Ionic dynamics in computer simulated molten LiNO3. II. Tumbling and spinning motions of nitrate ions

Abstract

The microdynamics of orientational motion (the tumbling of the symmetry axis and the spinning about the axis) of nitrate ions has been investigated in a molecular dynamics simulation of molten LiNO3. Simple Coulomb potential with Born type repulsion has been adopted and all degrees of freedom of vibrating nitrate ions have been included in the simulation. The relaxation of the orientational correlation functions is studied in the cumulant expansion scheme, and a correlation function which reflects only the spinning motion is separated by the method of Lynden-Bell et al. It is found that the angular momentum correlation function for the perpendicular component C⊥J(t) has a pronounced negative portion, which is clear evidence of librations in a temporary solvent cage. C∥J(t) has a shallow negative lobe. The spinning motion is less hindered and the reorientation is faster. Both the tumbling and the spinning motions are found to be predominantly Markovian. The difference between the tumbling and the spinning motions is examined from the magnitude of the torque impulse as compared to the average angular momentum. The coupling between the environmental structure and the rotational behavior of a nitrate ion has been examined through selectively sampled correlation functions. It is found that the rotational diffusion caused by the tumbling motion is slow and that by the spinning motion is fast if the coordination number remains low.