Vibrational Anharmonicity in the Methyl Halides

Abstract

A simple model has been assumed for the anharmonic intramolecular potential function of the methyl halides; a quadratic potential in instantaneous curvilinear coordinates has been supplemented with Morse functions for the bond‐stretching coordinates. All parameters in this potential function are known from other work, and the model could therefore be used to estimate the anharmonic spectroscopic parameters of the methyl halides. These are discussed and compared with the experimental values where the latter are known. Systematic differences in the calculated anharmonic constants $X_{\mathrm{ss\prime }}$ for the symmetric and asymmetric CH stretching vibrations are noted, and an explanation is offered. The concept of group anharmonicities has also been investigated.