Proton magnetic relaxation studies of various guest molecules in clathrate hydrates

The rotational dynamics of guest molecules in clathrate hydrates is studied by varying both the cage geometries of the host lattice and the properties of the cyclic guest species, as dipole moments, diameters, and moments of inertia. Proton spin-lattice relaxation rates have been obtained over a temperature range 20-263 K. The relaxation is caused by intramolecular dipole-dipole interactions only. The asymmetric relaxation rate versus inverse temperature curves have been fitted by using the Davidson-Cole spectral density J(omega tau, beta). Shape parameters beta between 0.1 and 0.3 are found for the reorientation of guests in their cages. Correlation times lying between 2.57 x 10(-12) and 4.97 x 10(-10) s and low activation energies indicate high guest mobility due to weak van der Waals interactions between guest and host. The reorientation of the guest molecules depends on the different cage types and guest geometries as well. The motions are specific for each cage type. For different guest molecules in the same cage type the correlation times increase with increasing diameters and moments of inertia of the molecules.