1997- Simplified concepts in spectroscopy and photochemistry

8/7/2019 1997- Simplified concepts in spectroscopy and photochemistry

1/85

US Army Corpsof EngineersWaterways ExperimentStation

Technical Report IRRP-97-3April 199

Installation Restoration Research Program

Simplified Concepts in Spectroscopyand Photochemistry

by Mohammed C?asim

.

Approved For Public Release; Distribution Is Unlimited

Prepared for Headquarters, U.S. Army Corps of Engineers


2/85

The contents of this report are not to be used for advertising,publication, or promotional purposes. Citation of trade namesdoes not constitute an official endorsement or approval of the useof such commercial products.The findings of this report are not to be construed as anofficial Department of the Army position, unless so desig-nated by other authorized documents.

@ PRINTED ON RECYCLED PAPER


3/85

Installation Restoration Technical Report IRRP-97-3Research Program April 1997

Simplified Concepts in Spectroscopyand Photochemistryby Mohammed Qasim

U.S. Army Corps of EngineersWaterways Experiment Station3909 Halls Ferry RoadVicksburg, MS 39180-6199

Final reportApproved for public release; distribution is unlimited

Prepared for U.S. Army Corps of EngineersWashington, DC 20314-1000


4/85

US Army Corpsof EngineersWaterways Experiment

/7,

9

Watenvays Expe riment Sta t i on Ca ta log ing -i n -Pub li ca t i on Da taQasim, Mohammed.Simplifiedmncepts in spectroscopy and photochemistry/by Mohammed Qasim ; preparedfor U.S. Army Corps of Engineers.84p. : ill. ; 28 cm. (Technical report; IRRP-97-3)Includes bibliographic references.1. Photochemistry. 2. Spectrophotometry. L United States. Army. Corps of Engineers.

Il. U.S. Army Engineer Waterways Experiment Station. 111.Installation RestorationResearch Program. IV. Series: Technical report (U.S. Army Engineer Waterways ExperimentStation) ; IRRP-97-3.TA7 W34 no.lRRP-97-3


5/85

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. viiIIntroductory Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Background andBasicConcepts . . . . . . . . . . . . . . . . . . . . . . . . . 2Plancks Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Photoelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Compton Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5de BroglieWavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Heisenberg Uncertainty Principle . . . . . . . . . . . . . . . . . . . . . . . . 6Schrodinger Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2Basic Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...11Lambert-Beer Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...11Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13Similarities and differences between absorption spectrophoto-meters intheW, VIS,and IR regions . . . . . . . . . . . . . . . . . . 13Similarities and differences between absorption and emissionspectrophotometers . . . . . . . . . . . . . . . . . . . . . . . . . . . ...13

Grating monochrometer . . . . . . . . . . . . . . . . . . . . . . . . . . ..14Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..163Regions of Electromagnetic Radiation . . . . . . . . . . . . . . . . . . . . . 19

X-Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...19Wand VISRegions oftheSpectra . . . . . . . . . . . . . . . . . . . . . ..21Singlet andtriplet electronic excited states . . . . . . . . . . . . . . . . . 21Fate ofelectronic excited states . . . . . . . . . . . . . . . . . . . . ...22Fluorescence and phosphorescence . . . . . . . . . . . . . . . . . . . . . 22Triplet sensitizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...26Quantum yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...26Piand sigma electrons . . . . . . . . . . . . . . . . . . . . . . . . . . ...304Vibrational and Rotational Excitations . . . . . . . . . . . . . . . . . . . . . 32

Infrared Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Fourier Transform Infrared Region . . . . . . . . . . . . . . . . . . . . . . . 33Microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35Nuclear Magnetic Resonance . . . . . . . . . . . . . . . . . . . . . . . . ..37Raman Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...40


6/85

5Relationships Between Structural Changes in Molecules and TheirUV and VIS (Wavelength and Molar Absorptivity) Spectra . . . .Solvent Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Extension of Conjugation of Polyenes and Similar Compounds . . .Polyenes Conjugated to Carbonyl Groups . . . . . . . . . . . . . . . .Branching and Methyl Group Substitutions of Conjugated Polyenesand Similar Compounds . . . . . . . . . . . . . . . . . . . . . . . . . .Polycyclic Aromatic Compounds . . . . . . . . . . . . . . . . . . . . .Interactions Between n Electrons of the Side Chain and r Electronsof Aromatic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . .Methyl Substitutions on the Benzene Ring . . . . . . . . . , . . . . . .Chlorosubstituted Benzenes . . . . . . . . . . . . . . . . . . . . . . . . .Di- and Trinitrobenzenes and Toluenes . . . . . . . . . . . . . . . . . .

6Examples of Applications of UV and VIS Spectroscopy to OrganicMolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Use of W and VIS Spectra to Study Organic Molecules . . . . . . .Example of the Use of Emission Spectra to Investigate Nature ofthe Excited States for 2-(p-dimethylarninobenzylidene)-4-butyrolactone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Using W and VIS Spectra to Follow the Course of a ChemicalReaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Use of a UV and VIS Stopped-Flow Technique for Reaction RatesMeasurements and Optimization of an Analytical Assay . . . . . .Example of Use of Stopped-Flow Technique . . . . . . . . . . . . . .Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Useful Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SF 298List of Figures

. . . 41. . . 41. . . 41. . . 42. . . 43. . . 44. . . 46. . . 46. . . 47. . . 47. . . 53. . . 53

. . . 59

. . . 61. . . 65. . . 65. . . 66. . . 69. . . 70. . . 72

Figure 1.Figure 2.Figure 3.Figure 4.Figure 5.Figure 6.

Sample cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...12Schematic diagram of an absorption spectrophotometer . . . . . 14Simplified schematic diagram typical for a single-beamemission spectrophotometer . . . . . . . . . . . . . . . . . . . . . . 15Diagram depicting population inversion . . . . . . . . . . . . . . 17Simplified diagram oflaser source. . . . . . . . . . . . . . . . . . 18Simplified energy diagram showing the fate of theelectronic excited states . . . . . . . . . . . . . . . . . . . . . ...23

iv


7/85

Figure7. Absorption and emission spectra of tram 2-(p-dimethyl-aminobenzylidene)+butyrolactone . . . . . . . . . . . . . . . . . 24Figure 8a. Structural representation of czk-trans photochemicalinterconversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..25Figure 8b. Structural representation of cis-trans photochemicalinterconversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..27Figure 9.Figure 10.Figure 11.

Schematic diagram of monochromatic irradiation system . . . . 29Simplified molecular orbital diagram of acetone . . . . . . . . . 31Sample IR spectra for tram and cis 2-(p-dimethyl-aminobenzylidene) -4-butyrolactone . . . . . . . . . . . . . . . . . 34

Figure 12.Figure 13.

Michelson interferometer . . . . . . . . . . . . . . . . . . . . . . . 35NMR spectra of the cis-trans isomers of 2-(p-dimethyl-aminobenzylidene)-4-butyrolactone . . . . . . . . . . . . . . . . . 39

Figure 14. Effect of interaction between m electrons of the aromaticring andnelectrons of the side chain . . . . . . . . . . . . . . . . 47Figure 15. Wavelengths of maximum absorption and structural varia-tionsof di-andtrinitrobenzenes . . . . . . . . . . . . . . . . . . . 50Figure 16. A comparison of wavelengths of maximum absorption ofdinitrotoluenes with corresponding trinitrotoluenes . . . . . . . . 51Figure 17. Relationships of wavelengths and structural differences insulfite anions of di and trinitrobenzenes and toluenes . . . . . . 52Figure 18. Absorption spectra of tram 2-(pdirnethylaminobenzylidene)-4-butyrolactone . . . . . . . . . . . . . . . . . . . . . . . . . . . ...54Figure 19. Representative spectra of 2-(p-dimethylarninobenzylidene)-

4-butyrolactone inheptane . . . . . . . . . . . . . . . . . . . . ...55Figure 20. Representative spectra of 2-(pdimethylaminobenzylidene)-4-butyrolactone in ethanol . . . . . . . . . . . . . . . . . . . . ...56Figure 21. Photostationary concentration ratios at wavelengths from4ooto330nm . . . . . . . . . . . . . . . . . . . . . . . . . . . ...57

Comparison of molar absorptivities of both isomers atwavelengths from400t0300nm . . . . . . . . . . . . . . . ...59Figure 22.Figure 23.Figure 24.Figure 25.

Jablonski diagram of pola.dnonpolar solvent mechanisms . . . . 61VIS and W spectra of Disperse Red 11, BSA, and GPSA . . . 62W and VIS spectra showing the progression of a columnchromatography separation of unreacted Disperse Blue 3from Disperse Blue 3-BSA conjugate . . . . . . . . . . . . . . . . 63

Figure 26.Figure 27.

Photographs of TLC on plastic coated sheet . . . . . . . . . . . . 64Absorbance rates of HZ02 and KI reactions at 350 nm . . . . . 67

v


8/85

Figure 28. Rates of a fast and slow two-step reaction of HZOZwithexcess iodide ions . . . . . . . . . . . . . . . . . . . . . . . . . . ..68

List of TablesTable 1.Table 2.Table 3.

Table 4.Table 5.Table 6.Table 7.Table 8.Table 9.Table 10.Table 11.

