INFRARED SPECTROSCOPY

INFRARED SPECTROPHOTOMETER

Classification of analytical techniques

1.Separation techniques →Chromatography

2.Spectrophotometric → SPECTROSCOPY

3. Electro analytical → Potentiometry,conductometry

4. Titrimetric analysis→ Titrations

SPECTROSCOPYSpectroscopy is the branch of science deals

with the study of interaction of electromagnetic radiation with matter.

Electromagnetic radiation is a type of energy that is transmitted through space at enormous velocities.

EMR→ANALYTE→SPECTROPHOTOGRAPH ↓ concentration should be lower

IR SPECTROSCOPY INTRODUCTIONInfrared spectroscopy (IR) measures the bond

vibration frequencies in a molecule and is used to determine the functional groups.

The infrared region of the spectrum encompasses radiation with wave numbers ranging from about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200µ.

Infrared region lies between visible and microwave region.

The infrared region constitutes 3 parts

a) The near IR (0.8 -2.5µm) (12,500-4000cm-1)b) The middle IR (2.5 -15µm) (4000-667cm-1)c) The far IR (15-200µm) (667-50cm-1)

most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region.

Wave number is mostly used measure in IR absorption because wave numbers are larger values & easy to handle than wave length which are measured in µm.

E = hν = hc/λ = hcν¯ It gives sufficient information about the structure of a

compound.

PRINCIPLE OF IR SPECTROSCOPYThe absorption of infra red radiation » excitation

of molecule.Vibrational transitions which are accompanied by

a change in dipole moment of the molecule are called infrared active transitions. Because they absorb the ir region.

Atoms or groups of atoms are connected by bonds. These are non rigid in nature.

Due to continuous motion of the molecule they maintain vibration with frequency.

Applied frequency = natural frequency of vibration

MOLECULAR VIBRATIONSThere are 2 types of vibrations.1) Stretching vibrations2) Bending vibrations• 1)Stretching vibrations: in this bond length

is altered.• They are of 2 types• a) symmetrical stretching: 2 bonds increase

or decrease in length.

b) Asymmetrical stretching: in this one bond length is increased and other is decreased.

2)Bending vibrations: • These are also called as deformations.• In this bond angle is altered.• These are of 2 types• a) in plane bending→ scissoring, rocking• b) out plane bending→ wagging, twisting

Scissoring:This is an in plane bending.In this bond angles are decreased.2 atoms

approach each other.

Rocking:• In this movement of atoms takes place in

same direction.

Wagging:It is an out of plane bending.In this 2 atoms move to one side of the plane.

They move up and down the plane.

Twisting:• In this one atom moves above the plane and

the other atom moves below the plane.

NUMBER OF VIBRATIONAL MODES

A molecule can vibrate in many ways, and each way is called a vibrational mode.

If a molecule contains ‘N’ atoms, total number of vibrational modes

For linear molecule it is (3N-5)For non linear molecule it is (3N-6)

Eg: H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

VIBRATIONAL FREQUENCYoccurs when atoms in a molecule are in

periodic motion while the molecule as a whole has constant translational and rotational motion.

The frequency of the periodic motion is known as a vibration frequency.

The value of stretching vibrational frequency of a bond can be calculated by the application of hooke’s law.

ν/c = ν¯ = 1/2пc[k/m1m2/m1+m2]1/2 = 1/2пc√k/µWhere, µ→reduced mass

m1&m2 →masses of the atoms

k →force constant

c →velocity of radiation

Factors influencing vibrational frequencies

Calculated value of frequency of absorption for a particular bond is never exactly equal to its experimental value.

There are many factors which are responsible for vibrational shifts

1) Vibrational coupling:• it is observed in compounds containing –CH2

& -CH3. EG. Carboxylic acid anhydrides amides aldehydes

2)Hydrogen bonding:Hydrogen bonding brings about remarkable

downward frequency shifts.Stronger the hydrogen bonding, greater is

the absorption shift towards lower wave length than the normal value.

There is 2 types of hydrogen bonding a) inter molecular→broad bands

b) intra molecular → sharp bands• hydrogen bonding in O-H and N-H compounds

deserve special attention.• Eg: alcohols&phenols

enols & chelates

3)Electronic effects:

In this the frequency shifts are due to electronic effects which include conjugation, mesomeric effect, inductive effect.

a) conjugation: conjugation lowers the absorption frequency of C=O stretching whether the conjugation is due to α,β- unsaturation or due to an aromatic ring.

b) mesomeric effect: a molecule can be represented by 2or more structures that differ only in the arrangement of electrons.

c) inductive effect: depends upon the intrinsic tendency of a substituent to either release or withdraw electrons.

INSTRUMENTATIONThere are 2 types of infrared

spectrophotometer, characterized by the manner in which the ir frequencies are handled.

1) dispersive type2) interferometric type In dispersive type the infrared light is

separated into individual frequencies by dispersion, using a grating monochromator.

In interferometric type the ir frequencies are allowed to interact to produce an interference pattern and this pattern is then analyzed, to determine individual frequencies and their intensities.

