Formation of Methanol Clathrate Hydrate in Simulated Interstellar Ices.

We report that methanol, widely regarded as an inhibitor of clathrate hydrate (CH) formation, can be incorporated into a structure-I CH relevant to interstellar ices, under ultrahigh vacuum (10-10 mbar) and cryogenic (125 K) conditions. In the experiment, layered dimethyl ether (DME)-methanol-water ices were deposited at 10 K and thermally annealed to 125 K. Reflection-absorption infrared spectroscopy (RAIRS) reveals the emergence of a distinct band at 1021 cm-1 that is absent in pure CH3OH and binary CH3OH-H2O ices. Density functional theory (DFT) calculations assign this band to the C-O stretching mode of methanol encapsulated in the 51262 cage of structure-I CH. This clathrate occurs only in sequentially layered ices and requires a clathrate-promoting guest, such as DME. Similar behavior was observed with ethylene oxide, and methanol consistently occupies the 51262 cage independent of the promoter molecule. RAIRS and DFT analyses further indicate that CH3OH···H2O hydrogen-bonded environments are involved in the methanol enclathration. These results identify a specific, morphology-dependent pathway through which methanol, traditionally considered a clathrate inhibitor, can be incorporated into the clathrate cages under nonequilibrium cryogenic conditions, with possible relevance to volatile trapping in thermally processed interstellar and cometary ices.