Pressure Effect on the Coupling Between Rotational and Translational Motions of Monosubstituted Benzenes in the Liquid State

The pressure dependence of the deuteron spin‐lattice relaxation times has been measured at 30°C in liquid pyridine‐4‐d₁, fluorobenzene‐4‐d₁, toluene‐4‐d₁, chlorobenzene‐4‐d₁, bromobenzene‐4‐d₁, and benzylcyanide‐4‐d₁. Viscosities at high pressures have also been measured at 30°C. In order to obtain information about the anisotropy of the intermolecular potential energy, the relaxation data were interpreted in terms of the parameter $\kappa$ introduced originally by Kivelson. There is a correlation between the magnitude of $\kappa$ and the van der Waals volume of the substituent on the benzene ring. From the results obtained it follows that molecular shape plays a decisive role in determining the degree of coupling between the rotational and translational motions in the molecular liquids investigated. Since the pressure dependence of the deuteron T₁ has also been determined for fully deuterated pyridine, fluorobenzene, chlorobenzene, bromobenzene, and nitrobenzene, comparison of these data with those obtained for the monodeuterated compounds allowed us to estimate the anisotropy of the reorientational motions. In agreement with our previous results, we have found that the pressure dependence of the intermolecular proton T₁ followed closely the viscosity changes in pyridine and fluorobenzene. Theoretical and experimental activation volumes have been determined both for the rotational and translational motions of the molecules investigated. A comparison of dielectric relaxation data with the NMR data indicates that the molecules reorient by small‐step diffusion.