Rotational dynamics of axially symmetric solutes in isotropic liquids. I. A collective cage description from molecular dynamics simulations

Abstract

An operational definition of collective cage variables previously introduced for liquid argon is extended, via a molecular dynamics study, to the rotational properties of axially symmetric molecules. Quantitative measures of the static and dynamic cage properties are extracted for liquid Cl 2 near the triple point. The collective cage variables are well described by the potential acting on an arbitrary molecule ͑i.e. solute͒ for a fixed configuration of the other molecules ͑i.e. solvent͒. A dynamic separability of the solute orientation relative to the cage potential and of the relative solute displacement is justified in part by the faster relaxation found for the latter. Large and persistent orientational cage potentials ͑ϳ15-20 k B T) lead to substantial alignment of the solute in the cage with an average local order parameter of 0.87. The reorientational correlation times for the cage are consistent with axially symmetric Brownian motion. The reorientational correlation times for the solute are nearly equal to the equivalent ones of the cage, consistent with the strong coupling of solute within its cage which leads to a collective reorientation of solute and cage ͑e.g. cage (2) ϭ1.4 ps, and solute (2) ϭ1.2 ps͒. Solute librations within the cage are much faster (libr (2) ϭ0.12 ps͒ and are comparable to the relaxation of the relative solute displacements (r ϭ0.15 ps͒. The solute angular momentum exhibits the fastest correlation time (J ϭ0.06 ps͒. While the orientational cage potential shows rapidly and slowly relaxing components (f ϭ0.14 ps and s ϭ2.87 ps, respectively͒, its dominant portion shows a very long persistence.

Time resolved four- and six-wave mixing in liquids. II. Experiments

“…On a longer time scale the thermal motion of the surrounding molecules causes the decay of this orientational anisotropy by diffusion. A theory that is close to this picture of liquid state dynamics is the itinerant oscillator model, [43][44][45] which was initially developed for the explanation of FIR absorption data. In this model, the optical field͑s͒ induce motions of an optically active oscillator which is coupled to an overdamped collective mode, modeling the many-body ͑diffusive͒ dynamics of the surrounding molecules.…”

CW-EPR Spectral Simulations

“…The SLE approach has been extended by the Freed group to a more general model that allows for motion of the environment that imposes local order on the spin label, as is the case for slowly tumbling spin-labeled proteins Polimeno, Moro, & Freed, 1995). That is, the tethered spin label is constrained by an ordering potential that is defined relative to the macromolecule, but the macromolecule itself is undergoing rotational diffusion on a slower timescale than the local label motion.…”

Dynamics of liquid benzene: A cage analysis

“…25 As in the cited papers, we base the cage analysis of liquid benzene here presented on MD simulations. 25 As in the cited papers, we base the cage analysis of liquid benzene here presented on MD simulations.…”

“…25 In both cases, it was possible to parametrize the collective cage variables by applying some simplifying assumptions on the instantaneous potential acting on probes, taking advantage of the strong confining effects by extracting the dominant harmonic terms with respect to the positional and orientational degrees of freedom. 25 In both cases, it was possible to parametrize the collective cage variables by applying some simplifying assumptions on the instantaneous potential acting on probes, taking advantage of the strong confining effects by extracting the dominant harmonic terms with respect to the positional and orientational degrees of freedom.…”