PROTON AND DEUTERON SPIN-LATTICE RELAXATION-TIMES IN HD-AR MIXTURES -A COMBINED EXPERIMENTAL AND THEORETICAL-STUDY

A combined experimental and theoretical study of the nuclear magnetic resonance (NMR) spin-lattice relaxation times for the proton and deute ron of HD in HD-Ar mixtures is presented. Spin-lattice relaxation time s for the proton and deuteron of HD in HD-Ar mixtures have been measur ed over the temperature range 180-420 K at several densities and mole fractions, and extrapolation to infinite dilution has been carried out . Theoretical values of the spin-lattice relaxation times associated w ith the HD-Ar interaction have been calculated using the XC(fit) poten tial energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105 , 2639 (1996)], transformed to allow displacement of the center-of-mas s of the HD molecule from its center-of-force. Both experimental and t heoretical results show that the density-dependence of the deuteron re laxation times lies in the linear regime, while that of the proton lie s in the non-linear regime. The experimental and theoretical results f or the relaxation times of the proton are in excellent agreement. The corresponding results for the deuteron are in good agreement (within a few percent); the agreement is, however, not as good as it is for the proton. These results indicate that the transformed XC(fit) potential energy surface represents the anisotropic part of the HD-Ar interacti on rather accurately. It is argued that the improved quality of the XC ( fit) potential energy surface for the heteronuclear HD-Ar interactio n is due to the large contribution that the isotropic part of the homo nuclear potential surface makes to the anisotropic part of the heteron uclear potential surface. (C) 1998 American Institute of Physics. [S00 21-9606(98)00615-1].