Properties of Various Regions of ElectromagneticRadiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...20Relative Quantum Yields for cis-trans Photoisomerization . . . 28Absolute Quantum Yields for cis-trans Photoisomerization of2-(p-dimethylaminobenzulidene)-4-butyrolactone in Heptaneand Alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...28Conjugated Polyenes . . . . . . . . . . . . . . . . . . . . . . . ...42Comparison of Polyene-aldehyde Structures . . , . . . . . . . . . 43Conjugated Polyene Systems with Addition of a MethylGroup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...43Increase in Wavelengths of Maximum Absorption . . . . . . . . 44Increase in Wavelengths of Maximum Absorption withIncrease in Size of the Aromatic Ring . . . . . . . . . . . . . . . 45Effect of Addition of Methyl Groups on the Benzene Ring . . 48Wavelength and Structural Variations of Chlorosubsti-tuted Benzenes . . . . . . . . . . . . . . . . . . . . . . . . . . . ...49Photostationary Composition of cis-trans Isomers of2-(p-dimethylaminobenzylidene)-4-butyrolactone atVarious Wavelengths of Irradiation . . . . . . . . . . . . . . . . . 58


9/85

Preface

The work reported herein was conducted by the Environmental Laboratory(EL), U.S. Army Engineer Waterways Experiment Station (WES), for Head-quarters, U.S. Army Corps of Engineers (HQUSACE). Funding was pro-vided by the HQUSACE Installation Restoration Research Program.Dr. Clem Meyer was the Installation Restoration Research Program (IRRP)Coordinator at the Directorate of Research and Development, HQUSACE.Technical Monitors were Messrs. George ORourke and David Backer. TheIRRP Program Manager was Dr. M. John Cullinane, Jr.The report was written by Dr. Mohammed Qasim, Environmental Restora-tion Branch (ERB). Dr. Mark Zappi initiated the project and providedsupport as well as technical help throughout its writing. The late Dr. HansJaffe, Chairman, Chemistry Department, University of Cincinnati, served asmentor during the work pertaining to cis-trans photoisomerization;Dr. Mary Jane Selgrade, Chief, Im.rnunotoxicology Branch, EnvironmentalProtection Agency, Research Triangle Park, North Carolina, worked on struc-tural identification of protein dye conjugations through ultraviolet and visiblespectroscopy and thin layer chromatography. Mrs. Mary Qasim constructed

tables, arranged final form of document, and verified technical informationand linguistics. Mr. Todd Miller, WES, contributed drawings of molecularstructures. Mses. Deitdre Walker and Evelyn Toro and Mr. Jimmy Boyd,WES, obtained stopped-flow spectra, and Ms. Walker verified technicalinformation.The work was conducted under the direct supervision of Mr. Daniel E.Averett, Chief, ERB, and under the general supervision of Mr. Norman R.Francingues, Chief, Environmental Engineering Division, and Dr. John W.Keeley, Director, EL.At the time of publication of this report, Dr. Robert W. Whalin was Direc-

tor of WES. Commander was COL Bruce K. Howard, EN.

vii


10/85

This report should be cited as follows:Qasim, M. (1997). Simplified concepts in spectroscopy andphotochemistry, Technical Report IRRP-97-3, U.S. Army Engi-neer Waterways Experiment Station, Vicksburg, MS.

7he contentsof this report are notto be usedfor advenising, pubiicataon,or promotional purposes. CID-on of trade namesdoesnotconstituteanoficial endomnnent or apptwwl of the use of such commemalproducts....Vlll


11/85

1 Introductory StatementPhotochemistry effects changesotherwise might not be attainable.cal interaction between matter and

in organic and inorganic compounds whichSpectroscopy demonstrates the photochemi-electromagnetic radiation and deals with allkinds of interactions of electromagnetic radiation with a molecule whetherthese interactions are absorption, emission, optical rotation, or light scattering(Levine 1983). These interactions generally offer valuable information aboutthe molecule being investigated.

Since spectroscopy is the quantum mechanical manifestation of the elec-tronic configurational changes in atoms and molecules (as it applies to primaryphotochemical processes), spectroscopic techniques are powerful tools fordetermining both structural and quantitative changes in organic and inorganiccompounds.The reason spectroscopic techniques can ascertain the structure of com-

pounds is because different structures absorb energies at specific frequenciesor wavelengths. In other words, absorbed energy is quantized by havingdiscrete energy levels and is specific to the particular structure. The intensity(a measure of molar absorptivity of absorbed light) is proportional to theamplitude of the wave and is also specific to the wavelength at which absorp-tion takes place, further, this intensity is a measure for determining whetherthe electronic, vibrational, or rotational transition is allowed. These conceptsand techniques will be discussed in this paper, with the purpose of providinguseful, practical information about photochemical and spectroscopic tools thatcan be used by researchers. Principles of quantum mechanics will be verybriefly presented; in general, the information presented in this paper is anabridged compilation from many sources and is by no means complete.There are two basic components of this paper. The first portion consists ofbasic concepts needed for minimal understanding of the field; the second is

comprised of a few ultraviolet (UV) and visible (VIS) applications to organicmolecules, showing representative uses and examples.

Chapter 1 Introductory Statement 1


12/85

Background and Basic ConceptsTounderstand some of theconcepts presented in this paper, it is necessaryto provide sufficient historical background to show their development.In 1801, Thomas Young, in his diffraction experiments, demonstrated the

wave nature of light when light waves from two adjacent pinholes in a boxinterfered with each other. James Maxwell, in 1860, advanced the idea thatlight is comprised of electromagnetic radiation, an idea proven in 1887 byHeinrich Hertz, who produced electromagnetic radiation from oscillatingelectrons in a wire. Hertz discovered the photoelectric effectthe ejection ofelectrons from a molecule when impinged by light. When the frequency isabove a certain minimum value, electrons are ejected from the molecule(Bromberg 1980). The unit of frequency, Hertz (cycles per second), wasnamed after him.Between 1885 and 1910, Balmer, Rydberg, Lyman, Baschen, Brachen, andPfimd observed different light spectra from the hydrogen atom consistent with

the equation:

wherenl and n2 = integers

R is the experimentally measured Rydberg constant, which is equal to109,678 cm-l.

(1)

Niels Bohr, in 1913, postulated that the angular momentum of the orbitelectron of the heated hyrdrogen atom is quantized and can have only valueswhich are integer multiples of h/27r. Bohr found that the derived Rydbergsconstant, R, is equal to 27 nze4/h3c,which is the same as the experimentalvalue (m and e are the mass and charge of the electron, respectively; c is thespeed of light, and h is Plancks constant). The above equation confirms thatthe energy between the states is equal toE2-E1=hv (2)

Although it was found that Bohrs theory applied only to the hydrogenatom and could not be applied to a many-electron system, it paved the way forquantum mechanics introduced simultaneously by Heisenberg andSchrodinger.2 Chapter 1 Introductory Statement


13/85

Plancks ConstantAt the turn of the century it became obvious that classical mechanicaltheories could not explain the spectral distribution emanating from a heatedcavity (black-body radiation) or the many experimental facts pertaining toatom-size moving particles.When a cavity of fixed size is heated to a certain temperature, the radiantenergy increases and reaches a maximum only at one definite frequency.When heated to a higher temperature, the value of the maximum radiantenergy increases with a slight shift toward higher frequency.In 1900, Planck derived an accurate equation for the curves of the rate ofradiant energy, M, emanating from a heated cavity of fixed size against fre-

quency, v, where v is the frequency of waves or cycles per second. Planckproposed that electromagnetic radiation is emitted or absorbed by oscillatingdipoles. Each oscillator cannot absorb or emit an energy value or quantitydifferent from a proton, hv (Alberty 1983). Planck calculated h by applyingthe experimental value of the cavity spectra to the radiant energy-frequencyequation. The present value of Plancks constant, h, is 6.626176 x10-34Joules .seconds (or Js). Planck explained that line spectra produced fromhot gases can be interpreted only as a difference between two energies

Ez -El=hv (3)

Planck called this discrete unit of energy, hv (which corresponds to the fre-quency, v), a quantum. Quanta can be only whole number multiples of hv.This equation is one of the most important equations in spectroscopy becauseit forms the basis for much later development in the field.Usefid forms of the above equation are

AE=hv

A+AE=h6

wherec = 2.998 x 108m per secA = wavelength in meters, m

(4 )

(5 )

(6)

Chapter 1 Introductory Statement


14/85

The speed of light, c, is the frequency, v, times the wavelength, X, and thewave number, ~, = reciprocal of X.

A useful relationship between temperature, T,and the wavelength, X, at whichthe maximum radiant energy takes place, can easily be derived as the Wiendisplacement law,hmmT = 2.898 x 10-3mK (Alberty 1983; Atkins 1990) (8)

Another easily derived equation,~ = ~T4 (9)

relates the total radiant energy per unit time per unit area, M, emitted from acavity with fixed size to temperature, T, where a is the Stefan-Boltzmannconstant ,2~k40= = 5-6697 x 1o-8JSlm 2K-4 (Alber ty 1983; Atkins 1990) (10)15h3c2

Photoelectric EffectEinstein added to Plancks concept of electromagnetic radiation that eachbundle of light, in addition to being a wave, is also particle-like and has aunit, hv, which he called a photon. Einsteins equation for the photoelectriceffect can be written

wherev= minimum energy needed for the electron to escape a metal

surfacekinetic energy the electron possesses after it escapes thesurface

(11)

4 Chapter 1 Introductory Statement


15/85

The value of the energy, V, differs from one metal to another. Light absorp-tion by a molecule is independent of number of quanta impinging on the mole-cule per second (intensity) but does depend on the minimum threshold energypossessed by each quantum. Anincrease inthenurnber ofphotom impingingon a metal surface increases the rate of emission (intensity), whereas anincrease in the frequency of light increases the kinetic energy of the emittedelectron. In other words, when the frequency or energy is above a minimumthreshold, the energy of light will be transferred completely to the absorbingelectrons. According to Einstein, the absorption of a photon is localized; theprimary, or first, absorption is where one electron completely absorbs onephoton or quantum, h, which can cause the ejection of electrons.

