The Motion of Guest Molecules in Clathrate Hydrates

Abstract

The reorientation of molecules encaged in hydrates of structures I and II is controlled mainly by the cage geometry and the electrostatic fields of the water molecules. Departure from spherical symmetry of the short-range interactions between the guest molecule and the water molecules which form the cages leads to preferred orientations which become increasingly occupied at low temperatures. A modified Lennard-Jones potential energy calculation shows the preferred positions of Cl2 to lie off-center and close to the equatorial plane of the tetrakaidecahedral cage with the Cl atoms near the axial planes of symmetry. In an attempt to account for the remarkable reorientational freedom of polar guest molecules, the electrostatic fields of the water molecules are treated in detail. It is shown that the geometry of the cages causes the sum of the fields of the cage water dipoles to almost vanish at points near the cage center, but that this is not true of the resultant quadrupolar fields. It is suggested that the quadrupolar fields are also relatively small because the hydrogen bonding to four neighbors considerably reduces the quadrupole moment of the water molecule. The orientational disorder of the water molecules results in a wide distribution of resultant electrostatic fields in different cages. This appears to be the origin of the great width of the dielectric absorption observed at low temperatures in tetrahydrofuran hydrate and of the wide temperature range of variation of the proton magnetic resonance (p.m.r.) line width in tetrahydrofuran deuterate.