ROTATION OF NONSPHERICAL MOLECULES IN DENSE FLUIDS - A SIMPLE-MODEL DESCRIPTION

Generalized Langevin equations (GLEs) are used to calculate angular ve locity correlation functions (AVCFs) and orientational correlation fun ctions (OCFs) of asymmetric top molecules in the high friction limit. Two exponential memory functions are shown to fit simulated AVCFs sati sfactorily. These memory functions consist of an intense but quickly r elaxed contribution (in-cage rotation) and a small but slowly decaying contribution (due to collective modes). An exact analytical expressio n for an arbitrary rank asymmetric top OCF is obtained up to terms of order O(t(6)). This result allows establishment of the correspondence between the short-time value of the memory function and the averaged m agnitude of the external torque. Explicit formulas are derived for sym metric top OCFs when arbitrary (from slip to stick) boundary condition s are presumed. These formulas are proved to reduce to the conventiona l first cumulant expressions provided that molecular rotation around t he symmetry axis is driven by a Gaussian stochastic process, The exper imentally observed non-Gaussian behavior is suggested to be attributed to the preferential angular velocity reorientation due to a collision , and a convenient analytical model is proposed to describe this effec t.