Compton EffectThe Compton effect is when a monochromatic X-ray impinges on a graph-ite target and an X-ray photon collides with an individual electron, causing theappearance of an additional light component of somewhat longer wavelength

(less energy). Some energy is lost through collision, and the appearance ofthe light component of longer wavelength (light scattering) is caused by thisenergy loss.In summary, to account for the interaction of light with matter, the dualnature of light as waves and as particles must be recognized. To explain lightpropagation and its reflection, refraction, and interference, light must betreated as a succession of waves or electrical and magnetic oscillations perpen-dicular to each other and to the direction of wave propagation. However, tounderstand its Compton effect or its photoelectric effect, light must be treatedas bundles of particles with specific and discrete energies.

de Broglie WavelengthIn 1924, de Broglie derived his equation,

A=-!?-mu (12)by relating the wavelength, X, to is momentum, mu, where m and v are themass and the velocity of the particle, respectively. To do this, de Brogliecombined Einsteins equation E = md with the energy quantization equation,E = hv, substituting c/A for the frequency, v, where c = speed of light; inaddition, he used v, the speed of the particle, instead of c, the speed of light.By this equation,

Chapter 1 Introductory Statement 5


16/85

A=-!!- (12bis)mude Broglie hypothesized that the behavior of electromagnetic radiation appliesalso to matter; this same equation applies both to particles and to electromag-netic radiation. Later this was shown to be true experimentally. The signifi-cance of this equation is that it paved the way to quantum mechanics.

Furthermore, the de Broglie wave equation, by explaining the particle-wave dual behavior of an electron around the atom, confirms, in essence, theHeisenberg uncertainty principle that it is impossible to simultaneously deter-mine the electrons position and momentum with precision.

Heisenberg Uncertainty PrincipleThe uncertainty principle states that the product of the uncertainties in both

momentum and position of a particle is > Z/2 or

whereAP=Ax=h=h=

change in momentumchange in position (displacement of particle)Plancks constanth/27r

The energy derived from the above equation, upon rearranging, squaring,and dividing by 2 m, becomes

E= h2 (14)%ma2wherem = mass of the particlea h



17/85

E = 2h28ma 2The minimum energy,

(15)

obtained from the solution of the Schrodinger wavemechanics equation for a particle in a one-dimensional (1-D) box becomesidentical to the energy, E = h2/8ma2 (uncertainty principle) when the quantumnumber n = 1. This contrasts with classical mechanics energy equationswhere there are no restrictions on the value of n. Also the minimum energy(from the uncertainty principle) is identical to the energy obtained from thede Broglie equation. In addition, the de Broglie wavelength is identical to thewavelength obtained from the solution of the wave function of the Schrodingerequation.

The uncertainty principle also applies to the product of the uncertainties intime and energy

This equation is really the same as the first equation for the product of theuncertainties in momentum and position. The product, AE.At, it is importantin determining the lifetime, 7, which is the average span of time particle staysin an excited state before reverting to the ground state. Substituting Plancksquantitized energy,AE = hAv (17)

which is AE = 27r h = h/2mAv into the uncertainty principle equation,

yields

or T > (4TAV)-1

(19)

(20)

This establishes an inverse relationship between the lifetime of an excited stateand its spectral bandwidth expressed in terms of frequency.



18/85

Schrodinger EquationIn 1929, at the time Heisenberg was developing his matrix mechanics,Schrodinger was postulating his wave mechanics equation. It is noteworthythat although the two methods proved to be mathematically identical, theSchrodinger wave equation, which marked a complete break from classical

wave mechanics, was never derived but was judged by its correct results andpredictions which agree with experimental values obtained through spectro-scopic techniques.The Schrodinger equation can be written in its simplest form as

H+ = E+ (21)

whereH=

E=$ =

quantum mechanical Hamiltonian operator, which contains bothkinetic and potential energy componentstotal energytime independent wave function, which completely describes the stateof the system at a specific time

The significance of ~ is when used with its conjugate, **, over the differen-tial volume element, dT, at a specific time, t, the product, ~*#dT, is real andrepresents the probability for finding the particle within the volume element,dr.A simplification of the wave equation considers the motion of a particlewithin a 1-D box in the x direction in which the Schrodinger equationbecomes

1H$x = E#x (22)

and the probability density along the x axis can be #X*#Xor 142I~. Theprobability density is real and finite; the wave function, +, is well-behavedand a single-valued.

The classical Hamiltonian operator isH=12 _px+v2m x (23)



19/85

whereP = momentum, muVx = potential energyIn quantum mechanics px is replaced by

whereJ1i=-

h=~27rThe I-D Schrodinger equation becomes

-~%z $x + Vx$ = E$x2nu?!x2 (24)

To avoid discontinuity, # must = O at x = O and at x = a. Within thefree space of the 1-D box whose length is a, VX= O. Thus the wave equationreduces to

(25)

There are two satisfactory solutions to this equation for the allowed energies:

or

or42 = B1 Cos b

(27)

(28)

(29)



20/85

where

A, A1, BThe above

Therefore,

and B1 = arbitrary constants and k = ;- m2Tsolutions represent traveling waves, where h = _k

substituting fork givesh=-!!-mu

This wavelength, A, obtained as asolution of the Schrodinger equation, isidentical to the de Broglie wavelength.It can also be shown that within the box,

;-a=n~

which gives

E== 8ma2where

a= length of the free path of the particle in the 1-D boxn = integer quantum number with a minimum value of 1

This energy equation is the same as that obtained from the de Broglie or theHeisenberg uncertainty principle and contrasts with the energy equationobtained from classical mechanics where n can be of any value.

(30)

(31)



21/85

2 Basic Tools

Needed basic tools will be briefly discussed in this chapter, namely:Lambert-Beer Law (the thoretical basis forabsorbance-concentrationmeasure-ments); the spectrophotometer (first, different spectrophotometers for differentregions of the spectra, and second, basic differences between emission andabsorption spectrophotometers); the monochromator (the component of aspectrophotometer where wavelength separation takes place); and lasers.

Lambert-Beer LawSpectroscopy is a direct application as well as the manifestation of quantummechanical concepts. The Lambert-Beer Law constitutes the basis of spectro-scopic analytical methods and can be used for concentration measurements inthe W, VIS, and infared (IR) regions. Therefore, the Lambert-Beer is dis-cussed here and can be written as

10A=log7=ecl

whereA= absorbancez~ = intensity light, quanta s-l per unit area1 = intensity of the transmitted light not absorbed by the samplec = concentration of the sample in moles per liter1 = sample cell length in centimetersE = (which used to be called the molar extinction coefficient) = molarabsorptivity, a measure as to whether an electronic or vibrationaltransition of a molecule is allowed at a certain wavelength

(33)

Chapter 2 Basic Tools 11


22/85

Toderive the Larnbefi-Beer Law, simply consider alightbem with inten-sity 10impinging on a sample of concentration, c, in a cell which is 1 cmlong. 10decreases with small increments, d, of the cell length (Figure 1).ThendI = -kcIa

which becomesIIn ~ = kcl, or using base 10 logarithm

Z.log ~ = G c1 = the absorbance, A,

whereE = 2.3k

(34)

(35)

(36)

(37)

Figure 1. Sample cell

12Chapter 2 Basic Tools


23/85

Instrumentation

Spectrophotometers measure changes in the spectra; irradiation deviceseffect these changes.

It is important to explain the basic components of that powerful tool, thespectrophotometer. These components of the spectrophotometer depend firston which spectral region (IR, UV, VIS) is being investigated and secondaccording to whether absorption or emission is being studied.

Simi lar it ies and di ff er en ces b et w ee n t he a bs or pt ion sp ec t roph ot o-met ers in t he UV, VIS, and IR regionsThe basic components of a spectrophotometer in the IR, VIS, and Wregions are similar; the main differences are in the sources and sample cells.The source in the IR is a metal or ceramic heating element, which emits heat

or vibrational energy, whereas the source for the W is an electron tube, suchas a deuterium light, and the source used in the VIS region is a tungstenfilament light bulb.The sample cell in each one of the spectral regions must be transparent toelectromagnetic radiation in that region and also inert to the sample beingmeasured. In the IR region, the sample cell is made of salt, such as sodiumchloride or potassium bromide. Although the sample cell in the VIS regioncan be quartz (fused silica) or glass, a sample cell in the W region must betransparent to UV radiation and therefore is made only from fused silica.

Sim ilar i t ies and di f ferenc es bet w een t he absorpt ion and t he em issionspec t ropho tomete rFigures 2 and 3 represent schematic diagrams of single-beam absorptionand emission spectrophotometers, respectively. The typical components of anabsorption or emission spectrophotometer are: light source, monochromator,sample cell, detector, and convenient display device.The main difference between the absorption and the emission spectrophoto-meter is that in the absorption spectrophotometer, the source, monochromator,sample cell, and detector are all in a line with the beam emanating from thelight source, whereas in the emission spectrophotometer, a second monochro-

mator and the emission detector are in line with a slit opening in the samplecompartment perpendicular to the beam emanating ffom the light source.Thus, in the emission spectrophotometer, the sample becomes a secondarysource of radiation; the only light reaching the detector is that emanating fi-omthe sample itself.



