Trends in vibrational frequencies of guests trapped in clathrate hydrate cages

Raman spectra of ethane trapped in the small 5(12) cage of sII hydrate (at similar to70 MPa), isobutane trapped in the large 5(12)6(4) cage of sII hydrate, and the gauche form of n-butane trapped in the large 5(12)6(4) cage of sII hydrate were obtained for the first time. These new Raman results are combined with existing Raman and IR results for various guests to infer general trends in vibrational frequencies of guest molecules trapped in clathrate hydrate cages as a function of cage size, guest size, guest vibrational mode, and pressure. The observed trend in stretching frequencies of guests with cage size, which can be stated as the larger the cavity, the lower the frequency, is explained through the qualitative loose cage-tight cage model of Pimentel and Charles.