Instant Notes: Analytical Chemistry

However, when bonds are stretched they weaken, so a better model takes this

into account, and the molecules are treated as anharmonic oscillators. Thus,

where high energies are involved, larger energy transitions may occur, where

Dv=+2, +3, etc., giving the first overtone at a wavenumber approximately

double that of the fundamental, and so on.

The electrical field associated with the electromagnetic radiation will interact

with the molecule to change its electrical properties. Some molecules (for

example, HCl) have a dipole momentdue to charge separation and will interact

with the field. Others may acquire a dipole when they vibrate. For example,

methane, CH 4 , has no dipole, but when one of the CH bonds stretches, the

molecule will develop a temporary dipole.

Even if the molecule does not have a dipole, the electric field, E, may distort

the electron distribution and polarizethe molecule:

mind=aE

where mindis the dipole induced by the field, E, and ais the polarizabilityof the

molecule.

The rules governing transitions in the infrared region of the spectrum require

that, in order to absorb, the dipole moment of the molecule must change during

the vibration. Such vibrations are said to be IR active. For transitions to be

active in the Raman region, it is required that the polarizability must change

during the vibration. The transitions are then Raman active, orR active (Fig. 1).

Consider two simple diatomic molecules, nitrogen and carbon monoxide.

These molecules have only one fundamental vibration frequency, no. For

nitrogen it is 2360 cm-^1 , and for carbon monoxide 2168 cm-^1.

Since carbon monoxide has a permanent dipole, which will increase and

decrease as the molecule stretches and compresses, the vibration will interact

with IR radiation, and an absorption peak will be observed close to 2168 cm-^1.

Nitrogen has no dipole, and vibration does not produce one. Therefore, it will

not absorb IR radiation. This is of great importance, since IR spectra may be

recorded in air without interference.

However, when the nitrogen molecule vibrates, the bonding electrons are

distorted and the polarizability is changed. Therefore, it will give a spectrum

using the Raman technique.

In order to excite Raman transitions, energy comparable to the difference

between electronic energy levels must be supplied. This may be visible laser light

234 Section E – Spectrometric techniques

IR

E

'Hot IR' Rayleigh Raman Resonance

Raman

Upper excited

electronic state

Vibrational levels in

the lower

electronic state

Fig. 1. Energy levels for IR and R transitions.