Ab initio vibration-rotation-tunneling spectra and dynamics of H 2·F- and its isotopomers

Abstract

Ab initio computations of the potential energy surface (PES) of the ground electronic state of H2·F- have been performed as a function of the stretching F-H2 (R) and H2 rotation (θ) coordinates. Minima on the PES correspond to linear H-H-F- structures, while the transition state is T-shaped. The F- to H 2 distance increases in the transition state from 2.07 to 3.10 Å, demonstrating strong coupling between the θ and R degrees of freedom. The vibration-rotation-tunneling spectra are calculated by diagonalizing the five dimensional Hamiltonian matrix that describes free rotation of the triatomic (three coordinates) plus the internal θ and R motions. For total angular momentum J=0, the spacing between levels in the tunneling doublets increases from 0.029 to 6.74 cm-1 as the stretching quantum number n corresponding to R motion varies from 0 to 5. The splittings increase even more strongly with the bending quantum number. For J=1, each level in the tunneling doublets is further split by Coriolis forces. K-doubling is found to be an order of magnitude smaller than the tunneling splitting. In the symmetric isotopomers D2·F- and T2·F-, the tunneling splitting drops by 3 and 4 orders of magnitude compared to H2·F-, and thus becomes comparable with the K-doubling for J=1. Finally, incoherent tunneling, appropriate to condensed phase environments, of the H2F- system is also treated. © 1995 American Institute of Physics.