions. The process is called as dissociation. Positively charged ions are cations and
negatively charged ions are anions. Due to the presence of free ions, electrolyte solutions
behave as an electrically conductive medium. They represent conductors of kind two, in
which the electric current is conducted by free ions, as oppose to the free electrons in
conductors of kind one (e.g. metals). Common electrolytes consist of salts, acids or bases.
Electric properties of the conductor of kind one are described by Ohm's law
I =U/R
Where, I corresponds to a current, U is a voltage and R describes electric resistance.
This resistance depends on the intrinsic properties of a conductor and its shapeand it
is given as R =ρ .l/s
Where, R is resistance, ρ is specific resistance, is a conductor's length and s is a
cross-sectional area.
Every material is characterized by a specific resistance, ρ , that is given in units ofΩ
.m (Ω is ohm, a unit of electric resistance). Electrical properties can be expressed also
through the quantity, conductance G. It is the inverse of resistance.
G =l/R
Its unit is S (siemens), where, 1S=1/ Ω
A specific conductance k is defined as
G =k//l/s
Similar to the relation between the conductance and resistance, specific conductance
is inversely proportional to the specific resistance. Its unit is Sm-1
.
If the measurement is done at alternating current and voltage then the same
equations as for metallic conductors can be used for electrolyte solutions. Such a
measurement is then done with conduct meter. It consists of a conduct metric container and
platinum electrodes covered with a platinum black; they are formed by the three fillets in the
conductometer. The cross-sectional area‘s corresponds to the surface of electrodes and l is the
distance between them. Ratio l/S is an intrinsic parameter of each conductometric container
and is called resistive capacity of conductometric container C.
C =l/s
The conductance is then calculated from the equationG =k/C
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Where, resistive capacity C is experimentally determined using the solution with
known specific conductance. The KCl solution (concentration c = 0.1 mol.dm-3
) is used for
the same.
Apparatus: -
The conductivity meter is used to measure conductivity of an electrolyte or solution. It
consists of conductivity cell and conductivity meter to measure conductivity.
The conductivity cell consists of two electrodes (platinum plates) firmly held at a
constant distance from each other and are attached by electrical wires to the meter.
The conductivity meter consists of a Wheatstone bridge circuit as shown in the figure
2.1
Figure 2.1 Schematic Diagram of Wheatstone bridge Circuit
The source of electric current in the conductivity meter applies a potential to the
plates and it measures the electrical resistance of the solution. In order to avoid change of
apparent resistance with time due to chemical reactions (polarization effect at the electrodes)
alternating current is used. Some meters read resistance (ohm) while othersread in units of
conductivity (milli-Siemens per meter). Platonized electrodes must be ingood condition
(clean, black-coated) and require replating if readings of the standard solution become
inconsistent. Replanting should be done in the laboratory. The cell should always be kept in
distilled water when not in use, and thoroughly rinsed in distilled water after measurement.
The Cell Constant (Calibration):-
The size, shape, position and condition of the plates in the conductivity cell
determines the conductivity measured and is reflected in the cell constant (Kc), The values
for Kc are 0.1 to 2.0. The cell constant can be determined by using the conductivity meter to
measure the resistance of a standard solution of 0.01 mol.dm-3
KCl. The conductivity of the
solution (1.413 mS.cm-1
at 25ºC) multiplied by the measured resistance gives the value of Kc.
The cell constant is subject to slow changes in time, even under ideal conditions. Thus,
regular determination of the cell constant must be required.
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Temperature Correction
Conductivity of solution changes with the temperature. Conductivity of electrolyte
increases with temperature at a rate of 0.0191 mS/mºC for a standard KCl solution of 0.01
mol.dm-3
. For natural water, this temperature coefficient is approximately the same as that of
the standard KCl solution. Thus, more the sample temperature deviates from 25°C greater the
uncertainty in applying the temperature correction. Thus the temperature of a sample (+
0.1ºC) is recorded and the conductivity is measured at 25ºC (using a temperature coefficient
of 0.0191 mS/mºC). Most of the modern conductivity meters have a facility to calculate the
specific conductivity at 25ºC using a built in temperature compensation from 0 to 60ºC. The
compensation can be manual (measure temperature separately and adjust meter to this) or
automatic (there is a temperature electrode connected to the meter).
Magnetism:-
From the property of attraction in magnetite, the iron ore, the term ‘Magnet’ is
supposed to have been discovered. Earlier it was thought that this property was associated to
the elements of iron group only. But with remarkable discoveries like earth as a magnet, this
belief was slowly given up. Chemists, in recent times have started taking a great deal of
interest in magnetism with a view to study the structures of molecules. Theoretical
standpoint, quantum mechanics and study of physical properties of molecules gave more
significance to this field. The important properties, which ascertain the structure of
molecules, are ultra-violet spectra, infrared spectra, nuclear magnetic resonance spectra,
electron diffraction studies, x-ray diffraction studies, dipole moments, molar refraction, etc.
Measurement of magnetic susceptibility is also a useful tool in the elucidation of structure of
molecules.
Diamagnetism and Paramagnetism:-
Faraday, the founder of magneto chemistry revealed that electric current and
magnetic field exists together. Faraday concluded that all substances whether elements or
compounds possessed magnetic property in varying degrees. During investigation, he found
that some substances when placed in a magnetic field tend to move away from the region of
maximum field intensity (diamagnetic), while others move towards it(paramagnetic).
Diamagnetism is a universal property of matter. It depends upon the structure of the
atom which will be practically unaffected by temperature. The magnetic susceptibility of a
diamagnetic substance is independent of temperature and field strength. Paramagnetism is
observed among the transition group elements and varies inversely as the absolute
temperature. A paramagnetic substance possesses underlying diamagnetism, but generally
paramagnetism is so large that it completely masks diamagnetism. There is also a third type
called ‘ferromagnetism’ which is very rare in nature and occurs among a few metals, alloys
and compounds. It depends on both temperature and field strength.
Magnetism and Electronic Theory:-
Several attempts were made to interpret the magnetic phenomena depending on the
behaviour of the extra nuclear electrons in the atom on the basis of electronic theory. The
revolving electrons acquire two types of angular moments
1) Due to orbital motion in a closed circuit
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2) Due to spin around their own axis
The net result of these two moments gives rise to magnetic moments in an atom
which is multiples of a unit known as ‘Bohr Magneton’.
If the net magnetic moment of an atom is zero the substance is said to be
diamagnetic. However, if the atom has an incomplete shell, then the net of the spin and
orbital moment has a definite value. The substance is then said to be paramagnetic. An atom
will have a permanent magnetic moment if it has an odd number of electrons or if all the
electrons are not paired off.
Ferromagnetism is observed in lattice of magnetic particles with loose inter-atomic
binding and with parallel electron spins.
Diamagnetic susceptibility is the useful aspect for the structural elucidation of
molecular structure of organic substances6for the inorganic compounds. Gray and
Cruickshank suggested that diamagnetic susceptibility could be used with advantage to
explore the structure of those compounds, which cannot be investigated by any other physical
method. This claim has to be validated by a large amount of data on the subject. Magnetic
method has certain unique advantages over other physical methods. For example the
measurement of susceptibilities does not involve a difficult procedure and these
measurements can be made on substances in any physical state. Generally other methods
have certain restrictions.
Langevins Theory of Diamagnetism and Paramagnetism:-
Langevin laid the foundation of the modern electronic theory of magnetism in 1905.
He formulated an exact mathematical expression on the basis of this theory, which
satisfactorily explained diamagnetism and paramagnetism. This treatment covers the
behaviour of many paramagnetics, which do not obey curie law, and correlates many
phenomena of ferromagnetism. Langevin attributed this effect to the electrons revolving in
the closed orbits of the atom producing at distance magnetic effects similar to those arising
from a current circuit. If an external magnetic field is applied, a modification in the orbital
motion will be exhibited and diamagnetic effects are produced. However, diamagnetic effects
will be produced if the molecules behave as permanent magnets owing to their net magnetic
moment. Eventually, paramagnetic effect will be produced as a result of change brought
about by collision in the orientation distribution of molecules, which behave as permanent
magnets.
Magnetic Susceptibility:-
When a substance is placed in a magnetic field, it may or may not become
magnetized. If I is the intensity of magnetization induced and H is strength of magnetic field
inducing in it then the strength of the magnetic field in the material represented by B and
known as the magnetic induction, is given by-
B = H + 4 π I
In the above equation the ratio B/H is called the magnetic permeability, μ of the
material, which can be obtained as,
B = H + 4 π I
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B/H = 1 + 4 π I/H
As, B/H is μ, and I /H = k, hence
μ = 1 + 4 π k
Where, k is the volume magnetic susceptibility of the material.
A more useful quantity than k is the molar magnetic susceptibility, Χm obtained from
the equation,
Χm = k ∙ M/ρ = I M/H ρ
Where, M is molecular weight of the compound and ρ is the density.
Now, the volume I (magnetic moment produced per unit volume of the substance)can
be positive or negative (i.e. strength of the field B, in the material may be greater or smaller
than the applied magnetic field H). If I is negative, then Χm is also negative and the material
is said to be diamagnetic. If I is positive, then Χm is positive and the material is said to be
paramagnetic.
Electrons possess magnetic dipoles because of their spin. When electrons are paired
(i.e. their spins are antiparallel), then the magnetic field is cancelled out. Most organic
compounds are diamagnetic, since their electrons are paired; however, odd electron
molecules are paramagnetic.
In the study of coordination compounds, magnetic susceptibility has been used to
obtain information about the nature of bonds and the configuration of coordination
compounds. Organic compounds, which are paramagnetic, are generally free radicals (odd
electron molecules), and the degree of dissociation of the compounds such as hexaphenyl
ethane into triphenyl methane can be measured by means of magnetic susceptibility. In the
same way an atomic and structural refractions have been determined so that corresponding
diamagnetic susceptibilities can be calculated as the molar magnetic susceptibility, which has
both additive and constructive properties.
Methods for Measurement of Magnetic Susceptibility:-
The methods frequently used for susceptibility measurements depend directly or
indirectly on the measurements of this force exerted upon the specimen when placed in a
magnetic field. The methods are classified into two main groups,
1) Non-uniform field methods
2) Uniform field methods
Non-Uniform Field Methods:-
It is employed by Faraday. According to this method a non-homogeneous field with
an axis of symmetry was produced when the poles of the magnet are inclined towards each
other. If a substance is placed in such a non-uniform field in a region where the strength of
the field changes quickly with displacement along the axis of symmetry, then the substance
will be subjected to a force along the axis, which is given by the expression
fx = m∙X∙H∙ (dH/dx)
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Where, m is mass of the substance, X is susceptibility of the substance, H is field
strength, dH/dx is field gradient along the axis.
This method is quite sensitive but according to Hoare and Brindly, it requires very
precise centering of the specimen, which is very difficult to achieve with certainty. Moreover
the measurement of field gradient is also very difficult. This method was proposed by
Faraday and then it was modified by several other workers such as, Curie-Wilson, Oxley,
Bhatnagar-Mathurand Decker.
Uniform Field Methods:-
The substance will suffer an orienting effect, if it is placed in a uniform magnetic
field, unless the body is magnetically isotropic. The moment acquired by the body will be
proportional to the susceptibility per unit volume times the volume of the body multiplied by
the field. The substance however will not experience any displacement.
The force acting on the body is given by,
f = 1/2 k∙ H2/A
Where, A is cross sectional area of the sample, H is uniform field strength and k is
volume of susceptibility of the substance.
The force acting on the specimen can be measured in absolute units by this method.
Out of the methods employing uniform magnetic field, the most important ones are Guoy’s
method for solids and liquids and Quincke’s method for liquids only.
Guoy’s Method :-
The Gouy’s method is less sensitive than the Faraday’s method. It has following
advantages
1. It is easy to adjust and set the apparatus.
2. There is less chance of damage in transit.
3. Relatively small amount of the sample is required to determine the
measurements of the susceptibility.
4. Precise centering of the tube between the two magnetic poles is not necessary
because the method utilizes uniform magnetic field.
