Influence of cage structures on the vibrational modes and Raman activity of methane

Melanophlogite is a naturally occurring SiO2-based clathrate structure whic h has the same structure of type I gas hydrates. Two types of voids are fou nd in melanophlogite. Furthermore, as in the case of the gas hydrates, mela nophlogite traps gas molecules within the voids. In this work we present a joint theoretical and experimental investigation of the CH4 Raman spectra a ssociated with the enclathrated CH4 molecules. We find that the Raman inten sities of the totally symmetric hydrogen stretch modes are significantly pe rturbed by the presence of the clathrate cage and show that the calculated Raman spectra may be used to determine the concentration and location of th e enclathrated gas molecules. Relative to the gas-phase structure we identi fy two effects which are responsible for the change in intensity of the enc lathrated molecules relative to the gas phase. The polarizibility of the su rrounding cage acts to increase the Raman spectra of the stretch mode in th e pentagondodecahedra cage. However, in the lower-symmetry tetrakaidecahedr a cage, mixing between the hydrogen stretch mode and other optically silent molecular vibrations counteracts this effect and accounts for the differen t Raman intensities observed for the two types of voids. We suggest that si milar calculations and experiments on the gas hydrates may provide an in si tu diagnostic tool for determining the amount of natural gas contained with in the gas hydrates on the sea floor.