Ab Initio, Embedded Local-Monomer Calculations of Methane Vibrational Energies in Clathrate Hydrates
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- 作者：Chen Qu ; Joel M. Bowman
- 刊名：Journal of Physical Chemistry C
- 出版年：2016
- 出版时间：February 18, 2016
- 年：2016
- 卷：120
- 期：6
- 页码：3167-3175
- 全文大小：398K
- ISSN：1932-7455
文摘
We present an anharmonic, coupled-mode vibrational analysis of CH4 in four clathrate cages, 512, 51262, 435663, and 51264, 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 512 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 512 and 51262 cages.