Infrared spectroscopy is a type of absorption spectroscopy uses infrared electromagnetic radiations to   determine functional groups of the molecule. It is also known as vibrational –rotational spectroscopy.

IR spectroscopy is one of the most valuable methods for characterizing both qualitatively and quantitatively the multitude of inorganic compounds.

REGIONS OF INFRARED RADIATIONS

The infrared radiations refers to the region of electromagnetic spectrum which lies between the visible and microwave regions. However, thus region is divided into 4 section:

  1. The photographic region. This ranges from visible to 1.2µm.
  2. The very near infrared region. This ranges from 1.2µmto 2.5µm.
  3.  The near infrared region. This is also known as infrared region and ranges from 2.5µm to 25µm
  4. The far infrared region. This is known as rotation region and ranges from 25 to 300-400µm.
infrared spectroscopy

Only in vibration region, the infrared spectroscopy works which gives the important information about the vibrations of molecules so about the structure of the molecules.

REQUIREMENTS FOR INFRARED SPECTROCOPY

  • The compound or sample should have dipole moments.
  • If the compounds are enatiomers, they will show similar IR spectra.

PRINCIPLE OF INFRARED SPECTROSCOPY

IR active compounds will get energy in the form of infrared radiations and cause vibrational rotation between the atoms of the molecule. This atom absorbs radiations and gives the characteristics peak value.  IR spectrum of a chemical substance may be a fingerprint of a molecule for its identification.

Infrared spectroscopy involves examination of the twisting, bending, rotating and vibrational motions of atoms in a molecule.

Two types of vibrations in Infrared spectroscopy

  1. Stretching: in this type of vibrations, the atoms move essentially along the bond axis, so that the bond length increases or decreases periodically. Stretching vibrations are of two types:

a. Symmetric: bonds increases or decreases in length simultaneously

b. Asymmetric: in this one  bond length increased and other is decreased

2. Bending: there is a change in bond angles between the bonds with a common atom. They are also known as deformations. There is a change is bond angle. It is of 4 types:

a. Scissoring: there is a plane bending in which two atoms approach each other. Bond angle decreases.

b. Rocking:  in this movement of atoms take place in the same direction

c. Wagging: 2 atoms move to one side of the plane. They move in up and down the plane.

d. Twisting: one atom moves above the plane and another atom moves below the plane.modernforensic.in

infrared spectroscopy

INSTRUMENTATION OF INFRARED SPECTROSCOPY

The main parts of IR spectrometers are as follows:

  1. IR radiation source
  2. Monochromators
  3. Sampling if the substances
  4. Detectors
infrared spectroscopy

We will discuss the components one by one

  1. IR radiation source: As in other types of absorption spectrometer, infrared instruments require a source of radiant energy which provides a way for isolating narrow frequency narrow bands. In case of IR spectrometers the source of radiation should be such that it is intense enough for detections, extend over the desired wavelength. Although these radiations are continuous, only selected frequencies will be absorbed by the samples.

The various popular sources for IR radiations are:

  • Incandescent lamp
  • Nernst glower
  • Globar source
  • Mercury arc

2. Monochromators: IR radiation source emits radiations or energy of various frequencies. The Monochromators select the desired frequencies and eliminates radiations from other frequencies as the samples in IR spectroscopy absorbs at certain frequencies. This selection of frequency is achieved by means of Monochromators which are mainly of two types: 

  • Prism Monochromators:  any prism used as a dispersive element must be constructed of materials such as various metal halide salts which transmit in the infrared.  Sodium chloride is probably the most common prism salt because of its high dispersion in the range of 4-15um, which is of special importance in functional group studies.
  • Grating Monochromators: reflection gratings are more common than prisms and are preferred over transmittance grating. The gratings offer linear dispersion and may be constructed from a variety of materials.

3. Sample cells and sampling of substances:  as infrared spectroscopy has been used for the characterization of solid, liquid or gas samples, it is obvious that samples of different phase has to be handled. But these samples have to be treated differently.

  • Solid sampling: samples that are solids can be sampled as- solid run in solution, solid films, mull technique and pressed pellet technique.
  • Liquid sampling: samples that are liquids at room temperature are usually put frequently with no preparation, into rectangular cells made of NaCl, KBr or and their spectra are obtained directly.
  •  Gas sampling: the gas sample cell is similar to the cell for liquid sample because the surfaces within the light path are made from KBr, NaCl and so on.

 4. Detectors: detectors detect the intensity of unabsorbed infrared radiation.  

The various types of detectors utilized in Infrared spectroscopy are:

  • Bolometer
  • Thermocouple
  • Semiconductor detectors
  • Photoconductivity cell
  • Fourier transform system
  • Golay cell

APPLICATION OF INFRARED SPECTROSCOPY

  1. Determination of purity: IR spectroscopy has been used for establishing the purity of compound. Whenever a compound has an impurity, it reduces the sharpness of individual bands, causes the looks of additional bands.
  2. Shape or symmetry of molecule
  3. Isomerism in organic chemistry: IR spectra are useful for identifying isomers.
  4. Determination of molecular structure: it is extremely helpful in determining the molecular structure of unknown substances.
  5. Identification of organic compounds
  6. Identification of substances such as hydrocarbons.
  7. Identification of substances such as aldehydes, ketones, carboxylic acids etc.
  8. Examinations of old paining and artifacts
  9. Measurements of paints and vanishes

LIMITATION OF INFRARED SPECTROSCOPY

  1. By IR spectroscopy, it is not possible to know the molecular weight of substances.
  2. Generally, this spectroscopy does not provide information of the relative positions of different functional groups on a molecule.
  3. From the single spectrum, it is not possible to know whether it is a pure or a mixture of compounds.

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