Infrared Spectroscopy

Infrared spectroscopy is a type of absorption spectroscopy that deals with the infrared region of the electromagnetic spectrum. Infrared radiation which extends from 0.8 to 200μ affects only the vibrational and rotational energy levels.

The infrared region is subdivided into- 1.Near infrared region (NIR): extends from 0.8 to 2.5μ. It is concerned with low energy electronic transitions as well as vibrational and rotational changes. 2.Infrared region (IR): extends from 2.5 to 16μ (frequency 4000 to 625cm -1 ). It affects only the vibrational and rotational energy levels. 3.Far infrared region (FIR): extends from 16 to 200μ. It affects only the rotational energy level of the molecule.

Theory of infrared absorption The energy of a molecule consists of translational, rotational, vibrational and electronic energy. E = E electronic + E vibrational + E rotational + E translational Translation energy of a molecule is associated with the movement of the molecule as a whole, for example in a gas. Rotational energy is related to the rotation of the molecule, whereas vibrational energy is associated with the vibration of atoms within the molecule. Finally, electronic energy is related to the energy of the molecule's electrons.

IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. The possible vibrational modes in a polyatomic molecule can be visualized from a mechanical model shown schematically in the following figure:

Atoms are represented by balls and the bonds connecting them as springs. If the model is suspended in space and struck, the balls appear to undergo random chaotic motions. The springs will be found to stretch and contract or bend.

These motions are known as stretching and bending vibrations. The frequency with which these vibrations occur depends on: - the nature of the atoms joined by the bond - arrangement of atoms within the molecule (i.e. environment) - mass of the atom - strength of the bonds.

Again, if a spring connecting two balls is struck, it vibrates and this vibration, in turn, influences the rest of the system. In a similar way, if a chemical bond vibrating at a certain frequency is struck by Infrared radiaton of that same frequency, the vibration of that bond will increase by absorption of radiation, i.e. the gain in energy will create a greater amplitude of vibration.

Therefore, if a substance is irradiated by IR radiation, the wavelength of which is constantly changing, different bond will absorb energy at different frequencies. As a result, an Infrared absorption spectrum is obtained in which percentage of intensity of the transmitted IR light is plotted against the wavelength (or wave number) of that light.

As the frequency of vibration of a bond in a molecule depends on the nature of the atoms joined by the bond, arrangement of atoms within the molecule (i.e. environment), mass of the atom and strength of the bonds, no two molecule of different structure have exactly the same infrared spectrum. For this reason, the Infrared spectrum can be used for molecules much as a fingerprint can be used for humans. For example, -OH group of alcohols absorbs strongly at cm -1 -CO- group of ketone absorbs at 1710 cm -1 -CN group at 2250 cm -1 -CH3 group at cm -1, etc.

Molecular vibrations The positions of atoms in a molecule are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main categories of stretching and bending.

Stretching In this type of vibration, the distance between the two atoms increases or decreases but the atoms remain in the same bond axis.

Stretching vibrations are two types- Symmetric stretching: In this type, the movement of the atoms with respect a particular atom in a molecule is in the same plane. Asymmetric stretching: In this type, one of the atom moves toward the central atom while the other moves away from it.

Bending In this type of vibration, the position of the atoms changes with respect to the original bond axis. Bending vibrations are four types-

Rocking: Movement of the atoms takes place in the same direction. Scissoring: Two atoms move toward and away from each other. Wagging: two atoms move up and below the plane with respect to the central atom. Twisting: One of the atoms moves up the plane, while the other moves down the plane with respect to the central atom.

Instrumentation Two types of infrared spectrophotometer are in common use in the laboratory. 1.Dispersive infrared spectrophotometer 2.Fourier transform infrared spectrophotometer (FTIR)

Dispersive infrared spectrophotometer Dispersive IR spectrophotometer consists of the following parts. a)Radiation source b)Sample compartment c)Monochromator d)Detector e)Recorder

Radiation source An inert solid is electrically heated to a temperature in the range K. The heated material will then emit infrared radiation.

Sample compartment After preparation of the sample (solid, liquid or gas), it is inserted into a holder and then placed in the sample compartment in the path of the IR beam. For solutions, a reference cell is needed that contain solvent only. After passing through the sample and reference, the beams are chopped by a mirror which focuses each beam alternately into the entrance slit of the monochromator. Glass and plastics absorb strongly throughout the IR region of the spectrum. Thus cells are constructed by ionic substances like- ▫Sodium chloride ▫Potassium bromide

3. Monochromator: A monochromator selects specific frequency from other extraneous radiation using either a prism or a diffraction grating. Three types of substances are normally employed as monochromators: Metal Halide Prisms: Various metal halide prisms such as: KBr (12-25 micro meters), LiF (0.2-6 micro meters) and CeBr (15-38 micro meters) have been used earlier. But now a days they have become more or less obsolete.

NaCl Prisms (2-15 micro meters): Sodium Chloride prisms are of use for the whole of the region from cm -1. However, It gives low resolution at cm -1. Gratings: Gratings are commonly employed. It offers greater resolution at higher frequencies than the prisms.

Detector: The detector senses the difference and determines which frequencies have been absorb by the sample and which frequencies remain unaffected and send a signal which is amplified by an amplifier.