5. The determination of the strength of the magnetic field is generally avoided by
making measurements relative to a substance whose susceptibility is known
with accuracy.
Principle:-
In the Gouy’s method a specimen in the form of a uniform cylinder is suspended in
a uniform magnetic field in such a way that one of the ends is in the region of large field
intensity, while the other is in the region of negligible field strength, it experiences a force
due to this field. As a result of this force the specimen tends to move away from the stronger
part to the weaker part of the field, if it is diamagnetic and vice-versa, if it is paramagnetic.
This force can be determined with the help of a sensitive balance in terms of an apparent
change in the weight of the specimen produced due to the application of the magnetic field.
In order to eliminate the difficulty involved in the accurate measurement of the intensity of
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the impressed magnetic field, a substance whose susceptibility is known with certainty is
used as a standard for comparison. The procedure improves significantly with the accuracy of
the susceptibility values obtained by the Guoy’s method.
The Guoy balance represented in Figure 2.2 consists of two parts,
1) Analytical balance
2) Electromagnet
Figure 2.2 Schematic Diagram of Goy Balance
Analytical Balance:-
A 0.01 mg sensitive, air damped semi-micro ‘Mettler’ single pan balance, is used for
magnetic measurements. In order to minimize effects due to vibrations the balance is
enclosed in a wooden case with glass doors and kept on a marble platform. A phosphor-
bronze rod (R), with a small hook at its lower end is attached to the bottom of the pan (P)
through a U-shaped metal extension. The rod passes below the pan into a wooden case
enclosing the electromagnet. The lower end of the phosphor-bronze rod carries an inverted Y-
shaped stirrup (S). The latter supported the horizontal arms of the glass collar in which the
specimen tube (T) is suspended vertically between the poles of the magnet.
Electromagnet:-
In the Gouy’s method the electromagnet used has poles pieces of truncated conical
shape and nickel-plated. The magnet core is made of soft iron having high permeability and
low hysterics. The coils of the magnet are wound in eight sections placed in series of wooden
formers so that any number of turns can be used as desired. The windings are of gauge d.c.
These sections are made of brass spiders for providing ample ventilation. The current of one
ampere at 460 volts is used. The low current employed is beneficial as it minimizes the
heating of the magnetic coils. An ammeter, which could read up to 0.05 ampere and a
rheostat to adjust the current exactly are placed in series with the magnet.
Specimen Tube:-
The Pyrex glass specimen tube of 10 cm in length and with a uniform diameter of
about 0.5 cm is used in the method. The tube is having a ground glass stopper and a mark was
etched at a height of 7 cm from the base, which indicates a level up to which the substance is
to be filled. The tube is held vertically and centrally between the pole pieces in such a way
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that the flat base is between the pole pieces and the other end is in a region of negligible
magnetic field.
Experimental Determination of Magnetic Susceptibility
In the method the specimen tube was washed with chromic acid first and then
thoroughly with distilled water and finally with pure acetone and dried in an oven.
The dry specimen tube was suspended between the pole pieces for sometime for
temperature equilibrium and weighed in the absence of the magnetic field. The same was
weighed in the presence of a magnetic field when a current of 0.35 ampere was passed
through the magnet. The glass tube being diamagnetic showed reduction in weight. The
difference between these two weights represented the diamagnetic ‘pull’ acting on the
specimen tube. Mean of the three closely agreeing readings was obtained to represent the
pull. When the current passed through the magnet, heating of the coils was negligible because
current was passed for only a fraction of a minute. The tube was then filled with different
standard substances and similar apparent change in weight was determined. Deducting the
‘pull’ due to empty tube alone from the above weight, the magnetic force exerted on the
specimen alone was measured. In case of liquids as specimen, they should be allowed to
stand for some time to make them free from air bubbles. The solids used in 41this study were
carefully dried and finally powdered in non-ferrous agate mortar. Small amount of these
substances were taken at a time in the specimen tube and packed as tightly as possible with
the help of a closed fitting glass rod. The substance was packed up to the etched mark.
Adherences of any particles on the side of the tube were removed by means of filter paper
strip.
Precautions:-
During the magnetic susceptibility measurements, the following precautions were
taken
1. The field strength of the magnet was kept constant by maintaining constant
current.
2. The specimen tube was packed as uniformly and as firmly as possible.
3. The tube was suspended vertically between the poles so that the tube should
not touch them.
4. The equipment was installed in such a way that there was no effect of droughts
and convection currents.
Calculations for Magnetic Susceptibility:-
The magnetic susceptibility is determined from the force exerted on a specimen,when
placed in a magnetic field by the expression,
X =2 ℓ F/ (H12- H2
2) +V KmW /W
where, ℓ is length of the specimen tube, F is magnitude of force on the body, W is
mass of specimen, V is volume of the tube up to the mark, Km is volume susceptibility and
H1 and H2 are limits of applied magnetic field.
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During the measurement of the force, ℓ, V and Km remained constant when the
temperature variation in the pole gap is negligible. The magnitude of the current remains
constant with the help of a rheostat and hence H1 and H2 also remain constant. Hence,
X =α F + C/W
Where, for a given tube, α = 2 ℓ / (H12- H2
2) and C = V Km which are constants.
For a given substance, H1 and H2 are constants and therefore α is constant. If there is
no temperature variation in the pole gap during the measurement of the force, the
susceptibility of the medium (Km) in which the specimen is suspended is also constant. Thus
from the force ‘F’ on the specimen, mass susceptibility (Χ) can be measured when ‘α’ and
‘C’ are known.
DETERMINATION OF ‘C’:-
The value of ‘C’ is given by C = V. Km
Where, V is volume occupied by the specimen under investigation, Km is volume
susceptibility of air.
The volume susceptibility of air is known with accuracy; the value of ‘C’ could be
determined.
Determination of ‘Α’:-
It is difficult to measure the values of ℓ, H1 and H2 with great accuracy, hence ‘α’ is
determined by measuring the force exerted on a substance of known magnetic susceptibility.
For example, taking the susceptibility of conductivity water as - 0.712 x 10-6
c.g.s. and
determining the value of the force exerted on water and ‘C’ and the weight of water, the value
of ‘α’ is measured. Similarly a standard solid substance like potassium chloride (- 0.712 x 10-
6c.g.s.) was also used to determine the value of ‘α’. This is confirmed by determining the
value of magnetic susceptibility of different standard substances, whose susceptibilities are
known in the literature. Farquharson has measured the susceptibilities of several sulphur
compounds and compared them with their theoretically obtained values.
UV-Visible Spectroscopy:-
Introduction:-
Several molecules are studied by UV-Visible spectroscopy as they absorb
ultraviolet or visible light. Absorbance is directly proportional to the path length (b) and the
concentration (c) of the absorbing species. Beer-Lambert’s law states that,
A = abc
Where, ‘a’ is a constant of proportionality, called the molar absorbtivity.
An absorption spectrum will show a number of absorption bands corresponding to
structural groups within the molecule, as different molecules absorb radiation of different
wavelengths. For example, the absorption which is observed in the UV region for the
carbonyl group in acetone is of the same wavelength as the absorption from the carbonyl
group in diethyl ketone.
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Electronic Transitions:-
The outer electrons in the molecule are excited during the absorption of UV or
visible radiation. There are three types of electronic transitions
1) Transitions involving p, s, and n electrons
2) Transitions involving charge-transfer electrons
3) Transitions involving d and f electrons
When an atom or molecule absorbs energy, electrons are promoted from their ground
state to an excited state. In a molecule, the atoms can rotate and vibrate with respect to each
other. These vibrations and rotations also have discrete energy levels, which can be
considered as being packed on top of each electronic level.
Absorbing Species Containing, s and n Electrons :-
In organic molecules the absorption of ultraviolet and visible radiation is restricted to
certain functional groups (chromophores) which contain valence electrons of low excitation
energy. The complex spectrum is obtained for such molecules containing chromophores. This
is because the superposition of rotational and vibrational transitions on the electronic
transitions gives a combination of overlapping lines. This appears as a continuous absorption
band.
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σ →σ* Transitions:-
I n this transition an electron in a bonding s orbital is excited to the corresponding anti
bonding orbital. The energy required is large. For example, methane (which has only C-H
bonds, and can only undergo *transitions) shows an absorbance maximum at 125 nm.
Absorption maxima due to σ → σ* transitions are not seen in typical UV-Visible spectra
(200-700 nm).
n →σ* Transitions:-
The n →σ* transitions are possible in saturated compounds containing atoms with
lone pairs (non-bonding electrons). These transitions usually required less energy than σ→σ*
transitions. They can be initiated by the radiation whose wavelength is in the range of 150-
250 nm. Less number of organic functional groups exhibit n →σ* peaks in the UV region.
n→π * and π→π *Transitions:-
Most of organic compounds show transitions of n or electrons to the π * excited
state. This is because the absorption peaks for n→π * and π→π *transitions fall in an
experimentally convenient region of the spectrum (200-700 nm). These transitions need an
unsaturated group in the molecule to provide the π electrons.
Molar absorbtivities from n →σ*transitions are relatively low and range from 10 to
100 dm3 mol
-1cm
-1and the π→π* transitions normally give molar absorbtivities between 1000
and 10000 dm3 mol
-1 cm
-1.
The effect of solvent in which the absorbing species is dissolved is also the important
factor in the spectral studies. Peaks resulting from n→π *transitions are shifted to shorter
wavelengths (blue shift) with increasing solvent polarity. This arises from increased solvation
of the lone pair, which lowers the energy of the n orbital. Often (but not always), the reverse
(i.e. red shift) is seen for π→π* transitions. This is caused by attractive polarization forces
between the solvent and the absorber, which lower the energy levels of both the excited and
unexcited states. This effect is greater for the excited state and so the energy difference
between the excited and unexcited states is slightly reduced, resulting in a small red shift.
This effect also influences π→π* transitions but is overshadowed by the blue shift resulting
from solvation of lone pairs.
Charge-Transfer Absorption:-
Charge transfer absorption is observed in many inorganic species and such species
are called as charge-transfer complexes. These species have one component with electron
donating properties and another component with electron accepting properties. Absorption of
radiation then involves the transfer of an electron from the donor to an orbital associated with
the acceptor. Molar absorbtivities for charge-transfer absorption are large (more than 10000
dm3 mol
-1 cm
-1).
UV-Visible Spectrophotometer:-
The UV-Visible spectrophotometer is used in ultraviolet-visible spectroscopy. It
measures the intensity of radiation passing through a sample (I) and compares it to the
intensity of radiation before it passes through the sample (Io). The ratio I / Io is called the
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transmittance and is usually expressed as a percentage (%T). The absorbance (A) is based on
the transmittance
A = log10I0/It = - log10T
The UV-Visible spectrophotometer can also measure the reflectance. In this case,
the spectrophotometer measures the intensity of radiation reflected from a sample (I), and
compares it to the intensity of radiation reflected from a reference material (Io). The ratio I /
Io is called the reflectance and is usually expressed as a percentage reflectance (%R).
The basic parts of a spectrophotometer are source of radiation, holder for the
sample, diffraction grating in a monochromator or prism to separate the different wavelengths
of radiation and detector. The source of radiation is often a tungsten filament (300-2500 nm),
a deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm), xenon
arc lamps, which is continuous from 160-2000 nm; or more recently, light emitting diodes
(LED) for the visible wavelengths. The detector is typically photomultiplier tube, photodiode,
photodiode array or charge-coupled device (CCD). Photodiode detectors and photomultiplier
tubes are used with scanning monochromators which filter the radiation so that only radiation
of a single wavelength reaches the detector at one time. The scanning monochromator moves
the diffraction grating to ‘step-through’ each wavelength so that its intensity may be
measured as a function of wavelength. Fixed monochromators are used with CCDs and
photodiode arrays. As both of these devices consist of many detectors grouped into one or
two dimensional arrays, they are able to collect radiation of different wavelengths on
different pixels or groups of pixels simultaneously.
A spectrophotometer can be either single beam or double beam. In a single beam
instrument all of the radiation passes through the sample cell. Io must be measured by
removing the sample. This was the earliest design, but is still in common use.
