| Abstract | Molecular-dynamics simulations and first-principles calculations are employed to understand vibrational spectroscopy and molecular and electronic structure of the encaged hydrogen molecules in hydrogen clathrate hydrate. The molecular-dynamics simulations, using empirical potentials, are performed to generate collections of the clathrate water cages with different hydrogen occupancies. The first-principles calculations, using Density Functional Theory with B3LYP hybrid density functionals for exchange and correlation, are carried out to optimize the structures and to calculate the Raman shift and activity of the stretching mode of the encaged hydrogen molecules. The Raman spectra are computed by a weighted moving average over a number of different structural configurations for different hydrogen occupancies. The results show that experimentally observed Raman peaks around 4120-4125 cm-1 are from small cages with single H2 occupancy and peaks around 4125-4150 cm-1 from those in the large cages with one to four H2 molecules. The Raman peaks of hydrogen molecules in the doubly occupied small cages are expected to be around or above the gas phase frequency 4155 cm-1. Molecular structural analysis shows that the single hydrogen molecule in the small cages and single to quadruple hydrogen molecules in the large cage are encaged in loose cages, while double hydrogen molecules in the small cage are confined in a tight cage. Normal-mode analysis shows that there is limited vibrational coupling for H2 molecules in doubly to quadruply occupied large cages while a strong vibrational coupling is observed in the doubly occupied small cage. The isovalue maps of total electron density and electrostatic potential suggest significant electron sharing between hydrogen molecules and water molecules, and important interaction between hydrogen and water oxygen atoms for confining the hydrogen clusters. The results help explain experimentally observed Raman spectra of hydrogen clathrates and provide new insights into the confinement effect by the water host framework on vibrational, molecular, and electronic properties of hydrogen molecules in the cages of clathrate hydrates. |
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