Dynamics of Some He and Ar Clathrate Hydrates. Computer Simulation Study

Molecular dynamics simulation of solid solutions of He and Ar inice II at T 200 K has shown that amplitudes of water moleculeoscillations diminish when noble gas atoms fill the cavities ofthe hydrogen-bonded framework. The effect of Ar atoms ismore pronounced. Slow diffusion of He along trigonal axis isobserved when not all the cavities are filled. He and Ar atomsexert little effect on frequencies of translational and librationalvibrations of the water molecules. Type II empty gas hydrateframework (CSII) is quite stable at T 200 K. Amplitudes ofoscillations of water molecules which occupy differentcrystallographic positions are different. Filling of the cavitiesin the CSII framework with Ar atoms causes diminutionof the amplitudes of water molecule vibrations, and differencebetween amplitudes of vibration of molecules occupyingdifferent positions becomes less pronounced. Large cavities inthe CSII framework can accommodate two Ar atoms withoutdistortion. No diffusion of guest Ar atoms was observed at 200 Kin CSII framework.