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In a double-beam instrument, the radiation is split into two beams before it reaches
the sample. One beam is used as the reference; the other beam passes through the sample.
The reference beam intensity is taken as 100% transmission (or 0 absorbance)and the
measurement displayed is the ratio of the two beam intensities. Some doublebeam
spectrophotometers have two detectors (photodiodes) and the sample and reference beam are
measured at the same time. In other spectrophotometers, the two beams pass through a beam
chopper, which blocks one beam at a time. The detector alternates between measuring the
sample beam and the reference beam in synchronism with the chopper. There may also be
one or more dark intervals in the chopper cycle. In this case the measured beam intensities
may be corrected by subtracting the intensity measured in the dark interval before the ratio is
taken.
In UV-Visible spectrophotometry, most often samples used are liquids, although the
absorbance of gases and even of solids can also be measured. Samples are typically placed in
a transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape, commonly
with an internal width of 1 cm. (This width becomes the path length, b, in the Beer-Lambert’s
law). Test tubes can also be used as cuvettes in some instruments. The type of sample
container used must allow radiation to pass over the spectral region of interest. The most
widely applicable cuvettes are made of high quality fused silica or quartz glass because these
are transparent throughout the UV, visible and near infrared regions. Glass and plastic
cuvettes are also common, although glass and most plastics absorb in the UV, which limits
their usefulness to visible wavelengths. Specialized spectrophotometers have also been made.
These include attaching spectrophotometers to telescopes to measure the spectra of
astronomical features. In simpler instruments the absorption is determined at one wavelength
at a time and then compiled into a spectrum by the operator. A complete spectrum of the
absorption at all wavelengths of interest can often be produced directly by a more
sophisticated spectrophotometer.
Microspectrophotometry :-
The microscopic samples are studied by UV-Visible spectroscopy by integrating
an optical microscope with UV-Visible optics, white sources of radiations, monochromator
and sensitive detector such as charge-coupled device (CCD) or photomultiplier tube (PMT).
These are single beam instruments because only a single optical path is available. Modern
instruments are capable of measuring UV-Visible spectra in both reflectance and
transmission of micron-scale sampling areas. The advantages of using such instruments is
that they are able to measure microscopic samples but are also able to measure the spectra of
larger samples with high spatial resolution. Such instruments are used in the forensic
laboratory to analyze the dyes and pigments in individual textile fibres, microscopic paint
chips and the colour of glass fragments. Micro spectrophotometers are used in the
semiconductor and micro-optics industries for monitoring the thickness of thin films after
they have been deposited. They are also used in the field of materials science and biological
studies and for determining the energy content of coal and petroleum source rock by
measuring the vitrinite reflectance.
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Infrared Spectroscopy:-
Introduction:-
For structural determination, infrared (IR) spectroscopy is one of the most widely
used spectroscopic techniques and simply, it is the absorption measurement of different IR
frequencies by a sample positioned in the path of an IR beam. The main objective of IR
spectroscopic analysis is to determine the chemical functional groups in the sample. Different
functional groups absorb characteristic frequencies of IR radiation. Using various sampling
accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids
and solids. Thus, IR spectroscopy is vital and popular tool for structural elucidation and
compound identification.
IR Frequency Range and Spectrum Presentation:-
Infrared radiation is bound by the red end of the visible region at high frequencies and
the microwave region at low frequencies and it spans a section of the electromagnetic
spectrum having wave numbers roughly from 13000 to 10 cm–1
, or wavelengths from 0.78 to
1000 μm. The IR region is commonly divided into three smaller areas, near IR, mid IR, and
far IR.IR absorption positions are generally expressed as either wave numbers or
wavelengths. Wave number defines the number of waves per unit length. Thus, wave
numbers are directly proportional to frequency, as well as the energy of the IR absorption.
The wave number unit (cm–1
, reciprocal centimetre) is more commonly used in modern IR
instruments that are linear in the cm–1
scale. In the contrast, wavelengths are inversely
proportional to frequencies and their associated energy. At present, the recommended unit of
wavelength is μm (micrometers), but μ (micron) is used in some older literature.IR studies
are generally expressed in the form of a spectrum with wavelength or wavenumber as the x-
axis and absorption intensity or percent transmittance as the y-axis. Transmittance (T) is the
ratio of radiant power transmitted by the sample (I) to the radiant power incident on the
sample (Io). Absorbance (A) is the logarithm to the base 10 of the reciprocal of the
transmittance (T).
A = log10 (1 / T) = –log10T = –log10 (I / Io)
As transmittance ranges from 0 to 100%T whereas absorbance ranges from infinity to
zero, the transmittance spectra provide better contrast between intensities of strong and weak
bands.
Theory of Infrared Absorption:-
All the atoms in molecules are in continuous vibration with respect to each otherat
temperatures above absolute zero. When the frequency of a specific vibration of atoms in a
molecule is equal to the frequency of the IR radiation directed on the molecule, the molecule
absorbs the IR radiation. Each atom has three degrees of freedom, corresponding to motions
along any of the three Cartesian coordinate axes (x, y, z). A polyatomic molecule of n atoms
has 3n total degrees of freedom. However, 3 degrees of freedom are required to describe
translational motion, the motion of the entire molecule through space. Additionally, 3 degrees
of freedom correspond to rotation of the entire molecule. Therefore, the remaining (3n – 6)
degrees of freedom are true, fundamental vibrations for non-linear molecules. Linear
molecules possess (3n – 5) fundamental vibrational modes because only 2 degrees of freedom
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are sufficient to describe rotation. Among the (3n – 6) or (3n – 5) fundamental vibrations
(also known as normal modes of vibration), those that produce a net change in the dipole
moment may result in an IR activity and those that give polarizability changes may give rise
to Raman activity. Naturally, some vibrations can be both IR- and Raman- active. The total
number of observed absorption bands is generally different from the total number of
fundamental vibrations. It is reduced because some modes are not IR active and a single
frequency can cause more than one mode of motion to occur. Conversely, additional bands
are generated by the appearance of overtones (integral multiples of the fundamental
absorption frequencies), combinations of fundamental frequencies, differences of
fundamental frequencies, coupling interactions of two fundamental absorption frequencies
and coupling interactions between fundamental vibrations and overtones or combination
bands (Fermi resonance). The intensity of fundamental band is more than those of overtone,
combination and difference bands. The combination and blending of all the factors thus
create a unique IR spectrum for each compound.
Stretching and bending vibrations are the major types of molecular vibrations.
Infrared radiation is absorbed and the associated energy is converted into these types of
motions. The absorption involves discrete, quantized energy levels. However, the individual
vibrational motion is usually accompanied by other rotational motions. These combinations
lead to the absorption bands, not the discrete lines, commonly observed in the mid IR region.
Dispersive Spectrometers:-
It is introduced in the mid-1940s and widely used since, provided the robust
instrumentation required for the extensive application of this technique.
Spectrometer Components:-
It consists of three basic components, radiation source, monochromator and
detector. An inert solid heated electrically to 1000 to 1800°C is the common radiation source
for the IR spectrometer. Three popular types of sources are Nernst glower (constructed of
rare earth oxides), Globar (constructed of silicon carbide) and Nichromecoil. They all
produce continuous radiations, but with different radiation energy profiles.
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The monochromator is a device used to disperse a broad spectrum of radiation and
provide a continuous calibrated series of electromagnetic energy bands of determinable
wavelength or frequency range. Prisms or gratings are the dispersive components used in
conjunction with variable-slit mechanisms, mirrors, and filters. For example, a grating rotates
to focus a narrow band of frequencies on a mechanical slit. Narrower slits facilitate the
instrument to distinguish more closely spaced frequencies of radiation, resulting in better
resolution. Wider slits allow more radiation to reach the detector and provide better system
sensitivity. Thus, certain compromise is exercised in setting the desired slit width.
The detectors used in dispersive IR spectrometers can be classified into two
classes: thermal detectors and photon detectors. Thermal detectors include thermocouples,
thermistors and pneumatic devices (Golay detectors). They measure the heating effect
produced by infrared radiation. A variety of physical property changes are quantitatively
determined viz. expansion of a non absorbing gas (Golay detector), electrical resistance
(thermistors) and voltage at junction of dissimilar metals (thermocouple). Photon detectors
rely on the interaction of IR radiation and a semiconductor material. Non conducting
electrons are excited to a conducting state. Thus, a small current or voltage can be generated.
Thermal detectors give a linear response over a wide range of frequencies but exhibit slow
response time and lower sensitivities than photon detectors.
Spectrometer Design:-
In a typical dispersive IR spectrometer, radiation from a broad-band source passes
through the sample and is dispersed by a monochromator into component frequencies. Then
the beams fall on the detector, which generates an electrical signal and results in a recorder
response.
Most dispersive spectrometers have a double-beam design. Two equivalent beams
from the same source pass through the sample and reference chambers respectively. Using an
optical chopper (such as a sector mirror), the reference and sample beams are alternately
focused on the detector. Commonly, the change of IR radiation intensity due to absorption by
the sample is detected as an off-null signal that is translated into the recorder response
through the actions of synchronous motors.
Fourier Transform Spectrometer:-
For most of applications Fourier transform spectrometer has recently replaced
dispersive instruments due to its superior speed and sensitivity. It greatly extended the
capabilities of infrared spectroscopy and applied to many areas that are very difficult or
nearly impossible to analyze by dispersive instruments. Instead of viewing each component
frequency sequentially, as in a dispersive IR spectrometer, all frequencies are examined
simultaneously in Fourier transform infrared (FTIR) spectroscopy.
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Spectrometer Components:-
Radiation source, interferometer and detector are the three basic components in an FT
system. The same types of radiation sources are used for both dispersive and Fourier
transform spectrometers. However, to provide better power and stability, the source is more
often water-cooled in FTIR instruments.
To differentiate and measure the absorption at component frequencies a
completely different approach is taken in an FTIR spectrometer. The monochromator is
replaced by an interferometer, which divides radiant beams, generates an optical path
difference between the beams and then recombines them in order to produce repetitive
interference signals measured as a function of optical path difference by a detector. As its
name implies, the interferometer produces interference signals, which contain infrared
spectral information generated after passing through a sample. Michelson interferometer is a
most commonly used interferometer. It consists of three active components, moving mirror,
fixed mirror and beam splitter. The two mirrors are perpendicular to each other. The beam
splitter is a semi reflecting device and is often made by depositing a thin film of germanium
onto a flat KBr substrate. Radiation from the broadband IR source is collimated and directed
into the interferometer and impinges on the beam splitter. At the beam splitter, half the IR
beam is transmitted to the fixed mirror and the remaining half is reflected to the moving
mirror. After the divided beams are reflected from the two mirrors, they are recombined at
the beam splitter. Due to changes in the relative position of the moving mirror to the fixed
mirror, an interference pattern is generated. The resulting beam then passes through the
sample and is eventually focused on the detector. Thus, the detector response for a single-
frequency component from the IR source is first considered. This simulates an idealized
situation where the source is monochromatic, such as a laser source. As previously described,
differences in the optical paths between the two split beams are created by varying the
relative position of moving mirror to the fixed mirror. If the two arms of the interferometer
are of equal length, the two split beams travel through the exact same path length. The two
beams are totally in phase with each other; thus, they interfere constructively and lead to a
maximum in the detector response. This position of the moving mirror is called the point of
zero path difference (ZPD). When the moving mirror travels in either direction by the
distance λ/4, the optical path (beam splitter–mirror–beam splitter) is changed by 2 (λ /4) or λ
/2. The two beams are 180° out of phase with each other and thus interfere destructively. As
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the moving mirror travels another λ /4, the optical path difference is now 2 (λ /2) or . The two
beams are again in phase with each other and result in another constructive interference.
When the mirror is moved at a constant velocity, the intensity of radiation reaching
the detector varies in a sinusoidal manner to produce the interferogram output. The
interferogram is the record of the interference signal. It is actually a time domain spectrum
and records the detector response changes versus time within the mirror scan. If the sample
happens to absorb at this frequency, the amplitude of the sinusoidal wave is reduced by an
amount proportional to the amount of sample in the beam.
