A joint local mode and normal mode interpretation of vibrational anharmonicity in methyl bromide

Abstract

The gas-phase i.r. spectrum of CH 3Br has been studied up to 14 000 cm −1. Some 32 new vibration levels are located accurately, involving up to 5 quanta ( V= 5) of excitation in CH stretching. Reproduction of a total of 72 vibration levels is achieved through a joint treatment of CH stretching vibrations in a local mode basis, other vibrations in a normal mode basis, and with the inclusion of two Fermi resonances. The local mode approach satisfactorily accounts for the effects of the large Darling—Dennison vibrational interactions which occur between CH stretching modes. Fermi resonances between CH stretching and overtones of both methyl group deformation vibrations (ν 2 and ν 5) are treated explicitly. Interacting levels become quasi-degenerate at V = 3 in the case of 2ν 5, and at V = 5 in the case of 2ν 2. The analysis permits the determination of a complete set of 27 physically representative anharmonicity constants for CH 3Br, four of which are interrelated through the local mode model. Data reproduction between 600 and 14 000 cm −1 is achieved with an r.m.s. error of 2.55 cm −1.