Similarities between the tetrahydrofuran clathrate hydrate after pressure-induced amorphization and aqueous tetrahydrofuran solution: an <i>in situ</i> Raman and infrared spectroscopic study

Pressure induced amorphization (PIA) of the type II clathrate hydrate containing the tetrahydrofuran (THF) molecule as the guest molecule was observed by an <i>in situ</i> Raman and infrared (IR) spectroscopic study. When the THF clathrate hydrate (THF-CH) and deuterate (THF-CD) were compressed isothermally at 100 K, well below their decomposition points, amorphization occurred around 1.5 GPa. The spectra obtained from the measurements provided the following insights into the structural and dynamic changes of the THF-CH/CD during the amorphization process. First, the hydrogen bond distance of OH⋯O extends with amorphization, while the density around THF molecules increases. Although this may appear contradictory at first glance, it is consistent with the idea that the bond angles of OH⋯O decrease, suggesting "folding" of the water molecule network. Furthermore, the vibron mode spectra of THF molecules after amorphization closely resemble those of THF aqueous solutions, indicating the formation of hydrogen bonds between the ether-oxygen and the water molecules. The formation of these hydrogen bonds is predicted to induce rotational defects in the water molecule network, which likely accounts for the differences in structure and thermal properties compared to amorphous ice. After amorphization, annealing was conducted under conditions of 0.8-1.9 GPa and 170-180 K where a glass transition occurred. Observed spectral changes indicated that the OH⋯O distance increased slightly and the annealing gradually led to phase separation with weakening of the hydrogen bonds of the THF molecule.