24/85

SCHEMATIC DIAGRAM OF AN ABSORPTION SPECTROPHOTOMETER

I

II1. SOURCE2. SILT3. CONVEX LENS4. MONOCHROMf=TER5. SAMPLE CELL6. DETECTOR

Figure 2. Schematic diagram of an absorption spectrophotometer

Grat i ng monochr oma t orThe gratings of these monochromators consist of a larger number of paral-lel lines engraved on a metallic surface. When a beam of monochromaticlight falls on a grating, each line of the grating becomes a secondary source ofradiation, emitting light in many directions. At certain positions on the oppo-site side of the grating, wavelengths in phase will accentuate each other,whereas at other positions, wavelengths not in phase will form destructiveinterference with each other. Thus, a pattern of bright lines with dark spaces

between them will be formed. Since each wavelength has its own deflectionangle, the many wavelengths of irradiation will separate upon impinging onthe grating. Using Youngs experiment for wavelength interference, it can beshown that the action of an incident beam of light falling perpendicularly on agrating can be described by the following equations:

14 Chapter 2 Basic Tools


25/85

SIMPLIFIED SCHEMATIC DIAGRAM TYPICAL FOR ASINGLE BEAM EMISSION SPECTROPHOTOMETER

4u

r

II031 I*I F -II

5

8

--b--b

1. SOURCE2. SILT3. ABSORPTION MONOCHROMETER4. SAMPLE CELL5. CONVEX LENS6. EMKSiON MONOCHROMETER7. DECTECTOR

Figure 3. Simplified schematic diagram typical for single-beam emissionspectrophotometerdsin O for bright

2dsin O for dark

where

lines (n = 0,1,2,3...) (38)

spaces (n = 1,3,5...) (39)

n = order of deflectiond = distance between the linese = angle of deflectionA = wavelength of incident radiationThe last of the two above equations is the same as the Bragg equation used

in X-ray, crystallography, n~ = 2dsin 0, where n = order of diffraction andequals 0,1,2,3...; d = the spacing between particles; 6 = angle of X-raydiffraction; and 6 = wavelength of incident X-ray radiation.

15Chapter 2 Basic Tools


26/85

Lasers (Light Amplification by Stimulated Emissionof Radiation)Lasers are produced by stimulated emission when more than two electronicenergy levels are involved. An upper energy state, m, can be populated byvery intense irradiation with appropriate frequency whereas an intermediate

energy level, 1, can be populated indirectly by radiationless processes from theupper level. If there is no incident irradiation on the middle or intermediatelevel, this level might lose its excitation and drop down to the ground state, k,only through spontaneous emission, which is much less efficient than stimula-ted emission. This would, therefore, result in an inversion of populationwhere the intermediate state becomes more populated than the ground state(Figure 4).The laser system is placed in a cylinder with two highly polished parallelmirrors, one at each end and one of which is partially silvered. The distancebetween the system and the mirrors is proportional to integer wavelengths, n~,between the ground state, k, and the intermediate state (Figure 5).Low-intensity spontaneous emission from the intermediate state multipliesas it is reflected back and forth into the mirrors along the axis of the cylinder,inducing a coherent, intense, and very monochromatic laser beam along theaxis. It is important to note that in spontaneous emission, the molecule emitsin random direction and phase, whereas in stimulated emission, the emissionis in direction and phase of the beam.Among the many uses of lasers:a.

b.

c.

d.

Lasers, due to their monochromaticity and coherence, are selective forspecific photochemical reactions.Lasers can be used to study very fast reactions (picosecond) becauseof their short durations (Bromberg 1980).Lasers are perfect tools for detecting weak lines in Raman spectrabecause of their very high intensity and monochromaticity.Lasers can selectively ionize or dissociate a specific isotope in amolecule and therefore can be used for the separation of isotopes(Alberty 1983).

16 Chapter 2 Basic Tools


27/85

DIAGRAM DEPICTING POPULATION INVERSION

INTENSE

m

k ) RADIONLESS TRANSITION/

\

SPONTANEOUS EMISSION(VERY INEFFICIENT)

Figure 4. Diagram depicting population inversion



28/85

SIMPLIFIED DIAGRAM OF LASER SOURCE

2d IS THE CYLINDER LENGTHn~ IS WAVELENGTHn IS AN INTEGER

Figure 5. Simplified diagram of laser source

18 Chapter 2 Basic TOOLS


29/85

3 Regions of ElectromagneticRadiation

The useful spectra of electromagnetic radiation covers a wide range ofenergies. Each region, due to differences in energy, unfolds useful and variedinformation about the molecules being studied. Although these regionsrepresent different energies, the equations pertaining to these various regionsare quite similar. Table 1 summarizes the functions and indicates the magni-tude of energies, frequencies, and wavelengths for the different parts of theelectromagnetic spectra. These regions are briefly discussed, the main empha-sis being on electronic excitations, UV, and VIS spectra. The order indealing with most of the spectral regions is from shortest to longestwavelengths.

X-RaysX-rays are produced when a high-energy electron30 Kev, strikes a target atom, typically metal atoms. beam, more thanX-rays produce spectra

which show continuous intensity curves with superimposed sharp peaks. Thecontinuous curve and discrete sharp peaks can easily be understood throughquantum mechanics. The continuous part of a typical X-ray curve originatesas the electron is slowed or stopped; part of the electrons kinetic energy isconverted to electromagnetic radiation (X-rays) as it slows. However, whenthe electron is completely stopped by the particle, all of its kinetic energy iscompletely converted to X-rays (light quanta, hv) (Bromberg 1980). Increas-ing the energy of the striking electron manifests itself by shifting the X-raycontinuous spectra toward shorter wavelengths (higher energy).The discrete peaks are due to transitions of electrons between the innerorbits of the target atoms (unlike UV and VIS spectra, which are due to elec-tronic transitions between the outermost orbitals). When an electron is ejectedfrom an inner orbit, an electron from a higher orbit falls down to replace it,causing emission of X-ray radiation. X-ray crystallographers use specialdesignations for these inner shell electronic transitions, such as K. and KB and

as La and LB, etc.

Chapter 3 Regions of Electromagnetic Radiation 19


30/85

6

\L

\ ~ !:. +---4

IEK=EEETEI$\

.2_k ---- -1 ----- -1 ----- -1 . --- -I

, 1 -.., :,, C$ . . .., .-,, ,,, ,. ... ,, :$01 I

.*

,-1 :+: q:=~.*0)1.Co:2 -,0, ,:s::3::1.$ . . . .1 O . . .. 01 . . . :.1 . ;:::155 n

,,.,

..,

~.t, ,

------- ----t--------------t------.-------l-- :------1,., ,,*, ,... ,,,4 ,. .,;,, ,, -,;,,,4, ,,, ,8, ,, ,,, .:1: .-::::1~,,ct 1:x: :,--:, ~s, : :. 0 =,. ,c-, , ,

ox. ! *: : ;=;: :.IE, ,:: :.,. , ,

l. !,,,, t .,, ,,, ,.,, . ..-.,,.,1

I

,

* **

20 Chapter 3 Regions of Electromagnetic Radiation


31/85

Avery important relationship, which establishes the basis for the presentperiodic table of the elements, was Moselys finding (1913-1914) that thesquares of the atomic numbers of the various elements are directly propor-tionalto their KaX-ray frequency (Bromberg 1980).X-rays are useful inidenti~ing structures oforganic and inorganic crystalsby showing adjacent particles and determining the distances between. X-ray

crystallography takes advantage of the fact that X-rays are of the same magni-tude as bonds between atoms, approximately 10-10m by using Braggsequation,n~ = 2d sin O (40)

wheren=A =e=d =

integerwavelengthangle of diffractiondistance between particles

UV and VIS Regions of the SpectraThe ease of electronic transition from the ground to a higher excited statedictates whether the observed wavelength is in the UV or VIS region. UVand VIS spectra offer valuable information for identifying compounds, espe-

cially by the use of their emission (See paragraph, Fluorescence andphosphorescence).This region also gives information about the electronic behavior of com-pounds and about ongoing changes due to chemical reactions.W and VIS spectra are the most useful regions for concentration measure-ments through use of the Lambert-Beer law. Molar absorptivities depend onthe compound, the wavelength, and the solvent and are a measure as towhether an electronic transition is allowed. The magnitude of molar absorp-tivity is proportional to intensity which is, in turn, proportional to the elec-trons undergoing transition.

Single t and t ripl et e lec t r on ic ex c it ed st at esAn electronic excited state can be represented as 2S+ 1, where S is thetotal electronic spin quantum number. Each electron possesses + or- 1/2 spin quantum number.



32/85

In a singlet state, the electron spins cancel each other, S = Oor 2S+ 1= 1, whereas in a triplet state, the total electron spin, S = 1 or 2S + 1 = 3.For example, fluorescence emanates from singlet states where AS = O;however, with phosphorescence, AS # O.

Fat e of t he elec tron ic ex ci t ed st at es~enamolecule absorbs aphoton orqum_(hv) of Wor VIS light, anelectron is promoted from the singlet ground state, So, to an excited singletstate in about 10-15seconds. In about 10-1~seconds, some of the energy willbelostby internal conversion, andtheelectrons drop tothefirst excited sing-Iet state, S1. A typical lifetime of this first excited singlet state is between10-8and 10-7seconds.At this time, the electron might drop (due to nonradiative intersystemcrossing) to the first excited triplet state. This state, TI, generally has lowerenergy than that of the first excited singlet state, SI, but higher than the

ground singlet state, So. The electron in the first excited triplet state mightlose its energy and drop to the singlet ground state by the radiative process,fluorescence, or bynonradiative processes. Or, ifthefirst excited triplet stateis close enough to the first excited singlet state, the electron might gain suffi-cient vibrational energy to cross over to the first excited singlet state and thenlose its excess energy by delayed fluorescence to the ground singlet state.When the electronic inthefirst excited singlet state, energy might also belost by the radiative process of fluorescence. Eventually the electron will loseits excess energy due to: chemical reactions (isomerization, oxidation, decom-position, etc.); emission (fluorescence and phosphorescence); or heat. Thusseveral competing processes exist at the same time. (Figure 6).

