Optical refraction by clathrate hydrates

Abstract

The first known measurement of the refractive index of a clathrate hydrate, the structure II hydrate of tetrahydrofuran, is reported in this paper. The Lorentz-Lorenz equation underestimates the contribution of the guest molecule to the refractivity. Inclusion of the reaction field correction for a molecule in a cavity the size of the cage occupied by the guest molecule gives better agreement. The refractive index predicted for air hydrate, now known to be structure II, is slightly larger than for ice, in agreement with observation of air hydrate trapped in Greenland ice. Hydrate compositions may be determined from refractive indices and from accurate values of the unit cell dimensions.