Raman and Infrared Intensities in the Vibrational Spectra of Hydrocarbons. II. The CH Vibrations of Straight n-Paraffins

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Abstract
The Raman and infrared intensities of the CH vibrations of solid n‐paraffins are analyzed theoretically. The dependence of the relative intensity I of bands on the number N of C atoms is investigated experimentally. An empirical relation of the form I = ANp+BF(N), where A»BF(N), p=0 for factor group modes, p=—1 for terminal modes, and —3<p<—1 for ``inactive'' modes, is found and confirmed by the theory. The intensities of 5 infrared‐active factor group modes are calculated from 3 independent parameters only, in agreement with experiment. One of the parameters is the permanent CH‐bond moment whose magnitude is found to be 0.085±0.02 debye. The relative intensities of the Raman‐active twisting, wagging, and rocking modes are determined theoretically; one finds approximately: Itw:Iw:Ir=6:3:1. This supports previous assignments.
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