Thermodynamic stability of hydrogen clathrates

The stability of the recently characterized type 11 hydrogen clathrate [Mao, W. L., Mao, H.-K., Goncharov, A. F., Struzhkin, V. V., Guo, Q., et al (2002) Science 297, 2247-2249] with respect to hydrogen occupancy is examined with a statistical mechanical model in conjunction with first-principles quantum chemistry calculations. It is found that the stability of the clathrate is mainly caused by dispersive interactions between H-2 molecules and the water forming the cage walls. Theoretical analysis shows that both individual hydrogen molecules and nH(2) guest clusters undergo essentially free rotations inside the clathrate cages. Calculations at the experimental conditions -2,000. bar (1 bar = 100 kPa) and 250 K confirm multiple occupancy of the clathrate cages with average occupations of 2.00 and 3.96 H-2 molecules per D-5(12) (small) and H-5(12)6(4) (large) cage, respectively. The H-2-H2O interactions also are responsible for the experimentally observed softening of the H-H stretching modes. The clathrate is found to be thermodynamically stable at 25 bar and 150 K.