Quantum dynamics of H2, D2, and HD in the small dodecahedral cage of clathrate hydrate: Evaluating H2-water nanocage interaction potentials by comparison of theory with inelastic neutron scattering experiments

Abstract

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 (H(2)O)(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 et al., J. Chem. Phys. 123, 024507 (2005)] was employed. This PES, denoted here as SPC/E, combines an effective, empirical water-water pair potential [Berendsen et al., J. Phys. Chem. 91, 6269 (1987)] and electrostatic interactions between the partial charges placed on H(2)O 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 et al., 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)-H(2)O 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.

Quantum calculation of inelastic neutron scattering spectra of a hydrogen molecule inside a nanoscale cavity based on rigorous treatment of the coupled translation-rotation dynamics

“…This uniquely high degree of realism and detail in simulating the INS spectra is not achievable by the approaches in the literature. It is made possible by our ability to compute accurately the fully coupled quantum 5D T-R energy levels and wave functions of the entrapped molecule on the 5D intermolecular PES 11,12,15,16 and, as shown here, use them as input for calculating the spectra.…”

“…12 . At 20 K, the temperature of the experiment, only the lowest rotational states J = 0, 1 are populated, and therefore are the only initial states considered in the calculations.…”

“…τ ( R cm , θ ρ , φ ρ ) and the corresponding energy levels ǫ τ (τ = i, f ) of the 5D T-R Hamiltonian of the caged H 2 molecule 11,12,15,16 ; θ ρ , φ ρ specify the orientation of H 2 in the cage. In…”

Mechanisms for thermal conduction in hydrogen hydrate

“…Bačić et al have studied these effects in much detail for hydrogen-containing clathrates [48][49][50][51][52][53][54], including explicit path-integral MD simulation [54]; based on these studies, it should be stressed that the classical MD results at 100 K and below reported in the present work needs to be interpreted with some caution due to the limitations of classical MD to describe accurately H 2 motion at low temperatures.…”

HD in C 60 : theoretical prediction of the inelastic neutron scattering spectrum and its temperature dependence

“…Our approach to the rigorous calculation of the coupled five-dimensional T-R energy levels and wave functions of a hydrogen molecule inside fullerenes, applied in this study to HD@C 60 , has evolved in our group over a number of years, in the course of the theoretical investigations of the quantum T-R dynamics of H 2 and its isotopologues entrapped in the cages of clathrate hydrates [19][20][21], fullerenes C 60 and C 70 [2,5], and an open-cage derivative of C 60 (ATOCF) [22]. A detailed description of the quantum five-dimensional methodology is available in [18], and therefore only the salient features are given here.…”