We have performed rigorous quantum five-dimensional (5D) calculations and analysis of the translation-rotation (T-R) energy levels of one H2, D2, and HD molecule inside the small dodecahedral (H2 O)20 cage of the structure II clathrate hydrate, which was treated as rigid. The H2 - 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 H2 O and H2. The 5D T-R eigenstates of HD were calculated also on another 5D H2 -cage PES denoted PA-D, used by us earlier to investigate the quantum T-R dynamics of H2 and D2 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 H2 - H2 O dimer. The quality of the SPC/E and the PA-D H2 -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 H2 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 H2, D2, and HD, which have not been measured yet.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry