Composition-Dependent Formation and Dissociation of Structure I and Structure II Clathrate Hydrates of Trimethylene Oxide in Ultrahigh Vacuum

This study reports the formation of structure I (sI) and structure II (sII) clathrate hydrates (CHs) of trimethylene oxide (TMO) at different concentrations of the components under cryogenic conditions in ultrahigh vacuum. We codeposited TMO:H2O ices of 1:3 and 1:15 ratios at 10 K and thermally annealed them to 135 K. Using reflection–absorption infrared spectroscopy, we observed the formation of distinct CH structures from compositionally varied ices at 135 K. Although TMO occupied the 51262 cage of sI CH in the 1:3 mixture, in the 1:15 mixture, it occupied the 51264 cage of sII. The sI TMO CH demonstrated greater thermal stability compared to sII. Interestingly, the dissociation of these CH structures exerts different effects on amorphous solid water (ASW). While the sI CH dissociation does not lead to the expected conversion of ASW to crystalline hexagonal ice, the dissociation of sII CH crystallizes ASW. This observation may be attributed to the relative stability and host–guest hydrogen bonding of the CHs, as supported by density functional theory analysis.