Cage-dependent Raman bands of O2 molecules confined in clathrate hydrate structures.

The vibrational properties of guest molecules confined within clathrate hydrate (hydrate) cages are strongly influenced by the local positions of the guests within the cage framework. Herein, we report the O-O stretching Raman bands of O2 molecules encaged in structure-I, structure-II, and structure-H hydrates. Unexpectedly, the O-O stretching bands assigned to the larger 51262 and 51264 cages are blue-shifted relative to that of the smaller 512 cage. This observation contrasts with the conventional loose-cage/tight-cage model, in which the vibrational frequencies of smaller guest molecules confined in smaller cages generally appear at higher wavenumbers compared to those in larger cages. To clarify the origin of this anomalous Raman behavior, density functional theory (DFT) calculations were performed for O2, N2, and CH4 molecules confined within representative hydrate cages. The optimized structures and guest-position potential energy profiles reveal that O2 exhibits more pronounced cage-wall-proximal localization than N2 and CH4, particularly in the 51262 and 51264 cages. The combined Raman and DFT results indicate that O2 confined within these larger cages experiences a more repulsive local environment than that in the smaller cage. These findings indicate that the unusual Raman behavior of O2 is associated not only with cage size but also with cage-wall-proximal localization.