Stable Occupancy of Hydrogen Molecules in H_2 Clathrate Hydrates and H_2 + THF Clathrate Hydrates Determined by Ab Initio Calculations

摘要

Structure II clathrate hydrates of pure hydrogen and binary hydrates of THF + H_2 are studied using ab initio calculations to determine the stable occupancies of small cavities. Ab initio calculations are carried out for a double cavity consisting of one dodecahedron (small cavity) and one hexakaidecahedron (large cavity). These two cavities are attached to each other as in sII hydrates to form a double cavity. One or two H_2 molecules are placed in the small cavity and one THF (or 4H_2 molecules) molecule is placed in the large cavity. We have determined the binding energies of the double cavities at the MP2 level using various basis sets (3-21G, 3-21G(2p), 3-21++G(2p), 6-31G, 6-31G(2p), and 6-31++G(2p)). Different basis sets yield different stable occupancies of the small cavity. The results from the highest basis set (6-31++G(2p) with zero point energy corrections) indicate that the single occupancy is slightly more favorable than the double occupancy in both the cases of pure H_2 hydrates and THF + H_2 double hydrates.