Raman spectroscopic study of CO2 in hydrate cages.

The Raman spectra of CO 2 molecules in hydrate cages was studied for structure I CO 2 hydrate and structure II tetrahydrofuran (THF)+CO 2 binary hydrate. The results obtained for sII hydrate indicated that the Fermi dyad peaks of CO 2 in the small cage of THF–CO 2 hydrate are located at 1274 cm −1 and 1380 cm −1 . Numerical fitting of the sI CO 2 hydrate Raman spectrum shows CO 2 Fermi dyad peaks in the small cage are located at 1275 cm −1 and 1382 cm −1 . The unperturbed frequencies of the symmetric stretching v 1 and overtone bending v 2 modes were calculated for CO 2 in various states. It was found that the v 1 frequency does not follow the loose cage–tight cage model for CO 2 in hydrate cages. Explanations are given as to why such a relationship is not expected in terms of the incomplete sampling of the void space of the non-spherical cages by the linear guest molecule and the contribution of H-bonding to the guest–host interactions. [ABSTRACT FROM AUTHOR]