Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate

Raman spectroscopy has exhibited the C–H stretch (A1 mode) frequency ν1 of hydrated methanes at 2915 cm−1 for the 512 cage and 2905 cm−1 for the 51262 cage. These values are lower than the frequency of 2916.5 cm−1 in gaseous methane. In this paper, we theoretically examine the Raman spectra observed in methane hydrate by normal mode analysis using the independent molecule model. By a breakdown of the symmetry, the four frequencies in modes A1, E, T2 and T2 observed in gaseous methane are separated into nine frequencies in the hydrate. It is necessary to consider the anharmonic potential energy within methane and hydrogen bonding between methane hydrogen and water oxygen in order to get a result in qualitative agreement with experiment. The frequency in the 51262 cage is shifted downward in comparison to the one in 512, and the frequencies in the both cages are also shifted downward compared with the frequencies in gas. Calculations are also reported for the isotopic methane (CD4, 13CH4) hydrates.