Principle of Infrared Spectroscopy

            IR spectroscopy is the electromagnetic spectrum that lies between Visible and Microwave regions. From the instrumentation and application point of view, the IR region can be divided further into as,

  1. Near IR region (14290-4000 cm-1)
  2. Mid IR region (4000 – 400cm-1)
  3. Far IR region ( 400-100 cm-1)

When IR light is passed through a sample of an organic compound, some of the frequencies are absorbed, while other remaining frequencies are transmitted through the sample. If we plot the absorbance or transmittance against the wavelength, then we get the infrared spectrum. IR region in the range of about 10000-100 cm-1 is absorbed and converted by organic molecules into vibrational energy. The groups like c-c, c=o, c=c, c-H, o-H, etc. have different vibrational frequencies and due to their characteristic vibrational frequencies as an absorption band, identification can be possible as functional groups.

In IR spectroscopy, vibrational spectra are absorbed as a band rather than lines because a single vibrational energy change is accompanied by a number of rotational energy changes. There are two types of molecular vibrations: stretching vibration and bending vibration.

Stretching vibration is the rhythmical movement along the bond axis in which the interatomic distance is increasing or decreasing. There are two types of modes of stretching vibration: symmetrical and asymmetrical stretchings.

Bending vibration is the change in bond angle between bonds with a common atom. There are 4 modes of bending vibrations; scissoring, rocking, twisting, and wagging. Only those vibrations that result in a rhythmical change in the dipole moment are observed in the IR. There are 3n-6 degrees of vibrational freedom for non-linear molecules while for linear molecules, there is 3n-5 degrees of freedom. Each vibrational degree of freedom corresponds to the fundamental mode of vibration and each fundamental mode corresponds to a band.

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