A more complex interferogram is obtained as an extension of the same process to
three component frequencies which is the summation of three individual modulated waves. In
contrast to this simple, symmetric interferogram, the interferogram produced with a
broadband IR source displays extensive interference patterns. It is a complex summation of
superimposed sinusoidal waves, each wave corresponding to a single frequency. When this
IR beam is directed through the sample, the amplitudes of a set of waves are reduced by
absorption if the frequency of this set of waves is the same as one of the characteristic
frequencies of the sample.
The interferogram contains information over the entire IR region to which the
detector is responsive. A mathematical operation known as Fourier transformation converts
the interferogram (a time domain spectrum displaying intensity versus time within the mirror
scan) to the final IR spectrum, which is the familiar frequency domain spectrum showing
intensity versus frequency. During the mirror scan, the detector signal is sampled at small and
precise intervals. The sampling rate is controlled by an internal, independent reference, a
modulated monochromatic beam from helium neon (He-Ne) laser focused on a separate
detector.
Deuterated triglycinesulfate (DTGS) and mercury cadmium telluride (MCT) are the
two most popular detectors for a FTIR spectrometer. The response times of many detectors
(e.g. thermocouple and thermistor) used in dispersive IR instruments are too slow for the
rapid scan times (1 sec or less) of the interferometer. The DTGS detector is a pyroelectric
detector that delivers rapid responses because it measures the changes in temperature rather
than the value of temperature. The MCT detector is a photon (or quantum) detector that
depends on the quantum nature of radiation and also exhibits very fast responses. DTGS
detectors operate at room temperature while MCT detectorsare maintained at liquid nitrogen
temperature (77 °K). In general, the MCT detector is faster and more sensitive than the
DTGS detector.
Spectrometer Design :-
The basic design of the instrument is quite simple. The IR radiation from a broadband
source is first directed into an interferometer, where it is divided and then recombined after
the split beams travel different optical paths to generate constructiveand destructive
interference. Next, the resulting beam passes through the sample compartment and reaches to
the detector.
Most benchtop FTIR spectrometers are single beam instruments. Unlike double beam
grating spectrometers, single beam FTIR does not obtain transmittance or absorbance IR
spectra in real time.
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A typical operating procedure is described as follows
A background spectrum is first obtained by collecting an interferogram(raw data),
followed by processing the data by Fourier transform conversion. This is a response curve of
the spectrometer and takes account of the combined performance of source, interferometer
and detector. The background spectrum also includes the contribution from any ambient
water (two irregular groups of lines at about 3600 cm–1
and about 1600 cm–1
) and carbon
dioxide (doublet at 2360 cm–1
and sharp spike at 667 cm–1
) present in the optical bench.
Next, a single beam sample spectrum is collected. It contains absorption bandsfrom
the sample and the background (air or solvent).
The ratio of the single beam sample spectrum against the single beam background
spectrum results in a ‘double beam’ spectrum of the sample. To reduce the strong background
absorption from water and carbon dioxide in the atmosphere, the optical bench is usually
purged with an inert gas or with dry, carbon dioxide–scrubbed air (from a commercial purge
gas generator). The alignment of spectrometer which includes optimization of the beam
splitter angle is required when a sample accessory is changed as part of a periodic
maintenance.
Thermal Analysis:-
The thermal analysis is concerned with the change in temperature. The change in
temperature always causes some changes in the substance concerned, e.g. water is
transformed from liquid to solid (ice) when it is deprived of heat of 80 cal per gram and from
solid to liquid when it is given same amount of heat. When it is heated to 100oC i.e.a heat of
540 cal/gm is further given, it is transformed into gas (vapours). These heats are latent heats
of fusion and latent heat of vapourization respectively. Heated or cooledmatters undergo
various changes, not only in the state but also in enthalpy, weight, dimensions and electric
resistance.
According to the ICTA (International Confederation on Thermal Analysis) the
thermal analysis is a group of techniques in which physical property of a substance is
measured as a function of temperature whilst a substance is subjected to controlled
temperature programmed.
In these methods, the change in physical and chemical properties of the material
subjected to the programmed heating or cooling at the pre-determined rate in the controlled
atmosphere are monitored as a function of temperature. Thermoanalyticaltechniques include
several methods which measures the properties such as mass, energy, dimension, modulus of
electricity, dielectric constant, etc. Each technique is identified with the physical parameter
measured.
If the mass of a substance is measured as a function of temperature, then the
technique is called as Thermogravimetry (TG) whereas if temperature difference between the
sample and the thermally inert reference material is measured as a function of temperature,
the technique is called as Differential Thermal Analysis (DTA). These techniques are
discussed in lucid manner in several books and monographs
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Table 2.1 Common Thermal Analysis Techniques
Technique Property Acronym
Thermogravimetry Mass TG
Thermomagnatometry Apparent mass Tm
Differential Thermal Analysis Temperature DTA / TA
Differential Scanning Calorimetry Heat/Heat Flux DSC
Thermodilatometry Dimensions TD
Thermomechanical Analysis Mechanical Properties TMA
Dynamic Mechanical Analysis DMA
Dielectric Thermal Analysis Electrical Properties DETA
Evolved Gas Detection Volatiles EGD
Evolved Gas Analysis EGA
Out of several thermo analytical techniques only two techniques namely, TG and
DTA were employed in the investigation hence are briefly illustrated in the following
sections.
Thermogravimetry (TG):-
It is a technique in which the mass of the sample is monitored against a time or
temperature of the sample when it is subjected to a programmed temperature change in a
specified atmosphere. The plot of a mass change versus temperature is termed as a
thermogravimetric or a TG curve.
The continuous recording of mass change offers special advantage; in that there is
much less possibility of missing the step corresponding to the formation of the weakly stable
intermediate in the multistep thermal decomposition reactions.
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TG curves describe some characteristic temperatures called the inception (Ti),
inflection (Tm) and final temperature (Tf). Inception temperature indicates the onset of the
mass change in the sample and generally corresponds to the temperature at which the
cumulative mass change exceeds the sensitivity of the thermogravimetric system. The
inflection temperature represents the temperature at which the rate of mass changewith
respect to temperature is maximum. The final temperature corresponds to the temperature at
which the mass loss / gain in the given step is complete. These temperatures for a given
sample size vary with several experimental parameters such as the sample size, heating rate,
type of sample holder, ambient atmosphere, etc. and sample characteristics such as particle
size, etc. Literature study gives many examples demonstrating the role of these experimental
parameters on the nature of the TG curve.
The thermo balance is used for monitoring the mass change as a function of
temperature. Such system can also be employed to monitor the mass change in the sample
isothermally. The thermo balance consists of a balance and a furnace as the main
components. The former monitors the mass change and the latter heats the sample at the
programmed rate or isothermally. The other important accessories of a thermo balance are the
gas and vacuum manifolds, temperature programmer and sample carrier. Several types of
thermo balances with different sensitivities and load capacities are available commercially.
Thermo balances have to be calibrated for mass and temperature using the standard materials
recommended by ICTAC.
Differential Thermal Analysis (DTA):-
Differential thermal analysis is one of the simplest and oldest thermal techniques.
It is employed to study the physical and chemical transformations in the materials associated
with the energy changes. The technique is associated with the measurement of temperature
difference between the sample and the thermally inert reference material when both are
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heated simultaneously at the predetermined constant heating rate in controlled atmosphere.
The sample as well as reference material kept in identical sample holders are located in the
uniform temperature zone of the furnace. The sample holders are made of silica, alumina or
platinum group metal alloys. The choice of the holder / container depends on several factors.
The system being investigated should be physically and chemically compatible with the
container material. The container should not crack due to dimensional incompatibility of the
product of transformation with it. Moreover, the sample under investigation undergoing
physical or chemical transformation like melting, vaporization or decomposition should not
promote any side reactions, which are often experienced when the container made up of
platinum group metal alloys are used.
The sensitivity of DTA instrument not only depends on the type of thermocouple
used but also on the sample container, its shape and the material from which it is fabricated.
The most commonly employed thermocouples in DTA are made either of base metal alloys
or platinum group metal alloys. The former, which includes chromel / alumel thermocouple,
can be used in inert or oxidizing atmosphere up to 1150oC and has an average sensitivity of
40 μV/oC. Platinum metal and their alloys are however known to vaporize at higher
temperature with the formation of their volatile oxides above 1200oC. This could result in the
loss of these precious metals on prolonged use. Continuous use of such thermocouples at high
temperatures could also result in the change in the sensitivity of these thermocouples due to
change in the composition of the alloy wire. In DTA, temperature difference between the
sample and the reference (ΔT) is plotted against the temperature T of either the sample or the
reference. The temperature difference ΔT is plotted on Y-axis and the temperature of the
sample is plotted on the X-axis.
The peaks obtained in a plot of ΔT vs T can either be exothermic or endothermic
depending on whether the energy is released or absorbed during the physical or chemical
process under consideration. Conventionally, the peak resulting due to exothermic process is
plotted above X-axis and that due to endothermic process below the X-axis. The initiation
temperature of the peak (Ti) indicates the temperature at which the process begins; the peak
temperature Tm, corresponds to the temperature at which rate of transformation of the
substance with respect to temperature is maximum and at the termination temperature (Tf)
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refers to the temperature at which the process is completed. These temperatures for first peak
are marked. The area under the DTA peak gives the measure of the energy / heat released or
absorbed during the process and is directly proportional to the active mass transformed. The
ratio of area traversed at any temperature ‘T’ on the DTA peak to the total area corresponds
to the fraction converted ‘α’.
In high sensitivity mode DTA can also be used for determination of glass transition
temperature (Tg). Just as conventional TG balance can be adopted for isothermal
measurements, DTA instrument also can be employed for following the temperature change
in the sample when the system comprising of the sample and the reference holder is
maintained at constant temperature and the process is initiated in the sample. Such approach
has been adopted in the study of the kinetics of solid state reactions, assuming that the
amount of heat absorbed / evolved is proportional to the amount of material transformed.
The DTA curve is highly influenced by several experimental parameters such as
sample size, heating rate, material and shape of container, ambient atmosphere, etc. The
knowledge of the influence of these factors on DTA curve is extremely important in the
interpretation of DTA curves.
X-Ray Diffraction:-
X-ray diffraction has been in use since the early part of twentieth century for the
fingerprint characterization of crystalline materials and the determination of crystal structures
and is one of the most important and useful techniques in solid state science. XRD was used
to confirm the formation of the metal oxides. Thus by matching the pattern recorded for the
sample with that in the standard X-ray diffraction file, the unknown can be identified.
Theory of X-Ray Diffraction:-
There are two categories of solid materials, crystalline and amorphous. When the
atoms or molecules are arranged in regular fashion in three dimensions, it is known as
crystalline state, while if atoms are arranged in irregular fashion then it is amorphous state.
The relatively random arrangement of molecules in non-crystalline materials makes them
poor scatters of X-rays, resulting in broad diffused maxima in their diffraction patterns. The
X-ray patterns of the amorphous materials are quite distinguishable from those of crystalline
specimen, which give sharply defined diffraction patterns. In crystalline materials, periodic
arrangement of atoms in all three dimensions is observed and is repeated over long distances.
Such arrangement gives rise to the lattice planes with fixed distance between adjacent planes.
The distance between the planes are related to the lattice parameters, the cell edges and inter
planer angles, which are the fingerprints for its identification. The distance‘d’ between the
two successive planes in the crystal is determined experimentally by X-ray diffraction.
X-rays are electromagnetic radiations of wavelength around 1Ao. They are
produced when a beam of electrons accelerated through ~30 kV is allowed to strike a metal
target such as Cu, Mo and Cr, etc. The high energy electrons knock off from the orbit close to
the nucleus and the electrons from higher level jump to the vacant levels and energy released
during the transition appears as X-rays. These are termed as characteristic X-rays. The
background or white radiations are also produced due to interaction of high energy electrons
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with matter and their subsequent deceleration. These background X-rays are suitably filtered
and X-rays with energy in the range 0.01 to 10 nm are employed in the X-ray diffraction
studies.
