Study of molecular reorientation: Pressure and temperature dependence of deuterium relaxation in liquid CDCl3

Abstract

Deuterium NMR spin‐lattice relaxation measurements have been performed on the neat liquid CDCl₃ over the range 28°C<T8 N/m²). These measurements enable one to determine τ_θ,2, the correlation time for molecular reorientation about the axes perpendicular to the symmetry axis. The data are presented as a function of all three state variables, P, V, and T. An attempt is made to describe the data in terms of various simple models for reorientation including activation and free volume models. Each of these models predicts certain aspects of the data but fails in other areas. In particular, the activation model fails to predict the correct behavior at constant temperature and the free volume theories generally fail to predict the constant volume experiments. Deviations from the Debye and microviscosity theories are also noted. The mean time between collisions τ_coll was calculated from τ_θ,2 assuming both Gordon's J‐diffusion model and the equivalence of τ_coll and τ_J, the angular momentum correlation time. These values of τ_coll were then compared with the predictions of the cell model for liquids in which τ_coll is simply the mean free path divided by the mean velocity. Generally, the data did not fit the cell model; however, at constant pressure, the cell model did predict the changes in τ_coll quite well. Qualitative suggestions are made to account for the discrepancies. Finally, the possibility that H‐bonding exists in CHCl₃ and affects the generality of these results by changing the quadrupole coupling constant and disturbing the dynamics of the system is discussed. It is concluded that these effects are small if they exist at all.