Dynamic and static behaviors of CH4 and CO2 in small and large cavities of hydrate

We investigated the static structures and dynamic behaviors for guest molecules (CH4 and CO2) in small and large cavities which are composed of 20 and 24 water molecules, respectively, by B3LYP/6-311++G(d,p) level calculations in GAUSSIAN 09, and using quantum molecular dynamics (QMD) (NVT MD with semiempirical MO PM3 method). For the static calculations, the guest CO2 and CH4 molecules are around at the center of small and large cavities with weak H-bond formations of HOH⋯O2C and H2O⋯H4C van der Waals interaction systems. Calculated carbon NMR chemical shifts of the CH4 in the gas-state and in the small and large cavities reflected the C-13 experimental tendency, while the calculated carbon NMR chemical shifts of the CO2 in the three states almost correspond to the experimental value in the gas-state. For QMD calculations, we used a cluster model containing 73 water molecules, and examined dynamic behavior of guest molecules in the shell cluster model of 39 water molecules which own small and large cavities. The dynamic behavior of guest molecules are simulated from the trajectory distribution of molecular center of the mass due to the translational motion, and also analyzed using librational motions of guest molecules in the cavities.