Principle of X-Ray Diffraction:-
The X-rays are scattered by the electrons or atoms without change in the wavelength,
the phenomenon is called as X-ray diffraction. In crystals the inter planer distance is of the
same order of magnitude as that of wavelength of X-rays and hence the crystal acts as a
grating for X-rays. The diffraction of X-rays by crystals obeys Bragg’s law
nλ = 2d sinθ
Where, n is the order of diffraction, is the wavelength of monochromatic X-rays and d
is the inter planer distance. It is clear that for the given values of and , the inter planer
distance ‘d’ can be evaluated.
When a narrow X-ray beam strikes the surface of a crystal like NaCl at a glancing
angle (θ) it meets an array of ions in parallel planes AA, BB separated by a inter planer
distance‘d’. The incident radiation LM is reflected as MN from the plane AA and the incident
radiation PQ is reflected as QR from the plane BB and so on. The second radiation PQR has
to travel a longer path than the first radiation LMN, the extra path being SQT, from the
geometry of the crystal planes and the laws of optical reflection is 2d sin (θ). If the two
reflected radiations are in phase then the path difference has to be an integral multiple of
� , the wavelength of the X-ray.
In powder method, a monochromatic beam of X-ray falls on the finally powdered
substances to be examined. The diffracted beam may be detected either by surrounding the
sample with a strip of photographic film or by using a movable detector, such as Geiger
counter, connected to a chart recorder such as diffractometer, which gives series of peaks on
a strip of chart paper. The peak positions and peak heights obtained from the chart are useful
for phase identification and phase analysis.
X-Ray Diffraction in Thermal Studies:-
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During the course of thermal analysis of solid materials the characterization of
materials is essential. X-ray diffraction has been used in solid state reactions and thermal
decomposition. In decomposition reactions, TG and DTA may be used to determine
individual decomposition steps, e.g. thermal decomposition of calcium oxalate monohydrate
CaC2O4.H2O + CaC2O4 CaCO3 CaO
The decomposition occurs in three steps giving intermediates as anhydrous calcium
oxalate and calcium carbonate and finally to calcium oxide at 800oC. The characterization of
these decomposition products is further supported by X-ray diffraction patterns, which are
characteristics of each compound. The products of solid state reactions are usually in the
form of a powder or a sintered polycrystalline material. X-ray diffraction technique is the best
for analyzing the product since this technique explains which crystalline material or mixture
of phases are present. X-ray diffraction technique also provides the information about the
completion of the reaction.
References:-
1. AMC Technical Briefs, Ed. Michael Thompson, Analytical Methods Committee,
AMCTB No 29, The Royal Society of Chemistry, 45 (2008)
2. R. Kellner, J.M. Mermet, M. Otto and H.M. Widmer, ‘Analytical Chemistry’,
Wiley-VCH, 433 (1997).
3. G. Raj, ‘Advanced Physical Chemistry’, Goel Publisher House, Meerut, 110 (1995).
4. J.N. Murrell and A.D. Jenkins, ‘Properties of Liquids and Solutions’,
Wiley-Interscience, New York, (1994).
5. M. Faraday, ‘Experimental Researches in Electricity and Magnetism’, (1949).
6. E. Muller, J. Electrochem., 45, 593 (1939).
7. W. Klemm, J. Electochem., 45, 503 (1939).
8. F.W. Gray and J.H. Cruickshank, Trans. Faraday Soc., 31, 1491 (1935).
9. P. Langevin, Ann. Chim. Phys., 4, 70 (1905).
10. F.E. Hoare and C.W. Brindly, Proc. Roy. Soc., London, 152 A, 342 (1935).
11. P. Curie and J. Wilson, Phys., 197, 263 (1895).
12. A.E. Oxley, Trans. Roy. Soc., London, 214 A, 109 (1914).
13. S.S. Bhatnagar and K.N. Mathur, ‘Physical Principles and Applications of
Magneto-chemistry’, Macmillan and Co. Ltd., London, (1935).
14. H. Decker, Ann. Phys., 79, 324 (1926).
15. L.G. Guoy, Compt. Rend., 109, 935 (1889).
16. G. Quincke, Ann. Phys., 24, 347 (1885).
17. J. Farquharson, Phil. Mag., 14, 1003 (1932).72
18. F.A. Cotton and G. Wilkinson, ‘Advanced Inorganic Chemistry’, John Wiley and
Sons, New York, (1966).
19. L.J. Bellamy, ‘The Infrared Spectra of Complex Molecules’, Chapman and Hall,
2 (1980).
20. G. Herzberg, ‘Molecular Structure and Molecular Spectra’, Van Nostrand,
London, 1 (1950).
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21. J.R. Ferraro and L.J. Basilo, ‘Fourier Transform Infrared Spectroscopy:
Applications to Chemical Systems’, Academic Press, 2 (1979).
22. J.O. Hill, ‘Better Thermal Analysis and Calorimetry’, 3rd Ed., International
Confederation for Thermal Analysis, (1993).
23. A. Blazek, ‘Thermal Analysis’, Reinhold, Van Nostrand, London, (1973).
24. W.W. Wendland, ‘Thermal Analysis’, Interscience, (1985).
25. P.D. Garn, ‘Thermoanalytical Methods of Investigation’, Academic Press,
New York, (1963).
26. P. Daniel, ‘Thermal Analysis’, Kogan Page Ltd., London, (1973).
27. C.J. Keattch and D. Dollimer, ‘An Introduction to Thermogravimetry’, Heydon,
London, (1975).
28. M.D. Judd and M.I. Pope, ‘Differential Thermal Analysis’, Heydon, London,
(1977).
29. G. Litplay, M. Berenyll and E. Sarkany, Hung. Sci. Inst., 15, 31 (1968).
30. M.D. Karkhanavala, A.B. Phadnis and V.V. Deshpande, J. Thermal Anal., 2, 259
(1970).73
31. S.R. Dharwadkar, A.B. Phadnis, S. Chandrashekharaiah and M.D. Karkhanavala,
J. Thermal Anal., 18, 185 (1980).
32. L.V. Azaroff, ‘Elements of X-ray Crystallography’, McGraw Hill Co., New York,
(1968).
33. L.S. Dentglasser, ‘Crystallography and its Applications’, Reinhold Co. Ltd.,
Van Nostrand, London, (1977)
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Chapter 3
Experimental
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Experimental
Chemicals :-
Mixed ligand Zn (II) complexes were prepared by using 8-hydroxyquinoline (HQ)
as a primary ligand and different amino acids (HL) such as L-valine, L-asparagine, L-
glutamine, L-arginine and L-methionine as secondary ligand.
For synthesis of the complexes, analytical grade zinc chloride dihydrate were used
as such without further purification. Amino acids, L-valine, L-asparagine, L-glutamine, L-
arginine and L-methionine and 8-hydroxyquinoline were obtained from S.D. Fine
Chemicals, Mumbai. Solvents like ethanol, dimethylformamide (DMF) and
dimethylsulphoxide (DMSO) and laboratory grade chemicals whenever used were distilled
and purified according to standard procedures.
Apparatus:-
Analytical balance was used for the samples weighing more than 50 mg, while
semi-micro balance was used for the samples weighing less than 50 mg. All glasswares used
were made of pyrex glass and they are calibrated by standard methods.
Synthesis of Mixed Ligand Zinc (II) Complexes:-
The zinc (II) complexes were synthesized from zinc (II) chloride dihydrate, 8-
hydroxyquinoline (HQ) as a primary ligand and different amino acids (HL) such as L-valine,
L-asparagine, L-glutamine, L-arginine and L-methionine as secondary ligand.
An aqueous solution (10 cm3) of zinc (II) chloride dihydrate (136.29 mg,
1mmol) was mixed with ethanolic solution (10 cm3) of 8-hydroxyquinoline (145 mg,
1mmol). The mixture was stirred and kept in a boiling water bath for 10 minutes. To this hot
solution, an aqueous solution (10 cm3) of amino acid (1 mmol) was added with constant
stirring. The reaction mixture (1:1:1 molar proportion) was taken in water bath and heated
for about 10 minutes till the temperature reached to 50oC. The pH of the mixture was raised
by adding dilute ammonia solution in the reaction mixture and complex was obtained. Then
the mixture was cooled and solid complex obtained was filtered, washed with water
followed by ethanol. The complexes thus synthesized were dried under vacuum.
Elemental Analysis:-
The purity of the all ligands used for the synthesis of mixed ligand Zn (II)
complexes was ascertained by recording their exact melting points. The elemental analysis
of mixed ligand Zn (II) complexes for carbon, hydrogen and nitrogen was carried out on
Thermo Finnigan Elemental Analyzer, Model No. FLASH EA 1112 Series at Department of
Chemistry, I.I.T., Mumbai while copper and zinc in the complexes were estimated
complexometrically as per the methods described below.
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Complexometric Estimation of Zinc:-
i) Decomposition of the Zinc (II) Complex:-
The 20 cm3concentrated HNO3 was added to 0.35 gm of zinc (II) complex
(accurately weighed) and the mixture was evaporated to dryness in a beaker on a sand bath.
To the same mixture, few drops of HClO4 in 2 cm3concentrated HNO3 were then added and
the content of the beaker was again evaporated to dryness. The procedure was repeated till
the organic moiety was decomposed completely.
ii) Complexometric Titration:-
The 100 cm3 of dilute HCl was added by slow heating to the content of beaker
and the whole mixture was heated till boiling. In a conical flask 10 cm3
of above solution
was taken and 1 test tube distilled water and 10 cm3
of buffer solution of pH 10 was added to
it. The resultant solution was titrated with standard solution of EDTA by using Eriochrome
black T indicator till the colour change obtained from pink to light blue.
Physicochemical Studies:-
Conductance:-
All the mixed ligand zinc (II) complexes were found to be insoluble in water and
in common organic solvents viz. ethyl alcohol, acetone, chloroform, etc. but they are
partially soluble in DMF and DMSO. Hence 10-3
M solutions of all these complexes were
prepared by dissolving them in DMSO. The molar conductance of all mixed ligand zinc (II)
complexes were measured on an Equiptronics Autoranging Conductivity Meter Model No.
EQ-667 with a dip type conductivity cell fitted with platinum electrodes (cell constant = 1.0
cm-1
).
Magnetic Susceptibility:-
The Guoy’s method was applied to measure room temperature magnetic
susceptibility of all mixed ligand zinc (II) complexes. In this method Hg [Co(SCN)4] was
used as a calibrant. The room temperature magnetic susceptibility study was carried out at
Department of Chemistry, I.I.T. Mumbai. Then using Pascal’s constant5effective magnetic
moments were calculated after applying diamagnetic corrections for the ligand and metal
ions.
Electronic Spectra:-
The electronic spectra of all mixed ligand zinc (II) complexes in DMSO solution (10-4
M)
were recorded in the ultraviolet and visible region on Shimadzu UV / VIS-160
Spectrophotometer using a quartz cell of 1 cm optical path. Calibration of the instrument
was ensured with 0.0098% KMnO4.
Infra-red Spectra:-
Infra-red spectra of all the ligands and their mixed ligand zinc (II) complexes were
recorded in KBr disc on a Perkin-Elmer FTIR Spectrophotometer Model 1600 at
Department of Chemistry, I.I.T. Mumbai. The pellets were prepared by taking necessary
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precautions to avoid moisture. The instrument calibration with respect to wave number and
percent transmission was confirmed by recording the spectrum of standard polystyrene film.
From the spectra, the characteristic groups were assigned according to the different
frequencies.
Thermal Studies:-
Thermal studies (TG and DTA) of all mixed ligand zinc (II) complexes were
carried out in controlled nitrogen atmosphere on a Perkin-Elmer Diamond TG-DTA
instrument at Department of Chemistry, I.I.T., Mumbai by recording the change in weight of
the complexes on increasing temperature up to 900oC at the heating rate of 10
oC per minute.
