Coupled Vibrations
Groups in which two or more atoms of an element are bonded to a common atom are mechanically
coupled resulting in a band which is generally resolved into two components. These correspond to
symmetric and asymmetric modes of vibration. They also execute a variety of bending vibrations each
of which may be observed at a characteristic wavenumber. Examples include CH 3 , CH 2 , NH 2 , NO 2 and
COO–. The stretching and bending modes of a methylene group are illustrated in Figure 9.16. It should
be noted that asymmetric vibrations occur at a higher wavenumber than symmetric and that the
absorption is more intense. Another form of vibrational coupling occurs between the C—H groups in
both aromatic and alkene compounds resulting in bands which are characteristic of the substitution
pattern. In the case of out-of-plane bending, they are of a high intensity and are especially useful for
diagnostic purposes (vide infra).
Figure 9.16
Stretching and bending modes of a methylene group.Qualitative Analysis – The Identification of Structural Features
The value of infrared spectrometry as a means of identification of unknown compounds and to
investigate structural features is immense. Spectra are used in an empirical manner by comparison of
samples with known materials and by reference to charts of group frequencies. A simplified correlation
chart is shown in Table 9.8. The interpretation of infrared spectra is best considered by discussing the
prominent features of a representative series of compounds.
The spectrum of a saturated hydrocarbon, n-decane is shown in Figure 9.17. Four prominent bands arise
from C—H stretching and bending vibrations: asymmetric and symmetric C—H stretching (~ 2900 cm–
(^1) ), CH 2 bending and asymmetric CH 3 bending (~ 1460 cm– (^1) ), symmetric CH 3 bending (~ 1380 cm– (^1) ) and
rocking of all the CH 2 groups in the chain (~ 720 cm–^1 ). The position of the latter depends upon chain
length and is not observed if there are fewer than four methylene groups together. The C—H stretching
bands may be further resolved into components due to