Raman peak frequencies of fluoromethane molecules measured in clathrate hydrate crystals: Experimental investigations and density functional theory calculations

Systematic observations of fluoromethane clathrate hydrates were carried out by Raman spectroscopy. The series of fluoromethanes, i.e., methane (CH ₄), fluoromethane (CH₃F), difluoromethane (CH ₂F₂), trifluoromethane (CHF₃), and tetrafluoromethane (CF₄), were used as standard guest molecules to investigate the vibration modes of the guest molecules in the hydrate phase, since all of these fluoromethanes are included in the same crystal structure and share similar functional groups. In this study, both the C-H and C-F vibration modes of the guest molecules were systematically collected and assigned each peak based on the density functional theory (DFT) calculations. The Raman peak table obtained by the DFT calculations was useful for assigning the Raman peaks measured by the experiments. The assignment of the Raman peaks of the C-H stretching mode of each fluoromethane hydrate coincided well with those estimated both experimentally and theoretically in previous studies. The empirical "loose cage-tight cage" model of the Raman peak shifts allowed us to estimate the unperturbed frequencies of the C-H symmetric stretching mode on CH₃F molecules in the clathrate structure. Clathrate hydrates formed with deuterated water molecules indicated that the deuterium had little effect on the Raman spectra of the intramolecular vibration modes of the guest molecules within the experimental uncertainties.