Evidence of Guest-Guest Interaction in Clathrates Based on In Situ Raman Spectroscopy and Density Functional Theory

Isotopes are ideal substances for studying the intermolecular interactions in clathrates by replacing the atoms without destroying the geometry structure. When methane (CH₄) in the spatially homogeneous methane hydrate was replaced with deuterated methane (CD₄), 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 5¹² and 5¹²6² 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 CD₄ 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.