IMPROVEMENTS IN A CURVILINEAR INTERNAL COORDINATE MODEL FOR THE CH STRETCHING AND BENDING VIBRATIONS IN TRIHALOMETHANES

The interactions between the CH stretching and bending vibrations in C HX(3)-type symmetric-top molecules have been studied. The previous cur vilinear internal coordinate vibrational Hamiltonian (E. Kauppi and L. Halonen, J. Chem. Phys. 90, 6980-6992, 1989) has been improved by inc luding contributions arising from the nonlinearity of the redundancy r elation between the curvilinear internal coordinates. This affects the anharmonic kinetic energy terms of the pure CH bending Hamiltonian. V ibrational energy levels have been calculated variationally. The model has been applied to data for CHF3, CHCl3, CHBr3, and (CF3)(3)CH. The potential energy parameters have been optimized by a nonlinear least-s quares method using vibrational term values as data. In the cases of C HF3, CHCl3, and CHBr3 data for bath C-12 and C-13 isotopomers have bee n treated with one set of isotope invariant potential energy parameter s. Good fits have been obtained by using five or six adjustable parame ters. The values for the quartic bending force constants change consid erably from the values obtained by ignoring the nonlinear redundancy c ontributions. (C) 1994 Academic Press Inc.