Variation of the Intensity of the Methyl Symmetrical Bending Vibration in Paraffins

The integrated infrared absorption coefficients of the methyl C–H symmetrical bending vibration of a number of normal and branched paraffins have been studied. Several approaches to the correlation of intensity variations with structure are discussed and evaluated in terms of the experimental data. The conclusions are (a) the intensities of the branched and normal paraffins must be treated as two distinct classes, and (b) the branched paraffin group intensities must be further broken down into three types: (1) methyl groups adjacent to a tertiary or quaternary carbon atom having no other methyl groups directly adjacent, (2) methyl groups adjacent to tertiary or quaternary carbon atoms having other adjacent methyl groups directly attached, and (3) methyl groups at the ends of branches or chains of two or more carbon atoms in length. Integrated absorption coefficients are given for each type of group and approximate C–H bond moments are calculated from these values.