X-Ray Diffraction:-
X-ray diffraction pattern of decomposed zinc (II) complexes were recorded on X-
ray diffractometer (SIEMENS Model D-500) at 900oC in controlled nitrogen atmosphere at
the scanning rate of (2 = 10) min-1
using monochromatized X-ray beam of wavelength
0.15405 nm at Department of Chemistry, I.I.T., Mumbai.
References:-
1. A.I. Vogel, ‘Textbook of Practical Organic Chemistry’, Longmans Green and
Co. Ltd., London, 5th Ed., (1989).
2. D.D. Perrin, D.R. Perrin and W.L.F. Armarego, ‘Purification of Laboratory
Chemicals’, Pergamon Press Ltd., 2nd
Ed., (1980).
3. A.I. Vogel, ‘Textbook of Quantitative Inorganic Analysis’, Longmans Green and Co.
Ltd., 5th Ed., United Kingdom, (1989).
4. A.I. Vogel, ‘Quantitative Inorganic Analysis’, 4th
Ed., ELBS, (1965).
5. P.W. Selwood, ‘Magnetochemistry’, Interscience, New York, 2nd
Ed., (1956).
6. S.P. Ross and D.W. Wilson, Spectrovision, 4, 10 (1961).
7. K. Nakanishi, ‘Infrared Absorption Spectroscopy-Practical’, Holden Day Inc.,
San Francisco, (1962).
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Chapter 4
Results and discussion
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Results and Discussion
In the present work, mixed ligand zinc (II) complexes have been synthesized by
using 8-hydroxyquinoline (HQ) as a primary ligand and N and / or O donor amino acids
(HL) such as L-valine, L-asparagine, L-glutamine, L-arginine and L-methionine as
secondary ligands. The mixed ligand zinc (II) complexes have been synthesized by the
reaction of zinc (II) chloride dehydrate with 8-hydroxyquinoline and amino acids in 1:1:1
molar proportion as per the following reactions.
ZnCl2∙2H2O + HQ + HL → [Zn (Q)(L).2H2O] + 2 HCl
Where, Q is deprotonated N and O donor primary ligand, 8-hydroxyquinoline and L
is deprotonated N and / or O donor secondary ligands, different amino acids.
All the complexes in general are non-hygroscopic, stable solids, insoluble in
water and in common organic solvents such as ethyl alcohol, acetone, chloroform, etc., but
partially soluble in DMF and DMSO. This insolubility of complexes hampered the
molecular weight determination. Therefore molecular weights were computed using
analytical data. All mixed ligand zinc (II) complexes are yellow in colour.
All the mixed ligand zinc (II) complexes are thermally stable at least up to
266oC. They do not melt but decompose at high temperatures. The higher temperature
indicates a strong metal-ligand bond in the complexes (Table 4.1 ).
The purity of all mixed ligand zinc (II) complexes are studied by thin layer
chromatography. The studies carried out on the metal complexes show single spot indicating
that they are pure and true mixed ligand metal complexes rather than a mixture of the two or
more complex species.
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Table 4.1 :- Empirical formula, molecular weight , decomposition temperature and
colour of Zn (II) complexes
Elemental Analysis:-
Elemental analysis data (Table 4.2) shows that complexes of the type
[Zn(Q)(L)∙2H2O] are formed in 1:1:1 proportion by the reaction of metal salts, zinc (II)
chloride dihydrate with a primary ligand 8-hydroxyquinoline and secondary ligands L-
valine, L-asparagine, L-glutamine, L-arginine and L-methionine respectively.
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Table 4.2:- Elemental analysis data of Zn (II) complexes
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Conductance:-
All the mixed ligand Zn (II) complexes were found to be insoluble in water and in
common organic solvents viz., ethyl alcohol, acetone, chloroform, etc., but they were
partially soluble in DMF and DMSO. DMSO has a remarkable dissolving capacity. All the
complexes synthesized in the present work are partially soluble in DMSO. Thus 10-3
M
solutions of mixed ligand Zn (II) complexes were prepared in double distilled DMSO and
molar conductance values were measured. It is seen from the Table 4.3 that the molar
conductance values of these complexes in DMSO fall in the range 0.013 to 0.028 Mhos cm2
mol-1
, indicating their non-electrolytic nature.
Table 4.3:- Molar conductance of Zn (II) complexes
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Magnetic Susceptibility Measurements:-
In the Table 4.4 the results of the room temperature magnetic susceptibility
measurements of all the metal complexes are given. The following notations are used in all
the tables.
Χg = Specific magnetic susceptibility
Χm = Molecular magnetic susceptibility
By employing diamagnetic corrections, the magnetic moments of the mixed
ligand copper (II) complexes were calculated from the measured magnetic susceptibilities
which revealed their paramagnetic nature. The observed values for effective magnetic
moment (eff) expressed in B.M. (Table 4.4) are in the range 1.73 to 1.97. The magnetic
moment of the mixed ligand zinc (II) complexes were calculated from the measured
magnetic susceptibilities after employing diamagnetic corrections and revealed their
diamagnetic nature (Table 4.4).
Table 4.4:- Magnetic susceptibility data of Zn (II) complexes (c.g.s. units)
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Electronic Absorption Spectra:-
The electronic absorption spectra of mixed ligand Cu (II) and Zn (II) complexes
in DMSO solution were recorded in the ultraviolet and visible region (Figure 4.1 to 4.5) and
are summarized in Table 4.5. Electronic spectra of the metal chelates bear a close similarity
to those of the ligands and such type of results are reported by many researchers. Therefore
it is concluded that during complexation there is not much structural alteration of the
ligands. The electronic spectral studies of mixed ligand Zn (II) complexes show that the
bands in the range 272-280 nm (35714-36765 cm-1
) can be assigned to the π → π*transitions
of the aromatic chromophore of the ligand. The π → π* transitionsin the metal complexes
are observed at different positions indicating that the π electron system of the ligands
undergo alteration to a varying extent on coordination to different metal ions. The observed
bands in the range 333-339 nm (29499-30030 cm-1
) are attributed to the n → π*transitions
of the electrons of unshared electron pair on hetero atoms of the ligands. The electronic
spectra of the mixed ligand Zn (II) complexes also show charge transfer transitions in the
range 386-398 nm (25126-25907 cm-1
) in addition to the intra-ligand transition bands. These
bands may be attributed to the ligand to metal charge transfer (LMCT) transitions. As the
term implies, these transitions involve electron transfer from one part of the complex to
another which are fully allowed and hence give rise to much more intense absorption.
Table 4.5:- Electronic spectral data of Zn (II) complexes
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Figure 4.1:- UV –Visible Spectra of [Zn (Q) (Val.). 2H2O]
Figure 4.2:- UV –Visible Spectra of [Zn (Q) (Asp.). 2H2O]
Figure 4.3:- UV –Visible Spectra of [Zn (Q) (Glu.). 2H2O]
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Figure 4.4:- UV –Visible Spectra of [Zn (Q) (Arg.). 2H2O]
Figure 4.5:- UV –Visible Spectra of [Zn (Q) (Met.). 2H2O]
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Infrared Spectra:-
The FTIR spectra of the metal complexes recorded in KBr discs over the range
4000-400 cm-1. The IR spectra of the complexes were rather difficult to analyze due to
presence of many bands with fluctuating intensities. But on the basis of reported infrared
spectra of several N and / or O donor ligands, 8-hydroxyquinoline and their metal complexes
the important bands were assigned. It is observed that in the infrared spectra of all the metal
complexes there is absence of band due to O-H stretching vibrations of either the free –OH
group of 8-hydroxyquinoline or of the –COOH group of the amino acid. Therefore it can be
concluded that complexes are formed by the deprotonation of hydroxyl group of HQ and
carboxylic group of the amino acid moiety.A broad band observed in the region between
3300-3194 cm-1 due to asymmetric and symmetric O–H stretching modes and a weak band
in the range 1578-1570 cm-1due to H–O–H bending vibrations indicating presence of water
molecules furtherconfirmed by thermal studies.
In the infrared spectra of the metal complexes with 8HQ, there is the absence of
band 3440 cm-1due to the O-H stretching vibration of the free O-H group of HQ. Hence it
can be concluded that in the process of complex formation deprotonation of the hydroxyl
group of HQ moiety takes place to form M-O bond. The position of (CO) band in 8HQ
undergoes variation depending on metal complex under study. Charles et al. reported that for
several metal complexes with HQ, the (CO) band is observed at ~1120 cm-1. A strong (CO)
band observed in the range1111-1105 cm-1indicates the presence of oxine moiety in the
complexes coordinated through its nitrogen and oxygen atoms as uninegative bidentate
ligand.
The (C=N) mode observed at 1580 cm-1in the spectra of free HQ ligand is found
to be shifted to lower wave number in the range of 1500-1460 cm-1 in the spectra of
complexes, which indicates the coordination through tertiary nitrogen donor of HQ. The
coordination through ring nitrogen atom of HQ with the metal has been confirmed on the
basis of bands observed at the range of 508-504 cm-1 and 791-780 cm-1 that corresponds to
in plane and out of plane ring deformation modes respectively.
It is reported that, stability of amino acid complexes increases with the decrease in
the N-H stretching vibrational frequency. By comparing the spectra of free amino acids, it
has been proved that there is decrease in the N-H stretching frequency on complex
formation. Character and strength of the M-N bond has been correlated to the shift of N-H
stretching band. Broad band’s observed at range 3193-3086 cm-1 and 3060-3052 cm-1 are
assigned to N-H (asymmetric) and N-H (symmetric) vibrations respectively. In case of IR
spectra of free amino acid these bands appear at the range of 3040 and 2960 cm-1. This shift
of N-H vibrations to higher wave numbers, suggest that in the formation of metal
complexes, nitrogen atom of amino group coordinate to metal ion. Coordination through the
amino group of the amino acids has been further confirmed by the C-N symmetrical
stretching frequency. It is observed at 950 cm-1 in the spectra of free amino acids and found
to be shifted to lower wave numbers in the range of 914-910 cm-1in the spectra of the
complexes.
The coordination of carboxylic acid group via oxygen with the metal ion may be
indicated by the interpretation of the asymmetric and the symmetric mode of vibration of
(COO-) band. The asymmetric (COO-) band of free amino acids i.e. 1610-1590 cm-1 is
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shifted to higher wave number, in the range 1643-1602 cm-1 and the symmetric (COO-)
mode observed at 1400 cm-1 in the spectra of free amino acids is found to be shifted to
lower wave number in the range of 1373-1370 cm-1, in the spectra of complexes. Usually
the intense band is observed due to O-H stretching vibrations in the region 3650-3200 cm-1
indicates the presence of hydroxyl group in the molecule of carboxylic acid. In the spectra of
metal complexes, there is absence of bands due to O-H stretching vibrations indicating
bonding via oxygen atom of hydroxyl group to the metal ion. Some new bands of weak
intensity observed in the regions of 615-600 cm-1 and at 410 cm-1 may be ascribed to the
M-O and M-N vibrations respectively. It may be noted that these vibrational bands are
absent in the infra-red spectra of HQ as well as amino acids. The M-O bond has much less
covalent character than the M-N bond so the stretching bands of the former appear in low
frequency region.
The IR spectra of the complexes are shown in (Figure 4.6 to 4.10). Some of the
important IR bands and their assignments are shown in Table 4.6
Table 4.6:- Infra-red spectral bands (cm-1
) of Zn (II) complexes
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Figure 4.6:- FTIR Spectra of [Zn (Q) (Val.). 2H2O]
Figure 4.7:- FTIR Spectra of [Zn (Q) (Asp.). 2H2O]
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Figure 4.8:- FTIR Spectra of [Zn (Q) (Glu.). 2H2O]
Figure 4.9:- FTIR Spectra of [Zn (Q) (Arg.). 2H2O]
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Figure 4.10:- FTIR Spectra of [Zn (Q) (Arg.). 2H2O]
Thermal Studies:-
To understand the stages of decomposition, temperature range of decomposition
and decomposition products formed during the sequence of decomposition, the thermo-
gravimetric analysis (TG) and differential thermal analysis (DTA) was carried out for mixed
ligand zinc (II) complexes. The TG and DTA studies of the complexes have been recorded
in the nitrogen atmosphere at the constant heating rate of 10oC per minute upto 900
oC. TG
and DTA curves shown in Figure 4.11 to 4.20 and thermo analytical data is summarized in
Table 4.7 The thermal study indicates that the complexes show gradual loss in weight due to
decomposition by fragmentation with increasing temperature and they are thermally quite
stable.
