Quantum dynamics of H-2, D-2, and HD in the small dodecahedral cage of clathrate hydrate: Evaluating H-2-water nanocage interaction potentials by comparison of theory with inelastic neutron scattering experiments

摘要

We have performed rigorous quantum five-dimensional (5D) calculations and analysis of the translation-rotation (T-R) energy levels of one H-2, D-2, and HD molecule inside the small dodecahedral (H2O)(20) cage of the structure II clathrate hydrate, which was treated as rigid. The H-2- cage intermolecular potential energy surface (PES) used previously in the molecular dynamics simulations of the hydrogen hydrates [Alavi , J. Chem. Phys. 123, 024507 (2005)] was employed. This PES, denoted here as SPC/E, combines an effective, empirical water-water pair potential [Berendsen , J. Phys. Chem. 91, 6269 (1987)] and electrostatic interactions between the partial charges placed on H2O and H-2. The 5D T-R eigenstates of HD were calculated also on another 5D H-2-cage PES denoted PA-D, used by us earlier to investigate the quantum T-R dynamics of H-2 and D-2 in the small cage [Xu , J. Phys. Chem. B 110, 24806 (2006)]. In the PA-D PES, the hydrogen-water pair potential is described by the ab initio 5D PES of the isolated H-2-H2O dimer. The quality of the SPC/E and the PA-D H-2-cage PESs was tested by direct comparison of the T-R excitation energies calculated on them to the results of two recent inelastic neutron scattering (INS) studies of H-2 and HD inside the small clathrate cage. The translational fundamental and overtone excitations, as well as the triplet splittings of the j=0 -> j=1 rotational transitions, of H-2 and HD in the small cage calculated on the SPC/E PES agree very well with the INS results and represent a significant improvement over the results computed on the PA-D PES. Our calculations on the SPC/E PES also make predictions about several spectroscopic observables for the encapsulated H-2, D-2, and HD, which have not been measured yet. (c) 2008 American Institute of Physics.