The main parts of an IR spectrometer are as follows

1) IR radiation source2) monochromators3) sample cells4) detectorsInfrared sources: require a source of IR radiant energy.The rod is electrically heated to 1500K to

produce IR radiations. a) Nernst glower or Nernst filament: they are made up of

sintered mixtures of oxides of Zr, Th, Ce, Y, Er, etc.Used in near IR region.Intensity of radiation produced is more intense.Used for detection of carbohydrates & proteins.

b) globar source: made up of silicon carbide. They are self starters. used in middle IR region. used to detect simple functional groups. C) incandescent lamp: it is made up of

nichrome wire. used in near IR

instruments. Has a low spectral

emissivity. It is heated up to 1100K. Used to detect complex organic molecules d) mercury arc: high pressures are used. used in far IR region. intensity of radiation is

greater. used to detect inorganic

complexes.

e) tungsten lamp: they heated up to 3500K used in mid IR region. intensity of radiation is mild. used to detection of organic functionalgrps

MONOCHROMATORS• They convert polychromatic light into mono

chromatic light.• They must be constructed of materials which

transmit the IR.• They are of 3 types.• a) metal halide prisms• b) NaCl prisms• c) gratings

a) metal halide prisms: prisms which are made up of KBr are used in

the wavelength region from 12-25µm.LiF prisms are used in the wavelength region

from 0.2-6µm. CeBr prisms used in wavelength region from

15-38µm. b) NaCl prisms:Used in the whole wave length region from

4000-650cm-1. they have to be protected above 20•c because of

hygroscopic nature. c) gratings:• They offer better resolution at low frequency than prisms.

sample cells

Sample cells made up of alkali halides like NaCl or KBr are used.

Aqueous solvents cannot be used as they dissolve alkali halides.

Only organic solvents like chloroform is used.IR spectroscopy has been used for the

characterization of solid, liquid, gas samples.

detectors• They convert the radiation into electrical signal.• There are mainly 5 types of detectors used in IR.

Thermo couple detectors:• made up of bismuth& antimony coated

by metal oxides. If wires of 2 dissimilar metals joined head to tail, then a difference in temperature between head & tail causes a current to flow in the wire. This current proportional to the intensity of radiation.• also called as thermopile detectors.• material should be thermally active. used in dispersive instruments.• Response time is 30 seconds.• They give responses for all frequencies.

Thermister detectorsThese are made up of sintered oxides of Mn, Co, Ni. • The material sh0uld be thermally sensitive.• The response time is 4 seconds.

pyro electric detectors

• These are made up of TGS, LiTaO3.• They are used in FTIR instruments. • It involves multiple scanning.• Principle involved is electric polarization.

Golay detectorsThe material used is CO2.The material should be inert in nature.• used in non dispersive IR instruments.• Response time is 0.01 sec.• Principle involved is expansion of gases.

Photo conductive detectorsPhotoelectric detectors such as the mercury

cadmium telluride detector comprise a film of semiconducting material deposited on a glass surface, sealed in an evacuated envelope.

Absorption of IR promotes nonconducting valence electrons to a higher, conducting state.

These detectors have better response characteristics than pyroelectric detectors and are used in FT-IR instruments - particularly in GC - FT-IR.

DISPERSIVE INSTRUMENTSThese are often double-beam recording

instruments, employing diffraction gratings for dispersion of radiation.

These 2 beams are reflected to a chopper which consists of rotating mirror.

It sends individual frequencies to the detector thermopile.

Detector will receive alternately an intense beam & a weak beam.

This alternate current flows from detector to amplifier.

INTERFEROMETRIC INSTRUMENTS THE MICHELSON INTERFEROMETER:

It is used to produce a new signal of a much lower frequency which contains the same information as the original IR signal.

The output from the interferometer is an interferogram.

Radiation leaves the source and is split. Half is reflected to a stationary mirror and then

back to the splitter. The other half of the radiation from the source

passes through the splitter and is reflected back by a movable mirror. Therefore, the path length of this beam is variable. The two reflected beams recombine at the splitter, and they interfere .

interference alternates between constructive and destructive phases.

The accuracy of this measurement system means that the IR frequency scale is accurate and precise.

FOURIER TRANSFORM IR SPECTROMETER

In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector. The sample absorbs radiation of particular wavelengths.

An interferogram of a reference is needed to obtain the spectrum of the sample.

After an interferogram has been collected, a computer performs a Fast Fourier Transform, which results in a frequency domain trace (i.e intensity vs wavenumber).

The detector used in an FT-IR instrument must respond quickly because intensity changes are rapid .

Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used. Thermal detectors are too slow.

To achieve a good signal to noise ratio, many interferograms are obtained and then averaged. This can be done in less time than it would take a dispersive instrument to record one scan.

Advantages of Fourier transform IR over dispersive IRImproved frequency resolution Improved frequency reproducibility (older

dispersive instruments must be recalibrated for each session of use)

Faster operation Computer based (allowing storage of spectra

and facilities for processing spectra) Easily adapted for remote use (such as

diverting the beam to pass through an external cell and detector, as in GC - FT-IR)