Instant Notes: Analytical Chemistry

or NIR radiation. If the exciting wavelength matches the energy difference

between the electronic levels of the sample, the Raman signal is greatly enhanced

by the resonance Raman effect. Rayleigh scattering re-emits the exciting line. The

more intense emission due to fluorescence effects may mask the weak Raman

signal, but with NIR radiation fluorescent interference is much less.

As molecules become more complex, the number of possible vibrations

increases. For example, carbon monoxide, CO2, has three atoms arranged in a

line: O=C=O. This molecule does not have a dipole and may vibrate in three

ways.

(i) The symmetric stretch, denoted by v 1 , is where both oxygens are equidis-

tant from the central carbon, but the C-O bonds lengthen and contract

together. The dipole does not change, but the polarizability does, so this

vibration is IR inactive, but R active.

(ii) The antisymmetric stretch, v 3 , has one C-O bond stretching, while the other

contracts. The carbon atom moves as well so that the center of mass of the

molecule remains stationary. The dipole changes, but the polarizability

does not, so this is IR active but R inactive.

(iii) The bending vibrations, v 2 , may be resolved into two identical and mutu-

ally perpendicular components corresponding to two transitions of the

same energy (degenerate). It is necessary to think in three dimensions:

considering the page as a plane, then if the two oxygens go equally ‘down

the page’, while the carbon goes ‘up the page’ to balance, this is in-plane

bending. If the oxygens go ‘into the page’ and the carbon ‘out of the page’,

this is out-of-plane bending. These changes will be reversed as the vibra-

tion progresses. This vibration is IR active and R inactive.

The triangular molecule of water, H 2 O, also has three different vibrations,

corresponding to the same vibrational types. However, each involves a change

in dipole so all three are IR active. The Raman spectrum shows only one line

due to the symmetric stretch. These vibrations are shown schematically in Figure

2.

It is possible to extend these arguments to more complex molecules, but this

is only of value for studies of structural parameters such as the length and

E10 – Infrared and Raman spectrometry: principles and instrumentation 235

Schematic Dipole Polarizability Activity Wavenumber

(cm–1)

No change Change R active 1320

Change No change IR active 2350

Change No change IR active 668

Change Change IR & R active 3650

Change No change IR active 3756

Change No change IR active 1600

O : C = O

O = C = O

O

C

O

H

O

H

H

O

H

H H

O

Fig. 2. Schematic of the vibrations of the CO 2 and H 2 O molecules.