Molecular motions of fluorobenzene-d5 in the dense fluid region

Abstract

The angular position correlation time τ_θ and the angular momentum correlation time τ_J have been determined in fluorobenzene‐d₅ in the dense fluid region in the temperature range T/T_c from 0.5 to 1.1 and the density range ρ/ρ_c from 0.5 to 3.1. The correlation times are calculated from the experimental NMR fluorine and deuterium spin‐lattice relaxation times measured at temperatures 30–350°C and pressures 1–3500 bar. Since the densities were also determined it is possible to separate the effects of temperature and density on τ_θ and/or τ_J. Our finding that the angular momentum correlation time τ_J increases with increasing temperature at constant density is of particular interest. As τ_J is related to the time between collisions, the cell model and the Enskog theory are used to calculate the theoretical τ_J. In order to obtain the correct prediction of the observed temperature changes of τ_J at constant density, a temperature dependence of the hard sphere diameter is assumed. In agreement with our earlier results we find that the reorientational motion of fluorobenzene‐d₅ is well described by the extended J‐diffusion model. A brief discussion of the density and temperature effects on τ_θ and viscosity is also presented.