Infrared Band Assignments for the 2900 cm^−1 Region Methyl Group Vibrations in XC_6H_4COOCH_3, XC_6H_4CH_3, and XC_6H_4COCH_3 Molecules

The spectra of monomethyl aromatic compounds in which the methyl group is attached either directly to the aromatic ring or to an oxygen or a carbonyl group carbon on a sidechain exhibit a 150 cm−1 wide absorption region centering around 2920 cm−1. This region consistantly contains four bands, which have been assigned, in decreasing wave number order, to the following methyl group vibrations: the totally asymmetric stretch; the axially asymmetric stretch which, however, preserves its symmetry with respect to the molecule’s symmetry plane; the totally symmetric stretch; and the second harmonic of the asymmetric deformation. The exact position of this absorption region depends upon the point of attachment of the methyl group. Within certain series of homologous molecules, such as the para substituted methyl benzoates, the position of the region is also noticeably sensitive to the nature of the substituent.