| Summary: | Isotopes are ideal substances for studying the intermolecular interactions in clathrates by replacing the atoms without destroying the geometry structure. When methane (CH 4 ) in the spatially homogeneous methane hydrate was replaced with deuterated methane (CD 4 ), it showed a previously unrecognized strong anharmonic effect, identified by the Raman peak located at 1952.78 cm –1 . This was assigned to a coupled overtone of C–D in 5 12 and 5 12 6 2 cages on the basis of density functional theory. This coupling vibration was confirmed to be present also in methane hydrate by a peak around 3053.62 cm –1 its intensity is only 21.9% of that in the CD 4 system. This coupled vibration may have been observed in previous studies, yet without any solid evidence of its detailed assignment. Our work could provide a tool for characterizing the intermolecular behavior in the guest–host system; the proposed method should also be employed universally for similar isotopic supramolecular compounds.
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