There are different types of detectors that are used in the infrared region: Thermocouples (or Thermopiles) : The underlying principle of a thermocouple is that if two dissimilar metal wires are joined head to tail, then a difference in temperature between head and tail causes a current to flow in the wires. In the infrared spectrophotometer this current shall be directly proportional to the intensity of radiation falling on the thermocouple. Hence, the thermocouples are invariably employed in the infrared region, and to help in the complete absorption of ‘available energy’ by the ‘hot’ junction or receiver is normally kept cold.

Bolometers : These are based on the principle that make use of the increase in resistance of a metal with increase in temperature. For instance, when the two platinum foils are appropriately incorporated and radiation is allowed to fall on the foil, a change in the resistance is observed ultimately. This causes an out of- balance current that is directly proportional to the incidental radiation. Just like the thermocouples, they are used in the infrared region.

Recorder The amplified detector signal is recorded as % of transmitted light vs. frequency on a chart.

Preparation of sample IR spectra may be obtained from all phases: solid, liquid and gaseous. Vapor or gaseous phase: The vapor or gas is introduced into a special cell usually about 10cm long which is then placed directly in the path of one of the infrared beams. The end walls of the cell are usually made of sodium chloride which is transparent to infrared from 4000 to 650 cm -1.

Liquids: The liquids are usually examined as a thin film sandwiched between two polished salt plates made of NaCl or KBr. The pair of plates is inserted in a holder which is then placed in the path of the IR beam. It is the simplest of all procedures. KBr is transparent throughout the 4000 to 400 cm -1.

Solids Three methods are commonly used for preparing a solid sample for measurement of IR absorption. a) KBr disc: About 1mg of the sample is finely ground with about 300mg of KBr in a mortar. The mixture is pressed into a disk using a special mould and hydraulic press. Under pressure KBr melts and seals the compound into a matrix and form a KBr disk. The KBr disk can be inserted into a holder which is then placed in the spectrophotometer.

b) As Nujol mull (in the form of suspension): About 1mg of the sample is finely ground in a mortar with a drop of nujol mull. The mull is then placed between flat plates of NaCl. The main disadvantage is that nujol absorbs IR radiation and band appears at 2924, 1462 and 1377 cm -1. Therefore, if C-H vibration are to be examined, the nujol may be replaced with hexachlorobutadiene. c) In solution: The compound is dissolved to give a 1-5% solution in CCl 4 or CHCl 3. The solution is introduced into a special NaCl cell. A second cell of equal thickness but containing pure solvent is placed in the path of the other beam of the spectrometer in order to cancel the solvent absorption. For aqueous solutions, special calcium fluoride cells are used

Fourier Transform Infrared Spectroscopy (FTIR) In FTIR, instead of recording the amount of energy absorbed when the frequency of the IR radiation is varied by a monochromator, the IR radiation is guided through an interferometer. A diagram of a Michelson interferometer is shown below:

The Michelson interferometer consists of four arms. The first arm contains a source of IR radiation, the second arm contains a stationary mirror, the third arm contains a moving mirror, and the fourth arm is open. At the intersection of the four arms a beamsplitter is placed, which is designed to transmit half the radiation that impinges upon it, and reflect the other half. As a result, the light transmitted by the beamsplitter strikes the moving mirror, and the light reflected by the beamsplitter strikes the fixed mirror. After reflecting off their respective mirrors, the two light beams recombine at the beamsplitter and then leave the interferometer to interact with the sample and strike a detector.

The motion of the moving mirror causes the path length of radiation to vary. When the two beams meet at the beam splitter, they recombine, but the path length differences (differing wavelength content) of the two beams cause both constructive and destructive interferences. The combined beam containing these interference patterns is called the interferogram. This interferogram contains all of the radiative energy coming from the source and has a wide range of wavelengths.

A plot of light intensity versus optical path difference is called an interferogram. With the use of Fourier transformations it is possible to convert a signal in the time domain to the frequency domain (i.e. the spectrum). The fundamental measurement obtained by an FTIR is made in the time domain, which is Fourier transformed to give a spectrum.

Application of IR spectroscopy 1. Identification of functional group and structure elucidation Entire IR region is divided into group frequency region and fingerprint region. Range of group frequency is cm -1 while that of finger print region is cm -1. In group frequency region, the peaks corresponding to different functional groups can be observed. According to corresponding peaks, functional group can be determined.

2. Identification of substances Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic peaks are observed. It contains a very complicated series of absorptions. These are mainly due to all manner of bending vibrations within the molecule. This region of IR spectrum is called as finger print region of the molecule. Identification of a compound can be revealed by comparing its finger print region with the spectra of authentic sample with identical conditions.

3. Detection of impurities IR spectrum of the test sample to be determined is compared with the standard compound. If any additional peaks are observed in the IR spectrum, then it is due to impurities present in the compound. 4. Quantitative analysis The quantity of the substance can be determined either in pure form or as a mixture of two or more compounds.

All quantitative analysis is governed by I = I 0 e -  cd Where, I = Intensity of the transmitted radiation I 0 = Intensity of the incident radiation  = Molar absorptivity C = Concentration of the Sample (mole/litre) d = Thickness of the cell containing the sample therefore, log I 0 /I =  Cd or, A=  Cd Absorbance can be determined by constructing a calibration/standard curve.

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