Dynamics of chiral molecules in the liquid state: Computer simulation and field effects

Abstract

The molecular dynamics of the $R$ and $S$ enantiomers of fluorochloracetonitrile (HCFC1NC), and of the racemic mixture, are investigated with computer simulation. Some novel single-molecule rotation-translation cross-correlation functions are reported in a moving frame of reference defined by that of the principal molecular moments of inertia. Two off-diagonal elements of these cross-correlation matrices have an equal and opposite time dependence for the two enantiomers and vanish for all $t$ in the racemic mixture. The application of an external electric field to the chiral liquid generates laboratory-frame cross-correlation functions between the molecular center-of-mass translational momentum and the angular momentum of the same molecule. These cross-correlation functions are, therefore, seen in standard spectroscopic investigations of the molecular liquid state. As the intensity of the external field is increased, the following transient and field-on equilibrium effects become observable in the laboratory frame of reference: (i) Orientational rise transients are opposite in sign for the two enantiomers and vanish in the racemic mixture. (ii) These transients are field dependent, the dependence being qualitatively, but not quantitatively, that given by simple diffusion equations such as the Debye equation. (iii) At field-on equilibrium, the Grigolini decoupling effect is confirmed, i.e., the envelope of the oscillatory angular-velocity autocorrelation function decays more slowly than the field-off autocorrelation function at equilibrium as the external electric field strength is increased. (iv) The orientational fall transients are accelerated considerably with respect to the time dependence of the equivalent field-off orientational autocorrelation function. These results are interpreted with reduced-model theory, with use of Kramers equations with nonlinear potential terms.