On the molecular motion in liquid neopentane

Abstract

The molecular dynamics in liquid neopentane at 265°K has been investigated by cold neutron scattering experiments. The neopentane molecule is spherically symmetric, and the ratio of the moment of inertia and the molecular mass is fairly large. The assumption that the rotational and translational modes of motion are uncoupled should therefore be reasonable. Furthermore, the contribution from the torsional motion of the CH3 group is rather well separated from the scattered spectrum owing to the motion of the rigid molecule. For the rigid molecule motion the following result is then obtained: (1) The translational motion is reproduced only by models, which take the encaging effects into account. The more advanced models based on the memory function approach or on the linear response theory can only partly reproduce the observed spectra. A scaling procedure involving the use of liquid argon data fails to describe the liquid neopentane data. It seems that a model somewhere in between the simpler relaxing cage model and the more advanced memory function models should best describe the data. (2) The rotational motion is not quite reproduced by models assuming infinitely short collisions causing small angular reorientations during a period of free or slightly damped rotations between the collision events. It seems clear that in denser media the collision has to be included in the description of the rotational diffusion. The angular velocity autocorrelation function roughly resembles the linear velocity autocorrelation function, except for differing time scales. It therefore seems probable that an acceptable description of the rotational diffusion should involve not only a memory function characterized by the δ function in time but rather a function similar to those produced to describe translational motion in liquids.