Fl uo re sc e nc e a nd p ho sp ho re sc e nc eFluorescence and phosphorescence are two very important phenomena thatdeserve special attention. Fluorescence is the radiative emission of light fkoman electronic excited singlet state to the ground state, whereas phosphores-cence involves emission from the lowest excited triplet state (which is lowerthan the first singlet state) to the ground state, which causes phosphorescenceto appear at longer wavelengths. A fluorescence spectrum of a compoundappears to be a mirror image of its absorption spectrum and does not involvea geometric change of the molecule. A phosphorescence spectrum, on the

other hand, does not appear as a mirror image of the absorption spectrum ofthe compound and might be accompanied by a geometric change in themolecule.Figure 7 illustrates the absorption and emission spectra of tran.s 2-(p-dimethylaminobenzylidene)-4-butyrolactone and shows mirror imaging and



33/85

E

1. SINGLET GROUND STATE, So ~SJNGLET EXCITED, S n ABSORPTION TRANSITION2. SINGLfX, S1 ~SCjFLWRESCENCE3. TI~ so PHOSPHORESCENCE4. S1~ SO RADIATIONLESS DEACTIVATION5. s ~ TI ~ SI RAJJMnoNLEss INTERSYSTEM CROSSING6. Sz~ S1RADIATIONLESS INTERNAL CONVERSION

EY

Figure 6. Simplified energy diagram showing the fate of the electronicexcited states



34/85

24

9C

w7060

50

4030

20

100

(a)

ABSORPTION EMISSION (-196 C)

I I I I I I250 300 350 400 450 500 550 600 650nm

90

80

70

Il l~405IhO20

100

_ (b) /\EMISSION (-196 C) /IIABSORPTION 1,* Ihi;,; \I/ 1\/ I\; \\1 \\1 \.1I I J-\/ \,( 1-.

250 300 350 400 450 ~o 550 : 650(-Mnm

Figure 7. Absorption and emission spectra of trans2-(p-dimethy laminoben-zylidene)-4-buty rolactone

Chapter 3 Regions of Electromagnetic Radiation


35/85

delayed fluorescence and phosphorescence. A detailed study of the photoiso-merization of this molecule has been made, and therefore it will be used as amodel; discussion of its absorption and emission spectra is in Chapter 5regarding some applications of UV and VIS spectroscopy to organic mole-cules. A significant finding is that a trans isomer sample dissolved in MCIPin a sealed glass tube exhibited totally reversible green phosphorescence at196 C and delayed blue fluorescence at 78 C, demonstrating a clearexample of thermal intersystem crossing between the first excited singlet andtriplet states.

Figure 8a shows a structural representation of the photochemical intercon-version of cis-trans 2-(p-dimethylaminobenzylidene)-4-butyrolactone.Fluorescence is an allowedwhile phosphorescence is a for-bidden electronic transition,which accounts for the facts thatfluorescence is intense andinstantaneous (10-8 see) and that

phosphorescence is less intenseand much slower (10-2-I@ see). Therefore, otherquenching processes might beable to compete with phospho-rescence except at low tempera-tures where rates of quenchingare much reduced.Emission radiation from amolecule usually takes place at awavelength between 30 and50 nm longer than that of theexciting wavelength. Thisdifference, known as the Stokesshift, is due to loss of energy inthe excited state caused by

CH3, ,CH3NA/,~ H

Iv-\l~ cH3@Ct$vibrational relaxation (Gillespie 1985).

Other spectroscopic concepts that must be considered because they some-times constitute interference with fluorescence and phosphorescence are thelight scattering mechanisms that arise for various reasons, such as the Ray-leigh scatter. This light scatter is due to tie interface interaction between theexciting photons and the air cell, the cell wall solution, and the material sus-pended in the solution, making it more difllcult to separate the excitation fromthe emission spectra (Gillespie 1985).

Raman scattering (discussed later in more detail) appears at longer wave-lengths than that of the exciting light. This phenomenon is due to the

Figure 8a. Structural representation of ck-transphotochemical interconversion

Chapter 3 Regions of Electromagnetic Radiat ion 25


36/85

interaction of the bonds of the solvent molecules with the photons of the excit-ing light.Emission spectra (fluorescence and phosphorescence) are more sensitivethan absorption spectra for qualitative and quantitative measurements becauseof their higher intensity.

T r ip l et sens i t ize r sAbsorption of light energy by a compound used as a triplet sensitizer popu-lates the corresponding triplet state of the compound to be measured. This inturn results in p-type delayed fluorescence. A good triplet sensitizer is onewith a higher triplet, but lower singlet, state than those of the compound to besensitized. A good sensitizer also absorbs at a different wavelength than theother components in the solution. Using the triplet sensitizer technique, com-

pounds with concentrations as low as 10-9M could be detected. (Parker1966).

Quantum yieldThis useful concept can be defined in several operative ways. Forinstance, quantum yields can be evaluated for either absorption or luminesc-ence (fluorescence and phosphorescence). For absorption, quantum yield isthe fraction of a molecule that can absorb a quantum of radiation energy, i.e.,the number of molecules per quantum of energyor the number of moles perEinstein of energy absorbed. For fluorescence, quantum yield is defined asthe ratio of the number of molecules emitting radiation to the total number ofmolecules excited (Gillespie 1985) or, simply, quantum yield can be looked atas a ratio of photons emitted to those excited. Fluorescence quantum yield is

directly related to the mean lifetime of the excited singlet state, fluorescenceintensity, fluorescence rate constant, intersystem crossing, and to the rateconstant for internal conversion.Quantum yield calculations also shed light on nonradiative processes andtherefore on the fate of excited states and their mechanisms. In addition,large quantum yields are indicative of free radical involvement since theseproduce chain reactions.Quantum yield measurements were used to shed light on the photochemis-try involved in the cis-trans interconversion. Since quantum yield reflects theratio of the rate of photochemical isomerization to the rate of light absorption,

a knowledge of the number of quanta absorbed per unit time is necessary.The fraction of light absorbed by either isomer can be determined only if thelight flux incident on the system is known. This is turn was determined bychemical actinometry.



37/85

Since the values of the quantum yield measurements (between 400 and300 nrn) of the cis-trans photochernical isomerization of the sample com-pound, 2-(p-dirnethylaminobenzulidene)-4-butyrolactone, have been accuratelydetermined, and since quantum yield values (plus nuclear magnetic resonance(NMR) measurements) indicate no significant competing mechanisms with thephotochemical isomerization, this makes the compound a perfect chemicalactinometer (Figure 8b).

Quantum yields for the pho-tochemical interconversion weredetermined at room temperatureover a wide range of excitationwavelengths in the most intenseabsorption band and in bothabsolute alcohol (polar) and inheptane (nonpolar). The rela-tive quantum yields for the twoisomers in both solvent systemsat the various wavelengths ofirradiation are listed in Table 2and the absolute quantum yieldsare shown in Table 3. Thesequantum yields are derived fromthe ratios of the photostationarystate concentrations of the twoisomers multiplied by theirrespective molar absorptivities.Quantum efllciency of the cisis less than that of the trans iso-mer in both solvent systems,

w,, ,..3N

H

H

o

Figure 8b. Structural representation of cis-trans pho-tochemical interconversion

especially in heptane. Also, the quantum efllciency of both isomers, particu-larly that of the trans isomer, is lower in alcohol than in heptane. The quan-tum yields of the cis-trans isomers in both solvent systems were quite high;however, their sum was less than unity (although in heptane it was close toone). The quantum yields appeared to be independent of the irradiation wave-lengths for most of the range. In addition, thermal conversion from the cisisomer to the trans isomer showed that the singlet ground state of the trans islower in energy than that of the cis isomer.Although complete elucidation of the photochemistry involved in the cis-trans interconversion has not been accomplished, some important information

regarding the mechanism was obtained and is discussed under the fate ofelectronic excited states. Figure 9 represents a schematic diagram of a mono-chromatic irradiation system that affects photochemical changes while measur-ing the number of quanta corresponding to these changes.

Chapter 3 Regions of Electromagnetic Radiat ion 27


38/85

Table 2Re la ti ve Qu an tum Y ie ld s fo r c k -t ~a ns Ph ot oi some ri za ti on~c m q t x % t at photostationary state@ m ec x %c at photostationary stateHep tane SystemWave lengt h , y , I Exper imenta ld i rect ionnm t - c c - t400390

A lcoho l SystemExper imenta ld i rect ion

t- c c- t0.77 0.860.74 0.89

380 I I I 0.81 I 0.88370 I 0.501 i 0.60 I 0.78 I 0.79360 0.70 0.80 0.77 0.77350 0.70 0.81 0.78 0.75340 I 0.70 I 0.81 I 0.78 I 0.75340 0.70 0.75 0.81 0.71330 0.74 0.68 0.932 0.61 Not reliable due to the very small molar absorptivity of the trans isomer at thiswavelength.2 Not reliable because conversion to the photostationary state was not completelyattained, due to weak l ight intensity at this wavelength.

Table 3Absolut e Quant um Yields for cis-trans Photoisomer izat ion2 -(p -d im e th yl am i no be nzu li de ne )-4 -b ut y ro Ia c to ne i n H ep t an e a ndAlcoho l

Hept ane Syst em Alc ohol Syst emy , nm @t I @c @t @c400 (0.49) 0.42390 (0.37) 0.33380 I I 0.42 0.37 I 0.310.45370 I (0.65) ] 0.39 I 0.34 I 0.27360 (0.48) 0.38 0.34 0.27350 (0.49) 0.40 0.39 0.34340 I 0.50 I 0.39 I 0.42 I 0.37330 I 0.47 I 0.41 I 0.57 I 0.53Note: The least reliable quantum yield measurements were at the wavelengths presentedat the beginning and en d of the table, and were due to the low absorptivities of the iso-mer s a n t o t he l ow i nt en si ty o f t he light sourse.@t values between brackets were calculated from experimental @c and their relative quan-tum yields, where error in obtaining experimental @ has been amde. Good correlationcoefficients and standard deviation of the corresponding @c values indicate that thesecalculated @t values are quite accurate. Calculated on the basis of no photchemical reversal from Vans to cis at 380 nm.