Thermal Study on Zinc (II) Complexes:-
All the Zn (II) complexes show similar behaviour in TG and DTA studies. The
TG-DTA curves of these complexes show the loss in weight corresponding to two co-
ordinated water molecules in the temperature range 131-171oC, followed by simultaneous
weight loss due to amino acid and 8-hydroxyquinoline moieties in the range 245-560oC.
The DTA of the complexes display an endothermic peak in the range 131-171oC
which indicate the presence of two co-ordinated water molecules. As the temperature is
raised, the DTA curve shows a broad exotherm in the range 245-560oC attributed to
simultaneous decomposition of amino acid and 8-hydroxyquinoline moieties present in the
complexes. The formation of a broad exotherm is possibly due to simultaneous
decomposition of ligand moieties and their subsequent oxidation to gaseous products like
CO2, H2O, etc. Like most of the metal organic complexes, these complexes also decompose
to a fine powder of metal oxide i.e. ZnO . The constant weight plateau in TG after 610oC
indicates completion of the reaction. The ZnO formed was confirmed by X-ray diffraction
pattern of the decomposed product.
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Table 4.7:- Thermal data of Zn (II) complexes
Figure 4.11:- TG Curve of [Zn (Q) (Val.). 2H2O]
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Figure 4.12:- TG Curve of [Zn (Q) (Asp.). 2H2O]
Figure 4.13:- TG Curve of [Zn (Q) (Glu.). 2H2O]
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Figure 4.14:- TG Curve of [Zn (Q) (Arg.). 2H2O]
Figure 4.15:- TG Curve of [Zn (Q) (Met.). 2H2O]
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Figure 4.16:- DTA Curve of [Zn (Q) (Val.). 2H2O]
Figure 4.17:- DTA Curve of [Zn (Q) (Asp.). 2H2O]
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Figure 4.18:- DTA Curve of [Zn (Q) (Glu.). 2H2O]
Figure 4.19:- DTA Curve of [Zn (Q) (Arg.). 2H2O]
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Figure 4.20:- DTA Curve of [Zn (Q) (Met.). 2H2O]
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Figure 4.21:- XRD of decomposition Product of Zn (II) Complexes
On the basis of elemental analysis data and various physico-chemical studies,
Coordination number six is proposed for zinc (II) complexes. The proposed bonding and
structure for the zinc (II) complexes are shown in figure 4.22 to 4.26.
Figure 4.22:- Proposed structure of [Zn (Q) (Val.). 2H2O]
Figure 4.23:- Proposed structure of [Zn (Q) (Asp.). 2H2O]
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Figure 4.24:- Proposed structure of [Zn (Q) (Glu.). 2H2O]
Figure 4.25:- Proposed structure of [Zn (Q) (Arg.). 2H2O]
Figure 4.26:- Proposed structure of [Zn (Q) (Met.). 2H2O]
References:-
1. G.B. Ghosale, O.P. Sharma and R.B. Kharat, J. Ind. Chem. Soc., 55, 776 (1988).
2. S. Vatsala and G. Parmeswaran, Ind. J. Chem., 25, 1158 (1986).
3. W.J. Geary, Coord. Chem. Rev., 7 (1), 81 (1971).
4. E. Canpolat and M. Kaya, J. Coord. Chem., 55 (12), 1419 (2002).
5. R. Zhang, X. Yu Wu, F. Zhao and Y. Zhan, Polyhedron, 14 (5), 629 (1995).
M. Sonmez, Turk. J. Chem., 25, 181 (2001).
6. N.S. Bhave and R.B. Kharat, J. Inorg. Nucl. Chem., 42, 977 (1980).
7. K. Krishnakutty and P. Ummer, J. Ind. Chem. Soc., 66, 194 (1989).
8. A.B.P. Lever, ‘Inorganic Electronic Spectroscopy’, 2nd Ed., Elsevier Science
Publishers, Amsterdam, (1984).
9. M. Tumer, Synth. React. Inorg. Met. Org. Chem., 30 (6), 1139 (2000).
10. P.B. Chakrawarti and P. Khanna, J. Ind. Chem. Soc., 77, 23 (1985).
11. H. Beraldo, S.M. Kainser, J.D. Turner, I.S. Billeh, J.S. Ives and D.X. West,
‘Transition Metal Chemistry’, Ed. R. L. Carlin, Marcel Decker Inc., New York,
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22 (1997).
12. A.B. Lever, J. Chem. Edu., 51, 612 (1974).
13. J.E. Huheey, E.A. Keiter and R.L. Keiter, ‘Inorganic Chemistry’, 4th Ed., Harper
Collins College Publishers, New York, (1993).
14. A.B. Lever, ‘Inorganic Electronic Spectroscopy’, 2nd Ed., Elsevier, New York,
(1986).
15. S. Panda, R. Mishra, A.K. Panda and K.C. Satpathy, J. Ind. Chem. Soc., 66, 472
(1989). 148
16. V. Bhagwat, V. Sharma and N.S. Poonmia, Ind. J. Chem., 15 (A), 46 (1977).
17. M.S. Islam, M.S. Ahmed, S.C. Pal, Y. Reza and S. Jesmine, Ind. J. Chem., 34 (A),
816 (1995).
18. K. Nakamoto, Y. Morimoto and A.E. Martell, J. Am. Chem. Soc., 83, 4528
(1961).
19. K. Mohanan and N. Thankarajan, J. Ind. Chem. Soc., 7, 583 (1990).
20. A.K. Banerjee, D. Prakash and S.K. Roy, J. Ind. Chem. Soc., LII, 458 (1976).
21. N.V. Thakkar and J.R. Thakkar, Synth. React. Inorg. Met. Org. Chem., 30 (10),
1871 (2000).
22. B.V. Murdulla, G. Venkatnarayana and P. Lingaiah, J. Ind. Chem. Soc., 28 (A),
1011 (1989).
23. R.C. Charles, H. Freiser, R. Friedel, L.E. Hillard and W.D. Johnson, Spectrochim.
Acta., 8, 1 (1956).
24. V.S. Sharma, H.B. Mathur and A.B. Biswas, Spectrochim. Acta., 17, 895 (1961).
25. D.N. Sen, C. Mizushima, C. Curran and J.V. Quagliano, Spectrochim. Acta., 77,
211 (1955).
26. D. Segnini, C. Curran and J.V. Quagliano, Spectrochim. Acta., 16, 540 (1960).
27. M.M. Kennely, Spectrochim. Acta., 15, 296 (1959).
28. C.P. Prabhakaran and C.C. Patel, J. Inorg. Nucl. Chem., 37, 1901 (1975).
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30. J. Chacko and G. Parameswaran, J. Ind. Chem. Soc., 28 (A), 77 (1989).
31. S.E. Al-Mukhtar, Synth. React. Inorg. Met. Org. Chem., 30 (6), 997 (2000).
32. R. Thomas, J. Thomas and G. Parameswaran, J. Ind. Chem. Soc., 73, 529 (1996).149
33. K. Nakamoto, ‘Infrared and Raman Specta of Inorganic and Coordination
Compounds’, 4th Ed., John Wiley and Sons, New York, 233 (1986).
34. S.S. Patil, G.A. Thakur and M.M. Shaikh, Acta Pol. Pharm. Drug Res., 68 (6),
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Chapter 5
Biological activity
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Biological Activity
Introduction:-
In last few decades, Microbiology has emerged as key branch of life sciences and
it has played important role in the finding the solutions related to the problem of global
environment, the need of recycling the natural resources, technology of genetic engineering,
health and welfare of human beings. Microbiology deals with study of living organisms of
microscopic size, which includes bacteria, fungi, algae, protozoa and viruses. It deals with
their structures, reproduction, physiology, metabolism and classification. It also includes the
study of their distribution in nature, their relationship with each other and with other living
organisms, their ability to make physical and chemical changes in the environment and their
effects on human beings and animals. The microorganisms have important applications in
many areas such as industry, agriculture, food, shelter and clothing. Although relatively few
species of microorganisms are harmful to mankind and the animals, there are many species
which perform useful roles for the mankind. Microorganisms are useful for various
physiological and biochemical applications. Microorganisms are involved in processing of
domestic and industrial wastes, in making of yoghurt, cheese and wine and in the production
of penicillin and alcohol. Some microbes are helpful while some others are harmful to the
mankind or animals. Microorganisms can deteriorate materials, like iron pipes, glass lenses
and wood pilings. They can cause diseases like pneumonia, dysentery, cholera, typhoid,
tuberculosis, etc. and can cause decay of food.
Modern bacteriology has developed in quick succession from 1874 as a well-
organized science. Number of microbiologists work on bacteria with keen interest because
bacteria can be nourished, maintained inexpensively and safely in the test tubes except some
dangerous pathogens. Many bacterial species may be investigated as individual cells or as
populations that can multiply in a few hours from a single cell to thousands or millions
within a test tube. All bacteria are classified1as Gram-positive and Gram-negative
depending upon Gram staining property.
Gram Staining Method:-
In 1884, this method was introduced by Dr. Hans Christian Gram. In this
technique a fixed bacterial smear is subjected to staining reagents in the order of crystal
violet, iodine solution, alcohol and safranine or suitable counter stain. The bacteria which
retain the crystal violet and appear as deep violet in colour are termed as Gram-positive
bacteria while those which lose the crystal violet and are counterstained by safranine and
appear pink in colour are termed as Gram-negative bacteria.
Rationale:-
Antimicrobial agent is the one that interferes with the growth and metabolism of
the microbes. The term denotes ‘inhibition of growth’. Many antimicrobial agents show both
inhibitory (bacteriostatic) and lethal (bactericidal) activity depending on the concentrations
used. The biological activity of number of organic compounds used as antimicrobial agents
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depends on size, shape and structure of molecules. In fact antimicrobial activity is associated
with every functional group in the molecule or with the molecule as a whole. It is observed
that certain functional groups especially electron rich groups like aromatic or aliphatic
carboxyl, aldehyde, phenolic –OH, halogens, amino, etc. are responsible for antimicrobial
activity exhibited by them. The role of metal complexes as drugs in the biological systems
has been widely studied. Studies on the metal complexes and their biological activity have
been reported. The majority of the metal complexes possessing biological activity are
chelates. The activity of number of antimicrobial agents depends upon their ability to form
chelates. For example, 8-hydroxyquinoline (oxine) is a well-known chelating agent which
exhibits antimicrobial properties; on the other hand, 8-methoxyquinoline is not a chelating
agent hence does not show antimicrobial activity. Number of metal chelates of the transition
metals with sulphur containing ligands shows anticancer and antitumor activity. Many Schiff
base complexes are reported as antifungal agents.
Satpathy et al. have studied biological activity of the complexes of p,p’-bis (benzoyl
thiourea) benzene with Co(II), Ni(II) and Cu (II) salts. They concluded that complexes were
more active than the ligands due to complexation. In recent years, antimicrobial and
cytotoxic studies on some mixed ligand complexes have been studied.
Methodology
In the present work the antimicrobial activity of the ligands and their metal
complexes were studied. The methodology implemented for antibacterial activity is as
follows.
Antibacterial Activity:-
For the present work following bacterial strains were selected
1. Staphylococcus aureus: Gram-positive
2. Corynebacterium diphtheriae: Gram-positive
3. Salmonella typhi: Gram-negative
4. Escherichia coli: Gram-negative
Experimental Materials:-
Media:-
Nutrient agar and Muller Hinton broth.
Culture:-
Cultures in Muller Hinton broth (24 hrs. old).
The Culture Strains:-
Staphylococcus aureus: A pathogenic, enteric Gram-positive staphylococci, responsible for
boils, wound infection, pneumonia, etc.
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Corynebacterium diphtheriae: A pathogenic, nocardioform Gram-positive bacteria,
responsible for diphtheria in human.
