Molecular-dynamics study of structure II hydrogen clathrates.

Molecular-dynamics simulations are used to study the stability of structure II hydrogen clathrates with different H2 guest occupancies. Simulations are done at pressures of 2.5 kbars and 1.013 bars and for temperatures ranging from 100 to 250 K. For a structure II unit cell with 136 water molecules, H2 guest molecule occupancies of 0-64 are studied with uniform occupancies among each type of cage. The simulations show that at 100 K and 2.5 kbars, the most stable configurations have single occupancy in the small cages and quadruple occupancy in the large cages. The optimum occupancy for the large cages decreases as the temperature is raised. Double occupancy in the small cages increases the energy of the structures and causes tetragonal distortion in the unit cell. The spatial distribution of the hydrogen guest molecules in the cages is determined by studying the guest-water and guest-guest radial distribution functions at various temperatures.

[1]  The structure and dynamics of doubly occupied Ar hydrate , 2001 .

[2]  D. C. Rapaport,et al.  The Art of Molecular Dynamics Simulation , 1997 .

[3]  Hideki Tanaka,et al.  On the thermodynamic stability of clathrate hydrates IV: double occupancy of cages. , 2004, The Journal of chemical physics.

[4]  E. D. Sloan,et al.  Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate , 2004, Science.

[5]  Wendy L. Mao,et al.  Hydrogen storage in molecular compounds. , 2004 .

[6]  J. Tse,et al.  Thermodynamic stability of hydrogen clathrates , 2003, Proceedings of the National Academy of Sciences of the United States of America.

[7]  A. Buckingham,et al.  Quadrupole moments of some simple molecules , 1968 .

[8]  J. Tse,et al.  Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations , 2001 .

[9]  Hoover,et al.  Canonical dynamics: Equilibrium phase-space distributions. , 1985, Physical review. A, General physics.

[10]  T. Straatsma,et al.  THE MISSING TERM IN EFFECTIVE PAIR POTENTIALS , 1987 .

[11]  Victor V. Goldman,et al.  The isotropic intermolecular potential for H2 and D2 in the solid and gas phases , 1978 .

[12]  J. D. Bernal,et al.  A Theory of Water and Ionic Solution, with Particular Reference to Hydrogen and Hydroxyl Ions , 1933 .

[13]  B. Chazallon,et al.  In situ structural properties of N2-, O2-, and air-clathrates by neutron diffraction , 2002 .

[14]  Ho-Kwang Mao,et al.  Hydrogen Clusters in Clathrate Hydrate , 2002, Science.

[15]  J. Tse,et al.  Molecular dynamics simulation study of the properties of doubly occupied N2 clathrate hydrates , 2001 .

[16]  H. Mao,et al.  Structure and dynamics of hydrogen molecules in the novel clathrate hydrate by high pressure neutron diffraction. , 2004, Physical review letters.

[17]  R. C. Weast.  CRC Handbook of Chemistry and Physics , 1973 .

[18]  R. McMullan,et al.  Polyhedral Clathrate Hydrates. X. Structure of the Double Hydrate of Tetrahydrofuran and Hydrogen Sulfide , 1965 .

[19]  Berend Smit,et al.  Understanding Molecular Simulation , 2001 .

[20]  G. Ciccotti,et al.  Hoover NPT dynamics for systems varying in shape and size , 1993 .

[21]  J. Tse,et al.  Computer simulations of the dynamics of doubly occupied N2 clathrate hydrates , 2002 .

[22]  S. Nosé.  A unified formulation of the constant temperature molecular dynamics methods , 1984 .

[23]  J. Banavar,et al.  Computer Simulation of Liquids , 1988 .

[24]  F. Stillinger,et al.  Proton Distribution in Ice and the Kirkwood Correlation Factor , 1972 .

[25]  S. Sasaki,et al.  Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate , 2003 .