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

摘要

Melanophlogite is a naturally occurring SiO2-based clathrate structure which has the same structure of type I gas hydrates. Two types of voids are found in melanophlogite. Furthermore, as in the case of the gas hydrates, melanophlogite traps gas molecules within the voids. In this work we present a joint theoretical and experimental investigation of the CH4 Raman spectra associated with the enclathrated CH4 molecules. We find that the Raman intensities of the totally symmetric hydrogen stretch modes are significantly perturbed by the presence of the clathrate cage and show that the calculated Raman spectra may be used to determine the concentration and location of the enclathrated gas molecules. Relative to the gas-phase structure we identify two effects which are responsible for the change in intensity of the enclathrated molecules relative to the gas phase. The polarizibility of the surrounding cage acts to increase the Raman spectra of the stretch mode in the pentagondodecahedra cage. However, in the lower-symmetry tetrakaidecahedra cage, mixing between the hydrogen stretch mode and other optically silent molecular vibrations counteracts this effect and accounts for the different Raman intensities observed for the two types of voids. We suggest that similar calculations and experiments on the gas hydrates may provide an in situ diagnostic tool for determining the amount of natural gas contained within the gas hydrates on the sea floor. [References: 22]