39/85

-1ii ic)n5CJ

E

hi-I I 6 JuLLl-1 I

.

n

A

29Chapter 3 Regions of Electromagnetic Radiation


40/85

Pi and sigm a elec t ronsElectronic excitations or transitions inorganic molecules may involven,sigma or pi electrons:n electrons are the nonbonding or unshared electrons such as the four sp3electrons on the oxygen molecule in water or the two nonbonding sp3 elec-

trons on the nitrogen in ammonia.T electrons are the electrons in the pi orbitals which overlap (if they arebonding) side to side in double and triple bonds above and below the axisbetween two atoms.o electrons are the electrons in the sigma orbitals which overlap (if theyare bonding) head to head along the axis between the two bonded atoms. Thesigma electronic density has circular symmetry along the axis of the bond.All single bonds are sigma bonds.Examples of the last two types of electron bonding, for instance, are the

double bond between the two carbons in ethylene, which has one sigma andone pi bond, and the triple bond in acetylene, which has one sigma and two pibonds.The following are some observations involving n, sigma, and pi electrons:a. In organic molecules, one can predict from their UV and VIS spectrawhether double bonds are conjugated; less energy is required to excitea pi electron in conjugated double bonds than in isolated double bondsbecause the energy difference between the highest occupied and theunoccupied molecular orbitals is less for conjugated than for isolateddouble bonds. Examples of trends of spectral changes (wavelength and

molar absorptivities) due to structural changes are found in Chapter 5.b. Transitions involving n ~ T* are of lower energy than transitions fromn ~ O*because X* orbitals are of lower energy than the correspondingO* orbitals (Fessenden 1982; Atkins 1990).c. Electronic transitions from n - T* or n ~ a* are of lower molarabsorptivities (less intense) than transitions from m~ T* or fromo + O* (about 1 percent) because the n electrons have a different orien-tation in space and therefore, according to the selection rules, theirtransitions are forbidden.Figure 10 is a simplified molecular orbital diagram of acetone,(CH~)z C = O, Summarizing the n + T* and T ~ T* transitions of the nelectrons around the oxygen atom and the T electrons between the carbon andoxygen atoms in the carbonyl group, C = O. The diagram also shows theinvolvement of singlet and triplet states regardless of whether the particulartransition is allowed.

.



41/85

1+-e-+ I7xe

a)

I

-i-7 1-*

g00Nzs1-

.0

-1-1 -1-1 1-1,Ccf

I-al

wx1-



42/85

4 Vibrational and RotationalExcitations

Infrared RegionA photon in the IR region possesses less energy than a photon in the VISor UV regions (2-15 kcal/mole compared to 40-300 kcal/mole in the W and

VIS regions). Near IR photons interact with the vibration of bonds betweenatoms in a molecule. A molecule can absorb IR if it forms an oscillatingdipole moment which can interact with the electric field component of the IRand also if the vibrations of the molecule correspond to those of the IR vibra-tions. A molecule absorbs fixed but quantized vibrational energy, increasingthe vibrational amplitude of its bonds and, thereby, acquiring excited vibra-tional states.Two nuclei bonded covalentiy vibrate similarly to two balls attached by aspring. Double and triple bonds might be thought of as stronger. springs. Atequilibrium distance, a molecules vibrational energy is minimum, and adiatomic molecule might be considered as being at the bottom of a potential

energy well.The average distance between the two atoms represents the covalent bondlength prior to IR vibrational excitation. Different bonds or different func-tional groups absorb different but specific vibrational energies. For example,the O-H bond absorbs between 3,000 and 3,700 cm-l, corresponding to wave-lengths between 2.7 and 3.3 ~m, whereas the C-O bond absorbs between 900and 1,300 cm-l or between 8 and 11 pm.Also the same bond can absorb energy at more than one frequency or wavenumber, i.e., the reciprocal of the wavelength. The specific frequency ofabsorption depends on whether the bond is experiencing a stretching or a

bending mode of vibration. For example, an O-H bond absorbs at 3,300 cm-l(3.0 pm) due to its stretching vibrations, while the same O-H bond absorbs at1,250 cm-l (8.0 pm) due to its bending vibrations. Except where hydrogenbonding is involved, absorption bonds broaden, and intensity increases, infra-red is insensitive as to whether the substance is gas, liquid, or solid.

32 Chapter 4 Vibrational and Rotational Excitations


43/85

The amplitude of the wavelength or the vibrational intensity is dependenton whether the particular vibrational excitation transition is or is not allowed,according to the various selection rules. The allowed vibrational energy for aparticle, according to the selection rules, is given by the equation:

(41)

wherem = mass of the particlek = force constantv = vibrational quantum number, which has an integral value such as O,1, 2, 3, etc. (Barrows 1981)

1The above equation can also be written as ~ ~i~= (V + .-) hv, since therIkfrequency v = 27r zAnother restriction on the vibrational quantum number is that AU= &1,which means that transitions are allowed only between vibrationally adjacentenergy levels.

The uniqueness of IR spectra for each organic compound makes it apowerful tool for identification. Figure 11 shows the IR spectra of the exam-ple compounds, the cis and trans isomers of 2-(p-dimethylarninobenzylidene)-4-butyrolactone.

Fourier Transform InfraredDue to innovative technology, IR and

Region (FTIR)NMR dispersive spectrophotometersare being replaced by Fourier transform systems in which the resultant signalis an interferogram resulting horn the sum of individual signals caused byeach wavelength. Each point in the interferogram is a culmimtion of allfrequencies in the spectrum. FTIR spectrophotometers are very fast and offerhigh resolution.

Figure 12 represents a simplified Michelson interferometer for the FTIRspectrophotometer, showing the source, beam splitter, fixed and moving mir-ror positions, sample holder, and detector.

Chapter 4 Vibrational and Rotational Excitations 33


44/85

o 000000% m< v. m. W.ty o00 o o o oooo -- 8o-0N

0m

0.0

0Ui

0a.0.co

0.*

InC6od

-UJz+gllq

i- --- Em+4+MtH

} 1 I ,I 1 I 1 I r

r -r13

3aNva140sav

2w

ua)QEc%

34 Cha@er 4 Vibrational and Rotational Excitations


45/85

o4

3

1. SOURCE2. BEAM SPLITTER3. SAMPLE HOLDER4 . DET ECTOR5. FIXED MIRROR

a

5

6PO

/ bL2

7PI1II1IIIII

8P2I

I

6. MOVING MIRROR (lNITIAL POSmON, a=b)7,8. OTHER MIRROR POSITIONS

WAVELENGTH

Figure 12. Michelson interferometer

MicrowaveThe microwave region of electronic radiation deals with the pure rotationof molecules and can be used for the measurement of bond lengths and molec-ular angles. The following are summary statements of angular momentum,selection rules for the allowed rotational states, and the quantized rotationalenergy states:a. The total angular momentum, Iu, must satis@ the equation

Chapter 4 Vibrational and Rotat ional Excitat ions


46/85

(42)

whererotational quantum number~ = 1, 2, 3, etc.

u = angular velocityZ = momentum of inertia

whereF?$= mass of the particleTi = rotational radiusb. The vector component of the angular momentum in a directionimposed on the rotating particles by an electric or magnetic field ishgiven by M 2T

whereM = -J, -J+l, -J+2...O, 1..., Jc. Rotational transitions are allowed only between adjacent rotational

energy levels, or

(43)

Al =&l (44)

d. Rotational energy is easily derived horn the angular momentum and isgiven as

E h2=J(J+l) rot (Barrows 1981)82 I (45)



47/85

Nuclear Magnetic Resonance (NMR)When placed in a strong magnetic field, nuclei of atoms with magneticresonance moments (atoms having odd atomic numbers or atomic masses,such as hydrogen and carbon -13) act as small magnets.

More than one-half of these nuclei align themselves with the magnetic fieldwhere they will be in their lowest spin states. Upon absorbing the neededradio frequencies, they will then align themselves against the field, going intoa higher spin state. Transition to this higher energy state (magnetic reso-nance) depends on both the external magnetic field and on the nucleus nuclearmagnetic moment.Nuclear magnetic resonance is governed by the selectively allowed com-

ponents of the nucleus angular momentum in the direction of the field andis summarized by the Equation 45. AE = hv = 2~H, v = frequency, ~= nuclear magnetic moment, H = field strength.Since different nuclei in a molecule experience different environments dueto different neighboring atoms, they therefore absorb different radio frequen-cies. In other words, one might say that resonance takes place at differentfrequencies or at different field strengths.The separation of NMR signals, or the place where resonance occurs in themagnetic field as a result of atoms being in different environments, is calledchemical shift and can be shown as:

Hsample - H ra e,en ce x 1066= (46)H r@erencewhereHsample d re$erence = magnetic field strengths of the sample and referencecompounds respectively (Barrows 1981)The large single signal of the reference compound usually usedtetramethyl sikme (CH~)dSi, referred to as TMSis assigned zero, HZorzero parts per million @pm) on the radio frequency scale. TMS is a conve-nient reference because its 12 protons are all in an identical environment andare shielded from the magnetic field by their electrons; protons in TMS do nothave neighboring electronegative atoms which can deshield them by attracting

the electrons toward themselves.The more screened or shielded the protons are from the external magneticfield by their electrons, the larger (more upfield) the magnetic field needed tocause magnetic resonance, whereas the more protons are exposed ordeshielded, the smaller (more dowfileld) the magnetic field needed to causemagnetic resonance.



