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摘要
We present an anharmonic, coupled-mode vibrational analysis of CH4 in four clathrate cages, 5(12), 5(12)6(2), 4(3)5(6)6(3) and 5(12)6(4), employing a general, full dimensional, ab initio potential energy surface for CH4(H2O)(n) clusters. This potential is expressed in a many-body representation, truncated at the three-body level, for the water and the methane water interactions. The embedded local monomer model is used to determine the energies of the intramolecular vibrations of the confined methane. This model is validated by comparing the harmonic density of states using local-monomer and standard full normal-mode analyses for the 5(12) CH4@(H2O)(20) clathrate. The agreement in the region of the methane intramolecular vibrations is excellent. Vibrational self-consistent field and virtual-state configuration interaction theory is employed to calculate the vibrational energies of methane in four cages using the code MULTIMODE, and comparisons with experiment are given. The zero-point energy and wave function of the enclathrated methane molecule are obtained in full dimensionality, but with a rigid cage, using diffusion Monte Carlo calculations. The results indicate substantial rotational delocalization. Dissociation energies are reported based on these calculations for methane in 5(12) and 5(12)6(2) cages.
- 出版日期2016-2-18
