Effect of Pressure on the Intramolecular and Intermolecular Contributions to Dipolar Spin Relaxation in Benzene and Chlorobenzene in the Liquid State

The pressure dependence of the proton spin–lattice relaxation times has been measured at 23°C in liquid benzene and its mixtures with benzene‐d₆ up to 600 bar, and in liquid chlorobenzene and its mixtures with chlorobenzene‐d₅ up to 2000 bar. The separation of the intramolecular and intermolecular contributions to proton dipolar relaxation allowed us to study the effect of pressure on the reorientational and translational motions of the molecular liquids investigated. As predicted by the Bloembergen–Purcell–Pound (BPP) theory the intermolecular $T_1$ were found to follow the changes in the viscosity with pressure. On the other hand, the experimental pressure dependence of the intramolecular $T_1$ in benzene differs widely from the BPP theoretical prediction due to the relatively free rotation about the $C_6$ axis in benzene. Removal of the $C_6$ axis and introduction of a dipole in chlorobenzene result in agreement between theoretical and experimental intramolecular $T_1$ pressure dependences. As an example of a molecule with internal rotation some of the previous results on acetone were also discussed. The results of this study show that the molecular symmetry plays an important role in determining how well the Debye–BPP model describes the rotational motion. From molecular symmetry one can also infer on the degree of coupling between rotational and translational motions in liquids. Activation volumes for the intramolecular and intermolecular $T_1$ were calculated and compared with corresponding activation energies. ${\mathrm{(\Delta V‡)}}_{\mathrm{intra}}{\mathrm{\hspace{0.17em}\ll \hspace{0.17em}(\Delta V‡)}}_{\mathrm{inter}}$ in benzene and acetone, whereas ${\mathrm{(\Delta V‡)}}_{\mathrm{intra}}{\mathrm{\hspace{0.17em}\cong \hspace{0.17em}(\Delta V‡)}}_{\mathrm{inter}}$ in chlorobenzene. The data obtained also indicate that in many cases the activation volumes are pressure dependent.