Alkanes Containing the CH3 and CH2 Units

There may be other bands in the CH stretching region, due to overtones or combination bands, so that one should not expect to find only four distinct bands in this region. The spectra of alkane molecules also depend on the physical state of the sample. 

The scissors vibrations of the CH3 and CH2 groups also appear in a fairly constant region. Generally, the asymmetric CH3 scissors is very near the CH2 scissors, so that it is not possible to distinguish between these two vibrations. 

Finally, it can be seen that the CH3 and CH2 rocking vibrations appear between 1200 and 700 cm L We shall see later that these are 

Compound CH3 Stretch CH2 stretch CH3 scissors CH2 scissors Sym. CH3 rock CH2 rock CH2 wag Ref. 

CH3CH3 (gas) 2996M 2954S 2915S 1472S 1379W 1190 (Raman only) 822 5 

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