IR spectroscopy


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INFRARED SPECTROSCOPY BY R RAMESH REDDY 1styear-pharmaceutical analysis




SPECTROSCOPY Spectroscopy 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 INTRODUCTION 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.


PRINCIPLE OF IR SPECTROSCOPY 1. Infrared or vibrational spectroscopy is concerned with study of absorption of infrared radiation, which result in vibrational transitions. Applied infrared frequency=natural frequency of vibration, absorption of IR radiation take place and peak is observed. 2. finger print of molecule. CRITERIA FOR A COMPOUND TO ABSORB IR RADIATION Change in dipole moment Correct wavelength of radiation.


MOLECULAR VIBRATIONS There are 2 types of vibrations. Stretching vibrations 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.

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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

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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.

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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.


INSTRUMENTATION There are 2 types of infrared spectrophotometer, characterized by the manner in which the ir frequencies are handled. 1) dispersive type 2) 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.

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The main parts of an IR spectrometer are as follows 1) IR radiation source 2) monochromators 3) sample cells 4) detectors Source Monochromator Reference Sample Detector Readout device

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Infrared sources: require a source of IR radiant energy. 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 .

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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. It is heated up to 1100 K. Used to detect complex organic molecules

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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 3500 K used in mid IR region. intensity of radiation is mild. used to detection of organic functionalgrps

1. IR source :

1. IR source

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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

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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 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.

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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 detectors:

Thermister detectors These 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 detectors:

Golay detectors The 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 detectors:

Photo conductive detectors Photoelectric 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 INSTRUMENTS These 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.



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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 IR:

Advantages of Fourier transform IR over dispersive IR Improved 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)

Sample Preparation:

Sample Preparation Pure Liquid Solution Nujol Mull Method KBr Pellet

Pure Liquid:

Pure Liquid IR spectra of liquid compounds may be obtained either from the neat liquid or from a solution of the liquid in an appropriate solvent. Interference by solvent absorption is thereby avoided.

Pure Liquid:

Pure Liquid To run a neat liquid, therefore, one normally places a drop of the liquid on the face of a highly polished salt plate (such as NaCl , KBr , or AgCl ), places a second plate on top of the first plate so as to spread the liquid in a thin layer between the plates, and clamps the plates together in some suitable fashion.


Solution The spectrum of this solution may then be obtained either from a thin film of the solution spread between salt plates (as above), or in a liquid IR cell. If an IR cell is used, a cell of identical path length containing pure solvent is generally placed in the reference beam of the spectrometer, so that solvent IR bands are not obtained in the desired spectrum. If a reference cell is not used, the solvent bands must be ignored in interpreting the resulting spectrum.

Nujol Mull Method:

Nujol Mull Method In this method, the solid sample is thoroughly ground up, using an agate mortar and pestle, with a weakly absorbing, non-volatile liquid to form a thick paste called a mull . The paste is spread on the surface of a sodium chloride salt plate and is covered with another similar plate.

Nujol Mull Method:

Nujol Mull Method The sample thickness is adjusted by rotating and pressing the plates together to squeeze out excess material. It is very important that the sample be ground to a very fine particle size to reduce light scattering and salt plate scratching. The most common mulling agent is mineral oil ( Nujol ), which is transparent in the infrared except for narrow bands at 2900, 1450, and 1375 cm-1. An alternative mulling liquid, which does not absorb in these regions, is a perfluorokerosene , such as Fluorolube S.

KBr Pellet:

KBr Pellet In this method, the solid sample is finely pulverized with pure, dry (expensive, IR grade) KBr , the mixture is pressed in a hydraulic press to form a transparent pellet, and the spectrum of the pellet is measured. It is important that the solids be extremely finely divided and well mixed . The pellet is usually pressed in a special die that can be evacuated in order to avoid entrapped air, which causes cloudiness in the pellet.

KBr Pellet:

KBr Pellet A major advantage of this method is that KBr has no absorptions in the IR above 250 cm-1 , so that an unimpeded spectrum of the compound is obtained. A disadvantage of the method for coordination compounds is that Br- from the KBr can often replace ligands in the compound whose spectrum is desired . If this is not realized by the experimenter, misinterpretation of the spectrum will result


Refernce Instrumental methods of Analysis by Skoog .Heller . Crouch Y.R Sharma, Elementary Organic Spectroscopy , Pg.No 69-77. Gurdeep R . Chatwal Instrumental Method Of Chemical Analysis , Pg.No.2.29-2.82

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