Dynamics of some He and Ar clathrate hydrates. Computer simulation study

Molecular dynamics simulation of solid solutions of He and Ar in ice II at T - 200 K has shown that amplitudes of water molecule oscillations diminish when noble gas atoms fill the cavities of the hydrogen-bonded framework. The effect of Ar atoms is more pronounced. Slow diffusion of He along trigonal axis is observed when not all the cavities are filled. He and Ar atoms exert little effect on frequencies of translational and librational vibrations of the water molecules. Type II empty gas hydrate framework (CSII) is quite stable at T 200 K. Amplitudes of oscillations of water molecules which occupy different crystallographic positions are different. Filling of the cavities in the CSII framework with Ar atoms causes diminution of the amplitudes of water molecule vibrations, and difference between amplitudes of vibration of molecules occupying different positions becomes less pronounced. Large cavities in the CSII framework can accommodate two Ar atoms without distortion. No diffusion of guest Ar atoms was observed at 200 K in CSII framework.