Quantum chemical study for intermolecular potential energy in methane hydrate
Abstract
Hartree-Fock,DFT(BLYP,B3LYP,MPW1PW91) methods are used to study the intermolecular potential energy for the unit cell of methane hydrate structure-Ⅰ.The unit cell is treated as a regular dodecahedron including 20 water molecules and a methane molecule in the center.The geometry of methane is optimized by ab initio HF/6-31G(d,p) and the water molecules take the geometry of an ST2 model.The water-water hydrogen bond potential Ehb(l) and water-methane van der Waals potential Evdw(l) as functions of the side length l are calculated,keeping the molecular geometries of water and methane fixed.In all calculations we use the 6-31G(d,p) basis set.Basis set superposition error(BSSE) is corrected and the upper and lower limits of BSSE are determined for the water-water hydrogen bond energy.The oxygen-oxygen distance RO—O=0.280 nm,and the carbon-oxygen distance RC—O=0.392 nm obtained by BSSE-corrected B3LYP are closer to the experimental values 0.282 nm and 0.395 nm than other methods.The results indicate that the water-water pair hydrogen bond energy(30～36 kJ/mol) in gas phase hydrate structure-Ⅰ is stronger than the hydrogen bond energy(22.6±2.9) kJ/mol in the water dimer(H2O)2 or(21.7±0.5) kJ/mol in hexagonal ice,and the dodecahedron cell is a stable unit.These molecular interaction potentials provide a solid basis for the derivation of parameters in the Lennard-Jones and Kihara potentials,which are useful in molecular dynamics simulations of gas hydrates.
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