14.15 Nonallowed and Nonfundamental Vibrational Transitions
Because molecules are not perfect harmonic oscillators, the strict selection
rules are not followed perfectly. In some cases v2,3,4,....These transi-
tions are usually much weaker than for v1. In many cases, very bright light
sources like lasers must be used to detect these transitions.
Overtone absorptionscan be thought of as two or more vibrational quantum
number steps. These are occasionally written in terms of the vibration num-
ber, for example 2 3 or 2 10 to indicate that the vibrational quantum number
for 3 or 10 of a particular molecule changes by 2. Although also forbidden
formally, it is not unusual to find absorptions in a vibrational spectrum due to
other combinations of the normal vibrations of the molecule. These are called
combination bandsand can be the addition of two (or more) normal vibrations
of a molecule or even differences of two (or more) normal vibrations of a mol-
ecule. As might be expected, the larger the molecule, generally the greater the
possibility of the appearance of combination bands. This is true because the
larger the molecule, the less ideal it probably is.
For example, let us consider the vibrational spectrum of methylacetylene,
CH 3 CCH. The 15 normal vibrations reduce to 5 A 1 -labeled motions and
5 E-labeled motions. Table 14.6 lists the 10 unique vibrational frequencies of
methylacetylene. Also listed are other absorptions that are attributed to vari-
ous overtones and combination bands. The nonideality of the molecule per-
mits some of these combinations to appear with detectable intensity.
Table 14.6 also shows that, for the frequencies of the combination bands and
overtones, the frequencies of the fundamental vibrations are not a perfect ad-
ditive combination. This can make the assignment of combinations and over-
tones particularly tricky, especially for large molecules. Although there are
symmetry rules for determining what vibrational frequencies will interact
with what other vibrational frequencies, careful spectral studies using selective
14.15 Nonallowed and Nonfundamental Vibrational Transitions 503Table 14.6 Infrared absorptions detected for methylacetylene
Normal vibration label Symmetry species Vibrational frequency
1 A 1 3334.0
2 A 1 2941.0
3 A 1 2142.2
4 A 1 Not observed
5 A 1 930.7
6 E 3008.3
7 E 1452.0
8 E 1052.5
9 E 633.2
10 E 328.0
Combination bands Frequency
3 10 10 2135.0
3 2 10 2 10 2128.0
5 10 10 932.2
2 9 2 10 2 10 1258.2
2 9 10 10 1256.8
2 9 1255.0
aSource:D.R.J. Boyd, H. W. Thompson.Trans. Farad. Soc.,1954, 50: 212.