NMR behavior of the clathrate hydrate of tetrahydrofuran. I. Proton measurements

Abstract

The motion of guest THF and host water molecules has been studied to 2°K by proton CW measurements and by proton T 1 measurements of the THF-H 2O, THF- d 8-H 2O, and THF-1320 systems. Reorientation rates of THF and water molecules and the corresponding activation energies (0.92 and 7.2 kcal/ mole, respectively) agree well with dielectric results, as does the NMR evidence of a very broad distribution of orientational correlation times of THF molecules. The latter is attributed to the variability of electrostatic fields in different cages arising from disordered orientations of the water molecules. Second moments show that THF molecules effectively undergo isotropic reorientation near 250°K and become rigid only below 10°K. The exponential magnetization recovery curves observed at all temperatures show guest and host protons to have common spin temperatures. T 1 ϱ measurements of THF-d 8·17H 2O suggest that both reorientation and diffusion of water molecules contribute to relaxation, the former being dominant at lower temperatures in contrast to the case in ice Ih.