Dielectric and nuclear magnetic resonance properties of ethylene oxide hydrate to 2 K

Abstract

Dielectric and proton magnetic resonance properties of the clathrate hydrate of ethylene oxide (EO) between 2 and 270 K are reported. Arrhenius activation energies of reorientation are 32.4 kJ/mol for the water molecules, measured above 190 K, and 5.9 kJ/mol for encaged EO, measured below 40 K. The contribution of EO molecules to the equilibrium permittivity declines below 50 K. At 2 K the permittivity (4.1 ± 0.3) includes a large contribution from rotational oscillations of EO. The second moment of the proton absorption of EO in D₂O hydrate reaches the rigid lattice value (11.4 ± 0.4 G²) only below 5 K, where a complex line structure from interaction of the four magnetically-isolated protons is resolved. Above 230 K, EO molecules effectively undergo isotropic rotation. The width of the dielectric absorption and the large temperature range of n.m.r. line-narrowing suggest considerable variability in the rotational freedom of EO in different cages, as expected from the presence of orientational disorder of the water molecules.