C—H Bending Vibrations Methyl Groups

the C—H bonds are moving like the closing petals of a flower in II, one petal opens as two petals close. 

The symmetrical bending vibration ( 5sCH3) occurs near 1375 cm-1, the asymmetrical bending vibration (5asCH3) near 1450 cm-1. 

The asymmetrical vibration generally overlaps the scissoring vibration of the methylene groups (see below). Two distinct bands are observed, however, in compounds such as diethyl ketone, in which the methylene scissoring band has been shifted to a lower frequency, 1439-1399 cm 1, and increased in intensity because of its proximity to the carbonyl group. 

The absorption band near 1375 cm 1, arising from the symmetrical bending of the methyl C—H bonds, is very stable in position when the methyl group is attached to another carbon atom. The intensity of this band is greater for each methyl group in the compound than that for the asymmetrical methyl bending vibration or the methylene scissoring vibration. 

The band resulting from the methylene rocking vibration (p CH2), in which all of the methylene groups rock in phase, appears near 720 cm 1 for straight-chain alkanes of seven or more carbon atoms. This band may appear as a doublet in the spectra of solid samples. In the lower members of the n-alkane series, the band appears at somewhat higher frequencies. 

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