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.