Molecular Motion and 19F Relaxation in the Liquid Hexafluorides of Molybdenum, Tungsten, and Uranium

Abstract

In order to study molecular motion in liquid molybdenum, tungsten, and uranium hexafluorides, we have undertaken a 19F relaxation study in these liquids. Measurements of the relaxation times as a function of resonance frequency show that the dominant relaxation mechanism is the modulation of the spin—rotation interaction by the molecular motions. It is shown that the strengths of the spin—rotation interactions, which have not been measured directly, can be deduced very simply from the chemical-shift tensors if some approximations are made. The molecular motion is discussed with the help of a generalized Langevin equation in which it is not required that the particle under consideration be much heavier than the molecules interacting with it. It is thus found that the hexafluoride molecules in the liquid phase can rotate through an angle of about 1 rad at 70°C before their angular velocity changes and that this intermediate degree of rotation is consistent both with the low values of the entropies of fusion, which indicate that the rotations do not occur freely, and with the appearance of rotation—vibration structures in their infrared absorption spectra. The self-diffusion coefficients were also measured as a function of temperature. The relationship between the diffusion coefficients and the longitudinal relaxation times is not in agreement with Hubbard's theory, and this is attributed to the failure of both the Stokes' equation and the rotational diffusion model.