Spectrophotometry - IR Spectroscopy - Theory and Concepts

Welcome back.! :) My sincere apologies to all my readers for not having posted for a long time.

We were discussing the different types of spectrophotometers. We have already discussed about the instrumentation and applications of UV-visible spectrophotometry. In this post, we will have a look at (infra-red) IR spectrophotometry.

As we have seen earlier that visible light that we are able to see is just a small part of electromagnetic radiation spectrum. On the immediate lower side of this visible spectrum lies the infrared and on the other side lies the ultraviolet. So, in this post, we will be discussing about the absorption on the lower side of this spectrum. This infrared covers the range of 0.78nm and 1000nm of the electromagnetic spectrum.

We will try to understand the theory of infra-red absorption. So, to start with, we know that the infra-red radiation is of higher wavelength as compared to UV-visible region, so the electromagnetic radiation of this region constantly has low energy. Thus, infra-red radiation is associated with vibrational transitions of molecules. The atoms in molecules are in continuous vibration with respect to each other at temperatures above absolute zero.

Remember that the bond distance between the atoms in a molecule fluctuate to about ±0.5A˚.
Now, there are two kinds of vibrations as:

a. Stretching vibrations
b. Bending vibrations

The stretching vibrations are those where there is an increase or decrease in the bond length but the atoms remain in the same bond axis. The bending type of vibrations involves the changes in the positions of the atoms with respect to bond axis (here, such variations in bond angles may be about ±0.5˚). These vibrational transitions are low energy transitions and these energy levels correspond to the energies of the electromagnetic radiation in the infra-red region of the spectrum.

So, now we can ask a question as to ‘When does the molecule absorb radiation?' So, the answer is, when the frequency of a specific vibration equals the frequency of the IR radiation directed on the molecule, then the molecule absorbs radiation. 

Note: Difference in the presentation of IR spectra and UV-visible spectra:
Firstly, in the IR spectra, wave number is used rather than wavelength. Secondly, IR spectra are typically presented as percent transmission (transmittance x 100) versus wave number.

Modes of Vibration:

Each of the atoms has three degrees of freedom which corresponds to the motions along any of the three Cartesian coordinate axes (x, y, z). The theory of molecular vibrations predicts that an asymmetrical molecule will have 3n – 6 modes of fundamental vibrations where n is the number of atoms in that molecule. So, by this, the molecule methane (CH₄) will have 3 (5) - 6 i.e.; 9 fundamental modes of vibration.

The diagram shown above depicts the vibrational modes available for AX2 systems (were any atom is joined to two other atoms eg., NO₂, CH₂ etc.)

Normally each vibration mode absorbs at a different frequency. Thus, a CH₂ group will give rise to two C - H stretch bands which maybe symmetric or asymmetric. However, this is not always true. There will be some vibrations that may absorb at the same frequency and naturally, their absorption bands will overlap. Such vibrations are said to be degenerate. Also, there are vibrations whose absorption frequency may lie outside the normal infrared examined.

Till now we have seen about the vibrations which are fundamental. There are many other frequencies at which the bands can appear in an infra-red absorption spectrum.  Some of them are:
Overtone bands: These bands are generated by modulation of fundamental vibrations. Like, strong absorption at 800 cm^-1 may give rise to a weaker absorption at 1600 cm^-1.
Combinations or Beats: Another kind of modulation is when the two different frequencies x and y interact with each other (combinations). Such interactions may take place as x + y or as x - y. These resulting weaker absorptions are called beats.

You would agree that a particular kind of combination can occur in that particular compound and in none other. It is so because each compound has its own particular arrangement of atoms and so we can say that the combination bands are unique to a compound. Thus, the combination bands have extreme importance because they maybe signature or the fingerprint of a given compound. In other words, the IR spectra of no two compounds are alike or we can say conversely that substances giving the same IR spectra are identical. A large number of compounds fall in 900cm^-1 and 1400cm^-1. For this reason, this region is called the “fingerprint region”.

So, these were certain concepts and theory of IR spectrometry. In the next post, we will have a look at the instrumentation followed by sampling techniques for IR spectroscopy and lastly, applications of IR spectrophotometry.