48/85

Important structural information about molecules can be obtained fromtheir proton NMR spectra.For example:a. Thenumber ofdifferent enviroments (different sets ofpeak) givesthe number of the different kinds of hydrogen atoms in a compound.b. The chemical shift, 8, gives information about the different environ-ments and therefore about the different kinds of hydrogen atoms in acompound.c. The relative integrated areas of the different groupings of chemicalshifts reveal the relative number of hydrogen atoms in each group.d. The spin-spin coupling pattern reveals the number of hydrogen nucleiin groups which interact with each other.The cis and trans isomers of 2-(p-dirnethylaminobenzylidene)-4-

butyrolactone can be used to illustrate the previous points, showing how NMRtechniques can be usefil for structural determination. Figure 13 representsthe NMR spectra of the cis and tran.s compounds, respectively. The molecu-lar figures show arbitrary letter desigmtions of the protons in different envi-ronments. Figure 13 shows definite assignments of spectra lines to protons inthese two structures, giving their chemical shills and coupling constants.The singlet line of the six dimethylamino protons which are in the sameenvironment is the most intense line of the spectrum and has about the samechemical shift for both isomers, 181 cps relative to TMS.The lactone ring has two kinds of protons, B and M. The lines of the

B protons for both isomers are partially buried under those of the A protonsand are split into three sets of doublets by the adjacent lactonic protons, M,and by vinyl proton, Z. M is split into a triplet by B. As shown in Fig-ure 13, the chemical shifts for B and M of the cis isomer are a few cyclesupfield relative to TMS from those of the trans isomer.The aromatic protons X and Y show two doublets, which approximate anXZYZpattern with the characteristic mirror image. X has the same chemicalshift for both isomers. The chemical shift of Y is 34 cps fiwther downiieldfor the cis than for the trans compound, where the Y protons are cis to thecarboxylic oxygen and are therefore more deshielded than in the trans isomer,thus according for dowtileld shift.The Z proton in both isomers is split into a triplet by B. The chemicalshift for this proton is 36 cps further upfield for the cis isomer than for thetrans isomer, because within the cis isomer the vinyl proton is trans to thecarboxylic oxygen and is therefore less deshielded.



49/85

PPM, T2.0 3.0 4.0 5.0 6.0 7.0I I I I I

40 0 Cps 300CpsCs-2-(p-dimethylaminobenZyfidene}4-butyrolactone

in CDCIa, @s relativetoTMSstandard

I I 1 I 1 I8.0 7.0 6.0 5.0 4.0 3.0PPM,SPPM,%2.0 3.0 4.0 5.0 6.0 7.0

I I I I I I8.0 7.0 6.0 5.0 4.0 3.0PPM,6

Figure 13. NMR spectra of the cis-trans komers of 2-(p-dimethylamino-benzylidene)-4-buty rolactone



50/85

From these assignments, the structures of the cis and trans forms may bedetermined by the chemical shifts of the Yand Zprotons inthe two isomers.

Raman Spectra (Light Scattering)Rarnan spectroscopy is the study of a small portion of light scattered by amolecule, the light undergoing an exchange of energy with that molecule.Unlike absorption and emission spectroscopy where the energy of a photonmust correspond to the energy difference between two states of the moleculethephoton interacts with, Rarnanscattering (change ofdirection) of thecollid-ing photon can take place at any energy. The Raman effect emanates fromthe small fraction of scattered photons which undergo an increase or decreasein their energy due to their interaction with molecules. In contrast, the Ray-leigh scattering effect describes the majority of photons, which scatter due totheir collision with a molecule and do not undergo an exchange of energy withthat molecule.Rarnan shift is the difference of frequency in the photon before and afterthe scattering. The corresponding change of the photon energy is a measureof the energy spacing of the molecule. Since Raman scattered radiation isonly about 10-5(1 in 100,000) of the exciting radiation, a very intense mono-chromatic laser exciting beam is needed. As in emission spectroscopy, oneobserves Raman scattered radiation at a right angle to the incident beam.In certain cases, Raman scattering becomes the only available tool to giveinformation about a molecule. For example, unlike IR, Rarnan scatteringoffers information about nonspherical top, nonpolar molecules such as ben-zene, CCHG,and fluorine, Fz.

Raman vibrational spectroscopy can be more usefid for molecules in aque-ous systems since water shows only weak Raman scattering (between 300 and3,000 per cm-l) (Levine 1983). Rarnan scattering can be very informative asto hydrogen bonding in biological systems. Other examples of the usefulnessof Raman scattering are, for instance, that it shows that carbonic acid is car-bon dioxide dissolved in water rather than the associated acid, also that themercurous ion is two single, positively charged ions, Hg + - Hg+, rather thanone, Hg+.



51/85

5 Relationships BetweenStructural Changes in Mole-cules and Their UV and VIS(Wavelength and MolarAbsorptivity) Spectra

For various organic compounds, it is desirable to predict the wavelengthsof UV and VIS absorption maxima by correlating the spectra to changes in thestructural chromophores of these compounds. This section will show a fewrepresentative examples. Although wavelengths of absorption are onlyapproximate in some cases, they demonstrate definite trends.

Solvent EffectsStriking solvent effects were seen in our example of cis-trans 2-(p-dimethylaminobenzylidene)-4-butyrolactone in the polar and nonpolar solvents,alcohol and heptane, respectively. Generally, spectra in alcohol are used as areference point. Spectra in water occur at about 8 nm longer than those inalcohol. Spectra in dichloroethane, tetrahydrofuran, diethyl ether and in hep-tane occur at about 2, 4, 7, and 11 nm, respectivelyshorter than in alcohol.Therefore, it can be seen that spectra can be compared only within the samesolvent system.It can also be said that UV and VIS spectra show more hyperfine structuraldetails in nonpolar than in polar solvents.

Extension of Conjugation of Polyenes and SimilarCompoundsUV spectra show the presence of various chromophores such as conjugatedsystems and extent of conjugation. Generally the longer the conjugation, the

Chapter 5 Relationships Between Structural Changes in Molecules 41


52/85

longer thewavelength of maximum absorption andalso thehigher the intens-ity. Too, (as shown by the addition of the alkyl groups) the more substitutedthe compounds, the longer the wavelengths of maximum absorption.Alkenes and dienes without conjugated multiple bonds usually absorb lightat wavelengths in the vacuum W region (less than 200 nm). For example,ethene and 1,4-pentadiene absorb at 170 and 178 nm, respectively. Molecules

with conjugated multiple bonds absorb at higher wavelengths than 200 nm.For example, 1,3-butadiene absorbs at217nm(E = 21,000).When thenumber ofconjugated double bonds increases, both the wave-lengths of maximum absorbance andthe intensity increase. This increase inwavelength is due to the lowering of energy caused by resonance stabilization.Each double bond addition increases the wavelength of maximum absorptionbetween 30 and 35 nrn; compounds with more than eight conjugated doublebonds absorb in the VIS region. A comparison of the systems in Table 4shows that extending the conjugation of polyenes increases the wavelength ofmaximum absorption.

Table 4Conj ugat ed Po ly enes

Wavel eng t h o f Max imumAbsorpt ion. A Moles Abso r pt i o n, GJ

Compound (nm approx imat e) (approx imate)H2C = CH2 170 15,550ethyleneH2C = CH - CH = CH2 217 20,9001,3-butadieneH2C=CH- CH=CH-CH=CH2

I274

I50,000

t ram- 1 ,3 , 5-hexatrieneH2C = CH-CH=CH-CH=CH-CH = CH2 304 11,3,5,7-octatriene&carotene(precursor of vitamin A) 425 Cl =1.3 x 105eleven conjugated double bonds 477

Iycopene(red pigment of tomatoes) 470 .85 X 105eleven con jugat ed doub l e bonds 505

Units of molar adsorptively are M- .Cm-. However, these units generally are not given.

Polyenes Conjugated to Carbonyl GroupsSpectra of conjugated polyenes to carbonyl groups (in addition to the n~~*transition, which occurs at longer wavelengths, generally with lower molar

42 Chapter 5 Relationships Between Structural Changes in Molecules


53/85

absorptivities) have r + m* transitions similar to those of conjugated polyenesin which both wavelengths and molar absorptivities increase with increase ofthe extent of conjugation. Also, comparing the following polyene-aldehydestructures show that the extent of the increase of wavelength becomes slowerupon increasing the length of the conjugation (Table 5).

Table 5Compar is on o f Po ly en e-a ld eh yd e St r uc t ur esCompound I Wavelengt hsCH~-(CH=CH)l-CH=O J = 220 nmCH~-(CH=CH)z-CH=O A = 270 nmCH~ - (CH = CH)g-CH=O I J = 343 nmCH~-(CH=CH)5-CH=0 lJ=370nmCH. -( CH=CH), -CH=O ]J=415nm

Branching and Methyl Group Substitution ofConjugated Polyenes and Similar CompoundsA comparison of the conjugated polyene system in Table 6 shows that,because of hyperconjugation of the sigma orbitals with the pi orbitals, the

Table 6Conjugat ed Polyene Syst em s w it h Addi t ion of a Met hyl Group

ICompound I Wa ve le ng t hH2C = CH - CH = CHZ J=217nm1,3-butadiene c = 20,900H2C = C(CHJ - CH = CH2 A = 220 nm2-methyl-1 ,3-butadiene 1HZC = CH-CH=CH-CH~ I A=223 nm1,3-pentadiene c = 24,000CHZ = C(CHJ - C(CHJ = CHZ A = 226 nm2,3-dimethyl-l ,3-butadiene 1CH~-CH= CH-CH=CH-CH~ I A = 227 nm2,5-hexadiene 11,3-cyclopentadiene I A=239 nme = 34,000(CH3)2 C = CH - CH = C(CHJZ I A = 244 nm2,5-dimethyi-2,4-hexadiene c = 24,5001,3-cyclohexadiene I A = 256 nm1 Units of C, molar absorptivities, generally are not given.

