Raman Spectroscopic Studies of THF Clathrate Hydrate

Abstract

Raman spectra were collected for tetrahydrofuran (THF) hydrate, a structure II clathrate hydrate (CH) that was prepared from THF and a dilute solution of D2O in H2O (5% D2O) in a 17:1 ratio. The vibrational modes observed include (1) the uncoupled O−D vibrational modes of HDO in H2O between 10 and 170 K and (2) the THF ring breathing mode (predominantly C−C−C−C stretches) between 10 and 170 K. The low-frequency host lattice modes and the THF C−H stretching modes at 10 K are also presented. The spectral line attributed to the C−C−C−C vibrational mode of the enclathrated THF molecules broadened, and a pronounced shoulder developed on the low-frequency side as the temperature decreased. This anomalous behavior is attributed to the inhibition of rapid reorientational motion of the enclathrated THF molecules at T < 100 K. Analysis of the uncoupled O−D vibrations indicates that the frequency increases (and thus the hydrogen bond strength decreases) with increasing temperature. The vibrational frequency of the u...