Reduced-Dimensional Quantum Computations for the Rotational–Vibrational Dynamics of F – –CH 4 and F – –CH 2 D 2 (original) (raw)

2013, The Journal of Physical Chemistry A

Variational rotational−vibrational quantum chemical computations are performed for the F − −CH 4 and F − − CH 2 D 2 anion complexes using several reduced-dimensional models in a curvilinear polyspherical coordinate system and utilizing an accurate ab initio potential energy surface (PES). The implementation of the models is made practical by using the general rovibrational code GENIUSH, which constructs the complicated form of the exact rovibrational kinetic energy operator in reduced and full dimensions in any user-specified coordinates and body-fixed frames. A one-dimensional CF stretch, 1D(R CF ), a two-dimensional intermolecular bend, 2D(θ,φ), and a three-dimensional intermolecular, 3D(R CF ,θ,φ), rigid methane model provide vibrational energies for the low-frequency, large-amplitude modes in good agreement with full-dimensional MCTDH results for F − −CH 4 . The 2D(θ,φ) and 3D(R CF ,θ,φ) four-well computations, describing equally the four possible CH−F − bonds, show that the ground-state tunneling splitting is less than 0.01 cm −1 . For the hydrogen-bonded CH stretching fundamental a local-mode model is found to have almost spectroscopic accuracy, whereas a harmonic frequency analysis performs poorly. The 2D(θ,φ) and 3D(R CF ,θ,φ) rotational−vibrational computations on the T d -symmetric four-well PES reveal that in most cases F − − CH 4 behaves as a semirigid C 3v symmetric top. For the degenerate intermolecular bending vibrational states substantial splittings of the rigid rotor levels are observed. For F − −CH 2 D 2 the rotational levels guide the assignment of the vibrational states to either F − −H or F − −D connectivity.