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wavelength (cm) wavelength (nm)

The two main types of molecular vibrations associated with the absorption of infrared radiation involve the stretching and bending of bonds. Stretching is a vibrational movement in which the bond length alters and the two nuclei move harmonically relative to each other. For a compound involving two atoms there is only one bond that is capable of being stretched and in general terms it can be said that for a compound containing n atoms there are (n - 1) possible stretching effects. Bending vibrations present many more possible variations, the total number being defined as (2n - 4) for linear molecules and (2n - 5) for non-linear molecules.

Although a large number of vibrational changes are possible for any given molecule, not all of them will result in the absorption of radiation. An absorption maximum in the infrared spectrum of a compound can be demonstrated only when a vibration results in a change in the dipole of the molecule, a process for which energy is required. Carbon dioxide, for example, has a linear structure and can undergo two stretching vibrations (n -1) and two bending vibrations (2n - 4) (Table 2.2). An examination of the four vibrational changes possible will show that one stretching effect does not result in a change in the relative distribution of the charge associated with the molecule (dipole) and as a result will not show an absorption maximum. The other three vibrational changes do result in a change in the overall distribution of the charge and give absorption maxima in the infrared. However, the two bending vibrations are identical in energy level, although in a different plane, and therefore show the same absorption maximum.

Table 2.2 Fundamental vibrations of carbon dioxide

Structure

Vibrations

Absorption maximum

(cm-1)

O—C—O

None

Normal structure

o^c^-o

Stretching

None

O+C—O

Stretching

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