Proton and deuteron spin-lattice relaxation times in HD–Ar mixtures: A combined experimental and theoretical study

Abstract

A combined experimental and theoretical study of the nuclear magnetic resonance (NMR) spin-lattice relaxation times for the proton and deuteron of HD in HD–Ar mixtures is presented. Spin-lattice relaxation times for the proton and deuteron of HD in HD–Ar mixtures have been measured 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 with the HD–Ar interaction have been calculated using the XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)], transformed to allow displacement of the center-of-mass of the HD molecule from its center-of-force. Both experimental and theoretical results show that the density-dependence of the deuteron relaxation times lies in the linear regime, while that of the proton lies in the non-linear regime. The experimental and theoretical results for 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 interaction rather accurately. It is argued that the improved quality of the XC(fit) potential energy surface for the heteronuclear HD–Ar interaction is due to the large contribution that the isotropic part of the homonuclear potential surface makes to the anisotropic part of the heteronuclear potential surface.