Effects of X3 frame vibrations on the CH chromophore in CHX3 type molecules: Harmonic coupling in curvilinear internal coordinate approach

Abstract

Effects of harmonic coupling between X3 frame vibrations and the CH stretching and bending vibration chromophore in CHX3 type molecules have been investigated using a curvilinear internal coordinate model. The CH chromophore Hamiltonian used consists of a Morse oscillator Hamiltonian for the CH stretching vibration, two-dimensional isotropic harmonic oscillator Hamiltonian for the CH bending vibration, anharmonic bending terms, and Fermi resonance interaction terms between the stretching and bending vibrations. Van Vleck perturbation theory is used to take the harmonic coupling terms between the CH chromophore and the X3 frame vibrations into account. Analytical matrix elements for the perturbation theory contributions have been derived. The model has been applied to vibrational energy level and infrared absorption intensity data for CHD3. Potential energy parameters for the CH chromophore have been optimized by a nonlinear least-squares method. The model reproduces the experimental data well. Only small changes in the potential parameters are observed when the coupling terms due to D atom motions are introduced into the model. The overall conclusion is that it is a good first approximation to ignore the effects of harmonic coupling between the CH chromophore and the X3 frame vibrations in the curvilinear internal coordinate model for the CH chromophore in CHX3 type molecules.