Infra red Absorption Spectroscopy - Theoretical Principles

Theoretical Principles

Introduction

The term "infra red" covers the range of the electromagnetic spectrum between 0.78 and 1000 mm. In the context of infra red spectroscopy, wavelength is measured in "wavenumbers", which have the units cm^-1.

wavenumber = 1 / wavelength in centimeters

It is useful to divide the infra red region into three sections; near, mid and far infra red;

Region	Wavelength range (mm)	Wavenumber range (cm^-1)
Near	0.78 - 2.5	12800 - 4000
Middle	2.5 - 50	4000 - 200
Far	50 -1000	200 - 10

The most useful I.R. region lies between 4000 - 670cm^-1.

Theory of infra red absorption

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.

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

Molecular rotations

Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions.

Molecular vibrations

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

Stretching: Change in inter-atomic distance along bond axis

Bending: Change in angle between two bonds. There are four types of bend:

Rocking
Scissoring
Wagging
Twisting

Vibrational coupling

In addition to the vibrations mentioned above, interaction between vibrations can occur (coupling) if the vibrating bonds are joined to a single, central atom. Vibrational coupling is influenced by a number of factors;

Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds

Coupling of bending vibrations occurs when there is a common bond between vibrating groups

Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration

Coupling is greatest when the coupled groups have approximately equal energies

No coupling is seen between groups separated by two or more bonds

Review your learning

You should now understand how certain molecules absorb infra red radiation, and the effects that this absorption has. You should be familiar with the ways in which molecules can vibrate, and factors which influence how these vibrations interact with each other.

UV-Vis. Luminescence Spectroscopy - Instrumentation Quiz

I.R. Absorption Spectroscopy - Theoretical Principles

I.R. Absorption Spectroscopy - Theoretical Principles Quiz

Biosciences Homepage