IR SPECTROSCOPY

Under the guidance of- Presented by-DR.P.NAGARAJU, M.pharm, Ph.D. Bhavana.G.(Y15MPh223) Professor, I/II M.pharm Dept. Pharm. Analysis

INTRODUCTION IR Spectroscopy is one of the most powerful analytical technique which is used for chemical identification of substances.One of the most important advantage of IR spectroscopy over the other methods are it provides useful information about the structure of the molecule without evaluation. IR is useful for determination of organic and inorganic structure. IR extends between 0.8-2.5 microns/ 12800-4000 cm-¹.

IR REGIONS1. Near IR (Over tone Region) 2. Mid IR (Vibrational Region)3. Far IR (Rotational Region)

PRINCIPLE In this technique chemical substances show absorption in the IR region.

After absorption vibration takes place and forms closed packed absorption bands, called IR absorption spectrum .

Various bands are present in this spectrum which correspond to various functional groups. Thus IR spectrum of a chemical substance is a finger print for its identification. Band absorption is identified by –

V=C/λ ῡ=1/λ

Band intensities in IR spectrum is expressed by transmittance(T).

The spectrum is measured based on wave number and % intensity of transmittance.

THEORY For a molecule to absorb IR radiation, it has to

full fill Certain requirements :1. Correct wave length: A molecule absorb

radiation only when natural frequency is same as the frequency of incident radiation

2. Electric dipole: A molecule absorb radiation only when its absorption causes a change in its electric dipole.

The closer the atoms in a molecule , the greater will be the strength of the dipole, faster will be the rate of change of dipole, the higher will be the frequency of the vibration, and the more intense will be the absorption of radiation.

MOLECULAR VIBRATIONS

Mol. vibration divided into 2 main types:

FUNDAMENTAL VIBRATIONS

• Vibrations which appear as band in the spectra.

NON-FUNDAMENTAL

VIBRATIONS

• Vibrations which appears as a result of fundamental vibration.

FUNDAMENTAL VIBRATIONS Fundamental vibration is also divided into

types:

These are vibrations in which the bond length is altered i.e. increased/ decreased.

STRETCHING VIB.

1.Bending vibrations are characterized by a change in the bond angle b/w two bonds.2.It requires less energy so appear at longer wavelength.

BENDING VIB.

Now, stretching vibration is further divided into :

Symmetric• In this the 2 bonds

increase/ decrease in length, symmetrically.

Asymmetric• In this one bond

length is increased and the other one decreases.

Bending vibration is divided into:

IN-PLANE BENDING

• In this change in bond angle is observed. Bending of bonds takes place within same plane

SCISSORING

• Here bond angle decreases/2 atoms move towards centre of atom

ROCKING• Here bond angle is maintained,

but bonds move within plane in 1 direction.

OUT OF PLANE

BENDING• Out the plane of molecule.

WAGGING• In this both atoms

move to one side of plane.

TWISTING• In this one atom is above

the plane and the other is below the plane.

NON-FUNDAMENTAL VIBRATIONSNON-

FUNDAMENTAL

OVER TONES:These are observed

at twice the frequency of strong

band.Ex: carbonyl group.

COMBINATION TONES:

Weak bands that appear occasionally at frequencies that are sum/difference

of 2 or more fundamental bands.

FERMI RESONANCE:Interaction b/w

fundamental vibration & overtones or

combination tones.Ex:CO2

INSTRUMENTATION

INSTRUMENTATION

The usual optical materials, glass or quartz absorb strongly in the IR region, the apparatus for measuring of IR spectra is different from that of UV-visible regions. The main parts of IR spectrometer are as follows: 1. IR radiation sources 2. Monochromators 3. Sample cells and sampling of substances. 4. Detectors 5. Recorders

Schematic diagram of IR spectrophotometer is:

SOURCEThe instrument requires a source of radiant energy which provides isolating narrow frequency bands. The radiation source must emit IR radiation which must be 1. Steady 2. Intense enough for detection and extend over desired wavelength.