Salmonella typhi: A pathogenic, enteric Gram-negative bacteria, is the agent of typhoid.
Escherichia coli: A pathogenic, enteric Gram-negative bacteria, a component of the normal
adult human intestinal flora. Diarrhea caused by toxin producing E coli is major cause of
infant mortality in the third world. It also causes urinary tract infection.
Assessment of Bacteriostatic Activity:-
Bacteriostatic activity can be measured in solid or liquid media. It depends on
several aspects like the agent concerned, time of contact, temperature, nutritional
environment and type of organism under study.
The exposure time have major importance since in some cases an organism may
initially be inhibited from growing but can multiply afterwards. Hence it is necessary to
standardize the period of incubation or a long incubation period is used. The antimicrobial
activity of the metal salts, ligands and their complexes has been tested against the selected
strains of microorganisms by agar cup and tube dilution methods.
Methods of Testing:-
Agar Cup Method:-
In this method, a single compound can be tested against number of organisms or
a given organism against different concentrations of the same compound. It was found
suitable for semisolid or liquid samples and was used in the present work. In agar cup
method, a plate of sterile nutrient agar with the desired test strain was poured to a height of
about 5mm, allowed to solidify and a single cup of 8 mm diameter was cut from the center
of the plate with a sterile cork borer. Thereafter the cup was filled with the sample solution
of 1000 µg/cm3concentration. The test solution was allowed to diffuse in surrounding agar
by keeping in refrigerator for 10 min and the plate was incubated at 37oC for 24 hrs. The
extent of inhibition of growth from the edge of the cup was considered as a measure of the
activity of the given compound. By using several plates simultaneously, the activities of
several samples could be qualitatively studied.
Tube Dilution Method:-
The test compounds were subjected to in vitro screening against
Staphylococcus aureus, Corynebacterium diphtheriae, Salmonella typhi and Escherichia
coli using Muller Hinton broth as the culture medium. The test compound (10 mg) was
dissolved in DMSO (10 cm3) so as to prepare a stock solution of concentration 1000 µg/cm
3.
From this stock solution, aliquots of 50, 100, 150, 200 to ……, 1000 µg/cm3 were obtained
in test broth. Bacterial inoculums were prepared in sterilized Muller Hinton broth and
incubated for 24 hrs. at 37oC. The aliquots were dispensed (5 cm
3) in each borosilicate test
tube (150 x 20 mm). The bacterial inoculums 0.1 cm3
of the desired bacterial strain (S.
aureus, C. diphtheriae, S. typhi and E. coli) containing bacteria/cm3 was inoculated in the
tube. The tubes were incubated at 37oC for 24 hrs and then examined for the presence or
absence of the growth of the test organisms. The lowest concentration which showed no
visible growth was noted as minimum inhibitory concentration (MIC).Tetracycline was used
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as standard drug against Gram-positive and Gram-negative bacteria by similar screening
procedure. The solvent DMSO was also tested as control to see that it did not affect the
growth of the culture. MIC of tetracycline was found to be MIC of tetracycline was found to
be 1.5 µg/cm3 against S. aureus, 2.0 µg/cm
3 against C. diphtheriae, 1.5 µg/cm
3 against S.
typhi and 2.5 µg/cm3 against E. coli.
The results of microbial studies are presented in Table 5.1 to 5.3.
Table 5.1:-Antibacterial activity (mm) of Zn (II) complexes by Agar Cup Method
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Table 5.2:-MIC of metal salts, ligand and tetracycline
Table 5.3:-Antibacterial activity of Zn (II) complexes
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Results And Discussion :-
All the metal complexes were screened against Staphylococcus aureus,
Corynebacterium diphtheriae, Salmonella typhi and Escherichia coli.The agar cup method
revealed that zinc (II) complexes are more active against S. aureus and S. typhi as compared
to C. diphtheria and E. Coli (Table 5.1 and 5.2).
The minimum inhibitory concentration (MIC) of metal complexes ranges between
50-250 µg/cm3. The results show that, as compared to the activity of metal salts and free
ligands the metal complexes (Table 5.3) show higher activity (Table 5.4 to 5.5). The activity
of metal complexes is enhanced due to chelation . The chelation reduces considerably the
polarity of the metal ions in the complexes, which in turn increases the hydrophobic
character of the chelate and thus enables its permeation through the lipid layer of
microorganisms.The tube dilution method revealed that zinc (II) complexes are more active
against S. aureu sand S. typhi as compared to C. diphtheria and E. Coli.
Compared to standard antibacterial compound tetracycline, all the complexes show
moderate activity against selected strains of microorganisms.
Conclusion:-
The microbial study of zinc (II) complexes shows that the potency of the
complexes depend on various factors such as the composition of ligand, type of
microorganism and the ability of metal ion to coordinate with ligand.
References:-
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15. P.K. Panchal, H.M. Parekh and M.N. Patel, Tox. andEnv. Chem., 87 (3),
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Chapter 6
Summary
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Summary
The research study on “Synthesis and Studies on Mixed Ligand Complexes of
Zinc (II) with Amino Acids” represents the synthesis and characterization of some mixed
ligand zinc (II) complexes. A microbial study of these complexes has also been carried out.
The thesis comprises of mainly five chapters.
CHAPTER 1:-
This chapter provides basic information regarding coordination chemistry. It
comprises a data on ligating properties of many polydentate ligands including amino acids
and 8-hydroxyquinoline. It also includes introduction to mixed ligand complexes, their role
in biological systems and stability and dynamics with reference to formation of binary and
ternary complexes. To decide the aim of this study thorough literature survey was carried
out which revealed the following conclusions –
Extensive research has been carried out for the study of mixed ligand complexes
and their importance in various biological processes. It has been found that many ternary
complexes of some metals are important for activation of enzymes and they are used for
storage as well as for transport of active materials. The correlation between the stability of
the metal-ligand complexes with their anti-microbial activity has been studied. Antitumor
activity of some mixed ligand complexes also have been reported . 2Complexes of many
metals with 8-hydroxyquinoline have been studied for their biological activity. Metabolic
enzymatic activities for many metal complexes of amino acids have been reported. Many
researchers have studied characterization, antimicrobial and toxicological activity of mixed
ligand complexes of transition metals and actinide metal ions. Synthesis and characterization
of some transition metal complexes derived from amino acids have been reported. It is well
known that the copper complexes play important role in various biological processes. The
antibacterial and anti-fungal properties of zinc (II) and copper (II) complexes have been
reported. Recently synthesis, structural characterization and antibacterial studies of some
biosensitive mixed ligand copper(II) complexes have been reported. Many complexes of
copper(II) and zinc (II) metal ion have been investigated for their chelation and biological
properties. Antioxidative and antitumour properties of copper(II) and zinc (II) metal
complexes have also been reported. The spectral, magnetic and biological properties of
ternary complexes of zinc (II) metal ion with amino acid as secondary ligand have been
studied.On the basis of above investigation, it was decided to undertake the study of
synthesis and characterization of some mixed ligand zinc (II) complexes of the type
[Cu(Q)(L)]∙2H2O, and [Zn(Q)(L)∙2H2O]respectively, where Q represents the deprotonated
primary ligand, 8-hydroxyquinolineand L represents deprotonated N and O donor amino
acids viz. L-valine, L-asparagine, L-glutamine, L-arginine and L-methionine as secondary
ligands. These complexes have been screened for their antibacterial activities.
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CHAPTER 2:-
The theoretical of techniques and methods used for the characterization of mixed
ligand complexes are discussed in this chapter. It includes elemental analysis and
introduction, basics and instrumentation of conductometry. The theory of diamagnetism,
theoretical calculations of magnetic susceptibility and different methods used in the present
investigation such as Guoy’s method for magnetic study also has been discussed. It explains
basics and instrumentation of UV-Visible and IR spectroscopy. It also describes thermal
techniques used, such as thermogravimetric analysis and differential thermal analysis to
interpret structure of the complexes. To confirm nature of metal oxide after decomposition
of the complex X-ray diffraction method was carried out.
CHAPTER 3:-
The chapter explains procedure for the synthesis of mixed ligand zinc (II) complexes with
different ligands. It includes experimental method for the synthesis of metal complexes.
Synthesis of Mixed Ligand Complexes:-
Zinc (II) complexes have been synthesized by using 8-hydroxyquinoline (HQ)
as a primary ligand and different amino acids (HL) such as L-valine, L-asparagine, L-
glutamine, L-arginine and L-methionine as secondary ligands.The mixed ligand zinc (II)
complexes were prepared from zinc chloride dihydrate respectively with primary ligand
(HQ) and secondary ligands (HL) in 1:1:1 proportion, according to the following reactions
ZnCl2∙2H2O + HQ + HL → [Zn (Q)(L) ∙ 2H2O] + 2HCl
It also explains various techniques employed for the characterization of the metal complexes
such as elemental analysis, conductometry, magnetic susceptibility, UV-Visible spectra, IR
spectra, TG-DTA and X-ray diffraction.
CHAPTER 4:-
Elemental analysis, electrical conductance, room temperature magnetic
susceptibility measurements, spectral and thermal studies are employed for characterization
of mixed ligand complexes.
Characterization of Mixed Ligand Complexes:-
The chapter emphases on the results obtained and their interpretation in
characterization of mixed ligand zinc (II) complexes of 8-hydroxyquinoline (HQ) as a
primary ligand and N and / or O donor ligands (HL) such viz. such as L-valine, L-
asparagine, L-glutamine, L-arginine and L-methionine as secondary ligands. All complexes
are non-hygroscopic and thermally stable solids. The complexes are insoluble in water and
common organic solvents but partially soluble in DMF and DMSO. Colour of zinc (II)
complexes varies from yellow to greenish yellow. The decomposition temperatures of the
complexes are found to be in the range of 246-266oC indicating that they are thermally
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stable with a strong metal-ligand bond. The elemental analysis data of zinc (II) complexes is
consistent with their general formulation as 1:1:1 of the type [Zn (Q)(L)∙2H2O] respectively.
The electrical conductance studies of the complexes in DMSO in 10-3
M concentration
indicate their non-electrolytic nature. Room temperature magnetic susceptibility
measurements specify that zinc (II) complexes are diamagnetic in nature. Electronic
absorption spectra of the complexes show intra-ligand and charge transfer transitions
respectively. FTIR spectra show bonding of the metal ion through N- and O- donor atoms of
the ligand molecules. The spectra also indicate the presence of water molecules in the
complexes. The far IR region shows presence of bands due to metal-nitrogen and metal-
oxygen stretching vibrations.
The TG and DTA studies of the complexes have been studied in the nitrogen
atmosphere at the constant heating rate of 10oC per minute. The TG of all the metal
complexes indicates that they are thermally stable to varying degree. The complexes show
gradual loss in weight due to decomposition by fragmentation with increasing temperature.
All the zinc (II) complexes show similar behavior in TG and DTA studies. The
TG-DTA curves of these complexes show the loss in weight corresponding to two co-
ordinated water molecules in the temperature range 131-171oC, followed by weight loss due
to amino acid and 8-hydroxyquinoline moieties in the range 245-560oC simultaneously.
Thermal decomposition of all the metal complexes in inert atmosphere produces finely
divided metal powder which gets transformed to metal oxide spontaneously even in the
presence of traces of oxygen present in nitrogen gas used in the experiment. The constant
weight plateau in TG of zinc (II) after 610oC indicates completion of the reaction. Each
metal oxides formed was confirmed by X-ray diffraction pattern of the decomposed product.
On the basis of elemental analysis data and various physico-chemical studies, it is
suggested that zinc (II) complexes has coordination number six.
CHAPTER 5:-
The chapter contains brief introduction about the microbiology and screening
procedures for agar cup method and tube dilution method. Both the methods were
implemented for study of antibacterial activity of the complexes against S. aureus, C.
diphtheriae, S. typhi and E.coli. The antibacterial study was carried out by using the
tetracycline as a standard antibacterial compound and it was found that, the complexes show
mild activity against selected strains of micro-organisms as compared to standard
tetracycline.
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