Infrared spectroscopy 

The region between. 0.8μ to 200μ is called region infrared region of electromagnetic spectra. The spectra studied in this region is called infrared spectra. And this branch of chemistry is called Infrared spectroscopy. This region can be divided into three parts for spectroscopic study.

Vibrational transitions occur in a normal infrared region so these spectra are studied in the normal infrared region. Most of the organic compounds absorb in this region.

Experimental Arrangement


For near infrared and infrared region the source of radiation is Nernst Glower which is a 30 mm long and 2 mm diameter rod of oxides Zirconium, Cesium, and Thorium. This rod is heated up to 1500°C to 2000°C by electric current. For far infrared region the source id generally Welsback lamp or high-pressure mercury arc.


Prism or grating type monochromator are employed in infrared spectroscopy. It is not worthy that neither quartz nor glass is sufficiently transparent to infrared. Hence crystals of certain halogen salts which transmit infrared freely have been utilized for optical parts. Crystalline potassium bromide and cesium bromide are satisfactory for a far infrared region and lithium fluoride is suitable for near infrared region. the material of the sample cell is also the same as that of a monochromator.


Water cannot be used as a solvent since water itself absorbs energy in the infrared region (at 3710 and 1630 cm¹). Carbon disulfide, chloroform, carbon tetrachloride, benzene etc. can be used as a solvent. The sample may be in gaseous, liquid, solid state or solution but it should be free from water.


Thermopile or bolometer may be used as detectors.

1.Conditions for infrared spectra

For a molecule to show IR spectra, it has to fulfill certain conditions which are given below:

  1. A molecule can absorb radiation only when the frequency of vibration of the molecule (i.e. atoms or group of atoms) is the same as that of the incident radiation. It means Vradiaton = Vvibration.
  2. Electric Dipole: A molecule can only absorb IR radiation when there is a net change in its electric dipole (dipole moment) due to absorption of radiation. This change occurs only when the molecule has a certain permanent dipole moment. A molecule is said to have a dipole moment when there is a slight positive and slight negative electric charge on its component atoms. Molecules like H2, N2, O2 do not give IR spectra since they do not have the permanent dipole moment. Similarly, CO2 (C=O=O) does not show since its net dipole moment is zero, On the other hand, a molecule like HCl, H2O, NO2 etc which have some permanent dipole moment give IR spectra.

2. Uses of Infrared Spectroscopy

infrared spectroscopy has various applications in the chemical field. Some of the important applications are given below:

1.Determination of force constant of a bond:

I.R. spectroscopy can be used to calculate force constant of a diatomic molecule. Vibrations of a molecule can be treated as a simple harmonic oscillator for small amplitude frequency of such vibrations can be given

as ν = 1/2πc√k/π

or k = 4π²ν²C²μ

Where       C = Velocity of light (= 3×10¹ ms¹)

k = force constant

μ = m1m2/m1+m2 reduce the mass of molecule

ν can be measure from IR spectroscopy and thus force constant k can be calculated. Force constant and energies of some diatonic molecules are given in the table.

2. Identification of a compound:

Fingerprint technique can be used to determine the structure of a compound. In this technique, IR spectra of an unknown compound are compared whit that of a known compound.

3. Elucidations of a structure of a compound:

IR spectroscopy is used to determine the structure of a compound. Different functional groups and bonds absorb in definite regions i.e. each functional group or bond has its own characteristic absorption band. For example- C≡ N bond shows absorption at 2250 cm. It means if a molecule shows absorption at 2250 cm, its mean it has – C≡ N bond. Similarly, absorption at 1725 cm shows the pressure of>C = ) (carbonyl) functional group.

Due to different modes of vibrations, the same group shows absorption in different places in polyatomic molecules. Thus absorption spectrum becomes complex. As shown in table-

A general IR spectrum is shown in fig. Where all types of groups stretching, twisting, fingerprinting etc.

4. Determination of purity of a compound:

Every compound has its own characteristics IR spectrum. The pure compound shows clear and separate bands. If the impurity is there other bands will also appear. Thus absorption bands are observed regularity during the purifications of a compound until desired bands are observed.

5. Study of Reaction kinetics:

During a chemical reaction, some bands are formed and some are broken. Thus the observation of bands during a reaction gives the idea of the progress of a reaction.

6. Determination of shape of molecule:

IR spectroscopy is useful in determining the shape and symmetry of molecules. For example, the geometry of NO should be linear (like CO molecule) but its IR spectrum shows three peaks at 750, 1323 and 1616 cm which is like a nonlinear molecule (3N-6). Thus NO is nonlinear. Similarly, IR spectra show that XeF, XeF, and XeF molecules are linear, square planar and octahedral respectively.

7. Hydrogen bond:

IR spectroscopy is also useful in finding the presence of H-bond in a molecule. Generally in presence of H-bond in a molecule the absorption bond widens due to stretching vibration and wave number decreases. This technique can be used in differentiating intermolecular and intramolecular hydrogen bonding. Due to dilution absorption decreases due to intramolecular hydrogen bonding whereas absorption does not change due to intermolecular hydrogen bonding.

8. Dipole moment can be measured by IR spectra.
9. IR spectroscopy is also useful in the measurement of bond lengths and detection of free radicals.
10. IR spectroscopy can also be used in the determination of cis and transform.
11. Different tautomers have different characteristic groups so they can also be studied by IR spectroscopy.

In this particular article we have discussed Infrared spectroscopy, its uses and various conditions in detail.

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