Source Sample Monochromator

Recorder Detector

The various popular sources of IR radiation are: 1. Incandescent lamp: This is used for near IR instruments, this fails in far IR because it is glass enclosed and has low spectral emissivity.

2.Nernest Glower: It consists of hollow rod which is about 2mm

in diameter and 30 mm in length. The glower is made of rare earth oxides like

Zirconia, Yttria and thoria. It is heated to a temperature between 1000-1800c.

Dis-advantage:1. Energy is concentrated in near IR and Visible

regions of spectrum.

3.Globar Source: It is a rod of sintered silicon carbide which is

about 50mm in length and 4mm in diameter. When heated at a temperature between 1300

and 1700c, it emits radiation in IR region. Disadvantage: Less intense than Nernst glower.

4. Mercury Arc: It is effective in far IR region(wave number

<200cm-1) At shorter wavelengths, heated glass

envelope emits radiation whereas at longer wavelengths the mercury plasma provides radiation through quartz.

MONOCHROMATOR The radiation source emits radiations of

various frequencies. Desired frequencies absorb radiations at

desired wavelengths. TYPES OF MONOCHROMATORS (a) Prism Monochromator-

Single pass Monochromator: PROCEDURE: * Sample is kept near the focus of the beam

before the entrance slit. * The radiation from the source after passing

through sample, strikes the entrance slit which renders the radiation parallel and sends it to the prism.

* The dispersed radiation after reflecting from plane enters the 2nd slit and focuses on the sample cell, through which it finally passes into the detector section.

Double pass Monochromator: The double pass produces more resolution

than monochromator in the radiation, before it passes on to the detector.

Grating Monochromator: The grating monochromator has a series of

parallel straight lines cut into plane surface. Dispersion by grating follows law of diffraction.

n=d(sin I sin )

SAMPLE CELLS IR spectroscopy is used for characterization

of solid liquid or gas. Sampling of substances1)Sampling of solids: SOLIDS RUN IN SOLUTION.Solids may also be dissolved in non-aqueous solvents

provided there is no chemical interaction with the solvent.

It involves: (A) Solid films: If solid is amorphous then sample is deposited on surface of KBr/ NaCl by evaporation of solid.This tech. is used in qualitative analysis. (B) Mull technique:

Grinding sample Mix with nujol

• (C) Pressed pellet technique: Here finely ground solid sample is mixed

with KBr.

The mixture is then passed under high pressure in a press to form a small pellet. IR radiation is passed through it.

2)Sampling of liquids: Samples that are liquids at room temperature are directly put into the cells which are made up of NaCl, KBr and their IR spectra are obtained directly.

3)Sampling of gases: The gas sample is introduced in such a way that it should to react with reflecting surfaces.

DETECTORS These are used to detect signals and responses. The various types of detectors used in IR spectroscopy are: BOLOMETERS: In this electrical resistance of metal

increases with increase in temperature. When IR radiation falls on conductor change in resistance occurs.

THERMOCOUPLE: Here electric current flows between 2 metal wires which are connected at both ends.

The end which is exposed to IR radiation and is called as “Hot junction”.The other end is called as “Cold junction” which is away from light. A thermocouple is made by welding the 2 wires at ends. In IR 1 end is kept at constant temperature and is not exposed to radiation. The temperature difference generates P.D .

THERMISTORS: It is made of mixture of metal oxides. As temp. increases electrical resistance decreases.

GOLAY CELL:

APPLICATIONS OF IR 1) Determination of purity of compounds. 2) Shape and symmetry of the compound. 3) In industry for detection of impurities, and to produce satisfactory products. REFERENCES Instrumental methods of chemical analysis by Gurdeep R. Chatwal Sham K. Anand Instrumental analysis by Skoog https://en.wikipedia.org/wiki/Infrared_spectroscopy