A quantum dynamical study of CH overtones in fluoroform. II. Eigenstate analysis of the vCH=1 and vCH=2 regions

Abstract

In this series, the multiple time scales and mechanisms of intramolecular vibrational-energy redistribution (IVR) present in the CH overtones of fluoroform is investigated. In part II, we analyze the ab initio vibrational spectrum and dynamics of the vCH=1 and vCH=2 regions, explicitly treating all vibrational degrees of freedom. A wave operator sorting algorithm is used to develop an efficient vibrational basis. Spectral transformation, by a filtered-Lanczos method, is used to accelerate eigenstate analysis. The theoretical spectrum of the fundamental is nontrivial, predicting a close triplet of lines (width <6 cm−1) centered at 3048.3 cm−1 and a weak line 20 cm−1 upfield, due to coupling with a small subsystem of background modes: ν4±+ν5±+ν6±, ν2+ν4±+ν6±, and ν3+2ν5. Furthermore, these background states mix with one another and other nearby background states via resonances ω5≈(ω3+ω6), ω2≈ω5, and (ω4+ω6)≈(ω3+ω5). An effective Hamiltonian is used to assess chromophore–background and background–background coupling. In contrast, the first overtone exhibits no strong background resonances, rather the background modes function as a weakly coupled reservoir. IVR time scales and eigenstate convergence, with respect to the active space, are presented.