Good group frequencies

Vibrational frequencies, so-called good group frequencies, are important in applying infrared and Raman spectroscopy to many chemical problems that require the qualitative interpretation of infrared spectra. A group frequency is that mode of vibration associated with a particular bond or sets of bonds. 

The following four factors make significant contributions to the development of a good group frequency from a molecular vibration ... 

Overtones (integral multiples of the fundamental mode frequency) and sum tones (the sum of two different fundamental modes) are forbidden bands that are almost always very weak. Occasionally these bands are good group frequencies. 

Aldehydes. The aldehyde functional groups gives rise to several good group frequencies (Table 8.10 also see Chapter 8W, IR section. Fig. W8.25). 

The NO2 group also has three bending modes, which occur below 700 cm". The out of plane bending frequency falls between 670 and 610 cm" and the symmetric deformation frequency between 630—610 cm". The latter is very clearly seen in the Raman spectra where it is a good group frequency for the CNO2 group. These bands have been little studied in nitrates or nitramines. 

In Fig. 4.3(f) is illustrated a C=N vibration. Like the C=C vibration, the C=N is a good group frequency when the attached atom is carbon or a heavier atom. 

In the case of the C=CH2 group the CH2 oscillator deformation frequency is lower than the frequency of the attached C=C bond, so during this type of CH2 deformation the attached C=C bond hardly changes in length. The relatively constant interaction results in a good group frequency (- 1420 cm ). 

The CH2 twisting vibrations in CH2 chains have frequencies which are spread over a region like the wag and rock and have little intensity in the infrared. However, the in-phase CH2 twist vibration in CH2 chains is a good group frequency in the Raman spectrum at 1300 5 cm which is the high frequency limit for the CH2 chain twist vibrational frequencies. ... 

Not all vibrations are good group frequencies, and some group frequencies which maintain a fairly constant position for most... 

It should be noted also that usually only fundamentals make good group frequencies. The combination and overtone bands generally have medium to weak intensity and cannot always be described as being related to the motion of some isolated group. 

We shall also find good group frequencies for rings, such as those in benzene compounds. In most instances the vibration involves a CH group, so that again the small mass of hydrogen is the prime factor which makes these vibrations retain fairly constant spectral positions. ... 

Groups such as —C—NH2 also have distinctive group frequencies. Analysis of these group frequencies indicates that they are partly due to a combination of NH and C—or C=0 vibrations. Again, the force constants and mass factors involved in such a vibration remain fairly constant for the compounds in which the vibration is a good group frequency. 

The CH3 rock and CH2 wags and twists are usually not considered good group frequencies since, although there generally is a band near 1305 cm in normal alkanes that is a CH2 wagging vibration, they are weak and difficult to assign. 

We can summarize the group frequency assignments for normal alkanes by stating that the CH stretching and scissors vibrations and the CH2 rock near 720 cm are considered good group frequencies. 

In general the CH2 scissors is therefore a good group frequency, only slightly perturbed by its environment. 

Since the masses of carbon and oxygen are similar and the force constant of the single bond between them is equal to that of many other bonds, the C—O—C frequencies tend to couple with other frequencies and therefore are not good group frequencies. 

The aliphatic and aromatic amines have a number of distinct bands which can be used to identify the type of amine. For these amines the structural units NH and CN both have fairly good group frequencies. A brief resume of these vibrations is presented in... 

Group frequency correlations for fluorocarbons are quite limited. It appears that interaction between vibrations occurs more extensively for fluorocarbons than for the corresponding hydrocarbons. Because of this it is difficult to find vibrations for fluorocarbons which remain constant for a series of compounds. Even though the extensive interaction of frequencies of fluorocarbons does not lead to good group frequency correlations, this interaction results in quite distinct spectra for each different type of fluorocarbon structure, so that small structural differences can often be detected by studying the infrared spectra of these compounds. 

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