Spectroscopic Observation of Na Cations Entrapped in Small Cages of sII Propane Hydrate

Although numerous studies have been conducted on various ionic clathrate hydrates, in spite of its potential importance the inclusion of metal cations in continuous water-host frameworks has not yet been clearly identified by direct spectroscopic evidence. Here, a key question arises as to whether the small alkali metals such as Na+ can be entrapped because they are considered to be too small to be stabilized in the hydrate cages. In this study, we first suggested spectroscopic evidence for the enclathration of Na+ in a small cage of sII propane hydrate. First, we checked the overall structure of sII propane hydrate incorporated with NaSO3CH3 and NaSO3NH2 with powder XRD and 13C NMR. Next, we revealed the difference of chemical shift of 23Na between Ih and sII hydrate phases with solid-state 23Na MAS NMR as direct evidence of entrapped Na+ in 512 cages. In addition, we also checked 13C MAS NMR of the CH3SO3– anion and found that Na+ cations in a small cage could be stabilized with an enclathrated CH3SO3– ...