Improvements in a Curvilinear Internal Coordinate Model for the CH Stretching and Bending Vibrations in Trihalomethanes

Abstract

The interactions between the CH stretching and bending vibrations in CH X 3-type symmetric-top molecules have been studied. The previous curvilinear internal coordinate vibrational Hamiltonian (E. Kauppi and L. Halonen, J. Chem. Phys. 90, 6980-6992, 1989) has been improved by including contributions arising from the nonlinearity of the redundancy relation between the curvilinear internal coordinates. This affects the anharmonic kinetic energy terms of the pure CH bending Hamiltonian. Vibrational energy levels have been calculated variationally. The model has been applied to data for CHF 3, CHCl 3, CHBr 3, and (CF 3) 3CH. The potential energy parameters have been optimized by a nonlinear least-squares method using vibrational term values as data. In the cases of CHF 3, CHCl 3, and CHBr 3 data for bath 12C and 13C isotopomers have been treated with one set of isotope invariant potential energy parameters. Good fits have been obtained by using five or six adjustable parameters. The values for the quartic bending force constants change considerably from the values obtained by ignoring the nonlinear redundancy contributions.