The Rotational Oscillations of Tetrahydrofuran in Tetrahydrofuran Clathrate Hydrate

Abstract

The absorptivity of tetrahydrofuran clathrate hydrate in the range 70–7 cm−1 has been measured at several temperatures in the range 17–80 K. There are two broad bands with maxima at 25 and 38 cm−1 which are due to the rotational oscillations of tetrahydrofuran molecules in their cages. The integrated absorptivity yields an effective dipole moment for the oscillation of 1.63 D, which is close to the gas-phase value. The negative second moment of the absorptivity yields the contribution 0.105 ± ~0.007 to the low-frequency refractive index, in good agreement with a less accurate value from dielectric measurements. The orientational disorder of the water molecules causes a distribution of potentials hindering the rotational oscillations of the guest molecules, and a detailed analysis of the shapes of the bands yields directly the distribution of force constants for the oscillations.