Analytical Chemistry

Table 9.6 Wavenumbers of characteristic vibrations

(a) X—H Vibrations

X—H Stretch/cm–^1 Bend/cm–^1

C—H 2700 – 3300 1200 – 1500

600 –^900

O—H 3000 – 3700 1200 – 1500

N—H 3000 – 3700 1500 – 1700

S—H 2550 – 2600 700 – 900

(b) Saturated and unsaturated groups

Group Stretch/cm–^1

C—C, C—N, C—O 400 – 1300

C C, C N, C=O^1550 –^1900

C C, C N

2100 – 2400

bonds are not sufficiently great to enable characteristic group frequencies to be identified. Prominent
bands can be observed, however, for groups such as C–O where the dipole change during the vibration
is large. For groups with multiple bonds the change in dipole and hence the intensity of the band is
determined by the position of the group in the molecule. Thus little or no absorption due to the multiple
bond is observed for symmetrically substituted alkenes or alkynes, whereas a terminal unsaturated
group gives rise to a strong absorption.

Factors Affecting Group Frequencies

Hydrogen Bonding

Compounds containing proton donor groups such as O—H and N—H can be involved in intra- or

intermolecular hydrogen bonding in the presence of proton acceptors, e.g. O,N, halogens, C C. The
stiffness of the X—H bond is thereby lessened, resulting in a lowering of the stretching frequency, and
the band broadens and often intensifies. Conversely, the frequency of the bending mode is raised but the
effect is much less pronounced. Intermolecular hydrogen bonding is suppressed at elevated
temperatures but it is favoured by a high solute concentration. Intramolecular hydrogen bonding is also
reduced at elevated temperatures but it is unaffected by changes in solute concentration. These effects
are particularly significant in the spectra of alcohols, phenols, carboxylic acids and amines.

Adjacent Groups

The wealth of structural information that can be obtained from infrared spectra arises largely from
differences in band position, shape and intensity