Molecular motions of fluorobenzene-d5 in the dense fluid region

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Abstract
The angular position correlation time τθ and the angular momentum correlation time τJ have been determined in fluorobenzene‐d5 in the dense fluid region in the temperature range T/Tc from 0.5 to 1.1 and the density range ρ/ρc 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 τθ and/or τJ. Our finding that the angular momentum correlation time τJ increases with increasing temperature at constant density is of particular interest. As τJ is related to the time between collisions, the cell model and the Enskog theory are used to calculate the theoretical τJ. In order to obtain the correct prediction of the observed temperature changes of τJ 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 τθ and viscosity is also presented.