43Chapter 5 Relationships Between Structural Changes in Molecules


54/85

44

addition of a methyl group increases the wavelength of absorption by approxi-mately 4 to 6 nm.Increase in the wavelengths of maximum absorption because of hyperconju-gation of the methyl groups sigma orbitals with pi orbitals (conjugatedcarbon-carbon to carbon-oxygen pi system) are illustrated by the structures inTable 7.Tab le 7Inc rease in Wavelengt hs of Max im um Absorpt ion Compound A(nm )HzC=CHCCH~

IIo I ~+nl(219) I (Note: the n+nl is a3-buten-2-one WJ7(305) fo rbidden t rans i t i on )

H~C\ C= CH CCH~/ n pffl(zzz) 1,300H o n+rf(31 1) 373-penten-2-one IC= CHCCH~I u ~+171(236) 1,200HaC o n+rr(314) 584-methyi-3-penten-2-one1 Due to hyperconjugation of methyl groups sigma orbitals with conjugated ~i systems.

This trend of increase in wavelength and in molar absorptivities (becauseof increases in conjugation of the pi system and the substitution of alkylgroups) can be clearly seen when the spectra of the above compounds arecompared to that of the unconjugated parent compound, acetone:o

acetone n-l, X = 273 n e = 15279 nm189 nrn

Polycyclic Aromatic CompoundsExtension of polynuclear rings increases the wavelength of maximumabsorption. For example, benzene, mphthalene, anthracene, tetracene, andpentacene have corresponding absorbencies at 200, 221, 252, 275, and310 nm, respectively. Other wavelengths of absorbencies are shown inTable 8.

Chapter 5 Relationships Between Structural Changes in Molecules


55/85

=

,

I

07m

(003N

FcoN=

=

,

U3G

bmF3

,

,=



56/85

46

Extending thenumber of fused benzene rings is, in effect, an extension ofthe conjugated system. This lowers thedifference between the~+zl energylevels, which shows as longer wavelengths of maximum absorption.

Interactions Between n Electrons of the Side Chainand D Electrons of Aromatic Compounds

Strong interactions between n electrons of the side chain and z electrons ofthe aromatic ring lower the differences in energy levels, thereby exhibitinglonger wavelengths of maximum absorption. This effect is similar to thelowering of energy due to extension of the conjugation of the m system.Figure 14 shows this effect: compounds in the left column have moreinteraction between the n electrons of the side chain and the r electrons of thearomatic ring, whereas those in the right column do not possess as much n~xinteraction.

Methyl Substitutions on the Benzene RingTable 9 shows two opposing trends. One effect is that the addition ofmore methyl groups on the benzene ring inductively increases the wavelengthsof maximum absorption; the other is that this same increase in the number ofmethyl groups on the aromatic ring causes steric instability (which raises theenergy/lowers the wavelengths of maximum absorption). Note that this tabledoes not include all methyl substituted benzenes or all the electronic tran-sitions of the same compound, but it does contain wavelengths for similartransitions. For example, benzene also has a m~r* transition at X = 180 nm

(E = 55,000) and at A = 200 nm (E = 8,000), while toluene has a similartransition at A = 189 nm (E = 55,000) and also at X = 208 nm (G= 79,000).These two effects caused by the addition of methyl groups can be explainedas follows: methyl and other allgd substitutions on the benzene ring increasethe wavelength of maximum absorption because of the interaction of the sigmaelectrons of the methyl groups with the m electrons of the aromatic ring, thuscausing an inductive activation of the aromatic T system and a lowering ofenergy (more stability). However, because of the steric effect caused by highmethyl substitutions, part of this gained stability is lost when the compoundsare highly substituted as in the case of penta- and hexamethyl benzenes. This

loss of stability, which manifests itself as an increase in energy (a lowering ofthe wavelength), is due to steric interference created by the methyl groupsaround the aromatic ring. It is worth noting from Table 9 that p-xylene ismore stable than O-xylene (longer wavelengths), because the methyl groups inp-xylene are far apart and less sterically hindered. This table also shows that

Chapter 5 Relationships Between Structural Changes in Molecules


57/85

maximum stability is achieved in1,2,4,5-tetramethylbenzene,where inductive stability is moreimportant than steric effect.

ChlorosubstitutedBenzenesTable 10 is a list of the mostsignificant wavelengths of maxi-mum absorption of chloro sub-stituted benzenes. Generally,the more chlorine substituted,the longer the wavelength. Thisis caused by a lowering ofenergy due to the stabilizationgained by extension of the con-

jugated system by the n elec-trons on the chlorine atoms.These wavelengths represent acompromise between this conju-gation extension effect and twoopposing effects, one caused bythe electron-withdrawing induc-tive effect due to electronegativ-ity of the chlorine atoms and theother due to the steric effectcaused by high substitution ofchlorine atoms on the plane ofthe benzene ring.

Di- and Trinitroben-q zenes and Toulenes

Figure 15 shows variousstructures of di- and trinitroben-zenes with their respectivewavelengths of maximumabsorption. These wavelengthsrepresent the weighted results ofthe resonance, inductive, andsteric effects. As expected,dinitrobenzenes (which are lesssterically hindered and whichhave one less

NH 26/\ IANILiNE280 nm1400b/\ ISODIUM PHENOUTE287 run235 nm

o-0/ ,0CH3\ I/c\H0VANILUN ION351 nm6/\ I

MM+OXY8ENZENE274 runE2500220 nmE81OO

CH2CH3l-ETHYL-+ Mf=t+OXYBENZENE276 nm&1930224 func 1010

kH3HS0-4 kH3Cl-

ANIUNIUM ANIUNIUMHYDROGEN cHfl:ESULFATE=nm OHb/\ IPHENOL270 nm211nm

VANILUN279 nmCH 3

t )

/

\ I

TOLUENE262 fun&260208 nmlz7goo

Q/\ ICH2CHsEIIWLBENZENE261 nmS200208 runE 7520

Figure 14. Effect of interaction between m electronsof the aromatic ring and n electrons ofthe side chain



58/85

Table 9Effec t of Addi t ion of Met hyl Groups on t he Benzene Ring

EFFECT OF ADD IT ION OF MEfHYL GROUPS ON THEBENZENE RING

CH sCH3 O/\ ICH so/ CH ~\ IO\ ICH3

a

c / CH s

\ IH3 C CH3

BENZENE TOLUENE O-XYLENE P-XYLENE

k= 254 nmt3=212 A= 262 nm&.280 i=263nm&.160~=273nm&=230

~ = 267 nm&.5cx)~ = 275 nm&=500

CH 3CH3

d \I

H3 C CH s

CH a oCH sHc \ I30/ CH3\ I CH s( 2Hs CH s CH3HEXAMETHYLBENZENE1,3,5-TRlMElHYL-

BENZENE1,2,3,4-TEIT3AMETHYL-

BENZENE1,2,4,5-THRAMETHYL-

BENZENE

~= 267 nm&=220~=273nm&=230

~=266nmE =316~=273nm&=250

k= 268 nm&=630~= 272 nm&=630 ~= 275 nm&=190~= 276 nm&==636

48 Chapter 5 RelationshipsBetween Structural Changes in Molecules


59/85

Table 10Wav el engt h a nd St ru ct ur al V ar iat io ns o f Ch lor os ub st it ut edBenzenesCompound I Wave le ng t h , nmChlorobenzene 245, 251, 258, 264, 2721,2-dichlorobenzene 250, 256, 263, 270, 2771,3-dichlorobenzene 250, 256, 263, 270, 2781,4-dichlorobenzenel 258, 266, 273, 2801,2,3-trichlorobenzene 265, 273, 280, (285)1,2,4-trichlorobenzene 270, 278, 2871,3,5-trichlorobenzene2 273, 285, 2941,2,3,4-tetrachlorobenzene 274, 280, 2911,2,3,5-tetrachlorobenzene 274, 281, (290)1,2,4,5 -tetrachlorobenzene3 I 276,285,294Pentachlorobenzene I 288,289Hexachlorobenzene I 291, 3011 Least steric hindrance of the three dichlorobenzenes, therefore the most stable (longestwavelength).2 Least steric hindrance of the three trichlorobenzenes, therefore the most stable (longestwavelength).3 Least steric hindrance of the three tetrachlorobenzenes, therefore the most stable(longest wavelength).

electron-withdrawing nitro group than trinitrobenzenes) are of lower energies;their absorption wavelengths are longer. Also, a comparison of the threedinitrobenzenes shows the 1,4- and 1,2-dinitrobenzenes to be the most and theleast stable, respectively (the compounds with the lowest energy/longest wave-lengths and the highest energy/shortest wavelength, respectively). This isbecause 1,4dinitrobenzene is the least sterically hindered. The most stericallyhindered is 1,2-dinitrobenzene, because the two nitro groups interfere witheach other in the plane of the aromatic ring.

A comparison of wavelengths of maximum absorption of dinitrotoluene tothose of corresponding structures of trinitrotoluenes shows that dinitrotoluenesabsorb at longer wavelengths and also, for both di- and trinitrotoluenes, thecompounds which are less sterically hindered absorb at longer wavelengths(less energy). These wavelengths, shown in Figure 16 are the results of theresonance, inductive, and steric effects.

Most deactivation of the aromatic ring

1997- Simplified concepts in spectroscopy and photochemistry

Documents

Transcript of 1997- Simplified concepts in spectroscopy and photochemistry