Electron-spin relaxation and molecular dynamics in liquids. II. Density dependence

A pressure‐dependent ESR relaxation study of the probe PD‐tempone dissolved in toluene‐d8 is described. Extensive results on rotational relaxation, in which ${\tau }_R\mathrm{(T,P)}$ is varied over more than two orders of magnitude, are presented. These results are found to be inconsistent with a simple Stokes–Einstein type η/T behavior modified with a nonzero intercept. However, the data were successfully fit by the empirical form ${\tau }_R\mathrm{=\; c\eta \beta (\rho -p̄)/T,}$ where β is the isothermal compressibility, c is a constant, and p̄ is an (empirical) reference density whose inverse can be thought of as an ’’expanded volume’’. Thus, ρ−? is a measure of the strength of the anisotropic intermolecular interactions acting on the probe, while ${\beta }^{\text{−1}}$ may be thought of as a measure of the total intermolecular interactions. These results for a solute of molecular size somewhat greater than that of the solvent molecules exhibit some differences when compared to a previous NMR study on neat toluene‐d8. The ε parameter introduced by Freed and co‐workers to fit their nonsecular spectral densities is found to be independent of temperature and pressure, and it is pointed out that this could be consistent with an intermolecular fluctuating torque model.