Raman spectroscopic study of CO2 in hydrate cages

•A method of making hydrate in which CO2 only occupies small cage is proposed.•The Raman peak of CO2 in the small cage of hydrate is identified at ~1274 cm−1.•The CO2 Raman spectrum does not follow the loose cage – tight cage model because the linear guest does not sample the entire volume of the asymmetric cage.

The Raman spectra of CO2 molecules in hydrate cages was studied for structure I CO2 hydrate and structure II tetrahydrofuran (THF)+CO2 binary hydrate. The results obtained for sII hydrate indicated that the Fermi dyad peaks of CO2 in the small cage of THF–CO2 hydrate are located at 1274 cm−1 and 1380 cm−1. Numerical fitting of the sI CO2 hydrate Raman spectrum shows CO2 Fermi dyad peaks in the small cage are located at 1275 cm−1 and 1382 cm−1. The unperturbed frequencies of the symmetric stretching v1 and overtone bending v2 modes were calculated for CO2 in various states. It was found that the v1 frequency does not follow the loose cage–tight cage model for CO2 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.