Rotational-dynamics of axially-symmetrical solutes in isotropic liquids .1. A collective cage description from molecular-dynamics simulations

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 Cl2 near the triple point. The collective cage variables are well described by the potential acting on an arbitrary molecule (i.e. solute) for a fixed configuration of the other molecules (i.e. solvent). 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 (∼15–20 kBT) 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. τcage(2)=1.4 ps, and τsolute(2)=1.2 ps). Solute librations within the cage are much faster (τ libr(2)=0.12 ps) and are comparable to the relaxation of the relative solute displacements (τr=0.15 ps). The solute angular momentum exhibits the fastest correlation time (τ J=0.06 ps). While the orientational cage potential shows rapidly and slowly relaxing components (τωf=0.14 ps and τωs=2.87 ps, respectively), its dominant portion shows a very long persistence.