EFFECT OF GUEST-HOST INTERACTION ON RAMAN-SPECTRUM OF A CO2 CLATHRATEHYDRATE SINGLE-CRYSTAL

The polarized Raman spectra of an artificial CO2:! clathrate hydrate s ingle crystal have been measured in order to examine the crystal-orien tation dependence of the Raman spectra. Since the crystal had crystall ographic facets, the orientation of the crystal was determined by usin g the Miller indices of the facets. When the angle theta between the p olarization plane of the incident laser beam and the direction of one of the [110] axes of the single crystal: varied, it was observed that the intensities of the peaks, which were caused by the Fermi resonance of the symmetric stretching mode and the overtone of the bending mode of CO2, and the O-H symmetric stretching vibration mode, varied with theta. Since the tetrakaidecahedron cage in the CO2 clathrate hydrate is distorted along the [100] axis, the variations of the scattering in tensities of the CO2? have been calculated by using a simple model tha t assumes that the CO2 rotates on the {100} plane in the tetrakaidecah edron cage. The results obtained from the experiments are consistent w ith the calculations made by using this model. It has been concluded t hat the anisotropy of the peak intensities of the CO2 show the influen ce of the cage geometry on the motion of the guest molecule.: The anis otropy of the O-H symmetric stretching vibration mode was interpreted with a five-body structure model. As the calculation with the model wa s consistent with the result obtained from the experiment, it was foun d that the anisotropy of the peak intensity of the O-H symmetric stret ching vibration mode was related to the arrangement of the water molec ules. We consider that the result indicates the influence of the motio n of the guest molecule on the surrounding hydrogen-bonded network. (C ) 1998 American Institute of ?Physics. [S0021-9606(98)02104-7].