Raman Spectroscopic Studies of THF Clathrate Hydrate

Raman spectra were collected for tetrahydrofuran (THF) hydrate, a structure II clathrate hydrate (CH) that was prepared from THF and a dilute solution of D 2 O in H 2 O (5% D 2 O) in a 17:1 ratio. The vibrational modes observed include (1) the uncoupled O−D vibrational modes of HDO in H 2 O 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 uncoupled O−D oscillator in hydrogen-bonded systems is sensitive to the hydrogen-bonded O−H- -O length, and thus the highest density of hydrogen bond length in THF structure II clathrate was found to be 2.766 and 2.778 Å at 10 and 170 K, respectively . The distribution of hydrogen bond lengths is also briefly discussed.