Diffusional anisotropy of simple sorbates in silicalite

Microcanonical ensemble molecular dynamics simulations are used to study an isotropy in diffusional and related dynamical properties for five spherical sorbates of varying size and polarizability in silicalite. Silicalite has straight and zigzag channels, parallel to the y and x-axes of the unit cell , respectively, which are interconnected in such a way that diffusion along the z direction is possible only by alternation of the sorbate between str aight and zigzag channel segments. Helium, the smallest and most weakly bou nd sorbate, is found to comply most closely with the behavior expected on t he basis of the simple random walk model developed to understand the geomet rical correlation between the principal elements of the diffusional tenser in silicalite (J. Karger, J. Phys. Chem. 1991, 95, 5558). The larger and mo re strongly bound sorbates, Ne, Ar, CH4, and Xe, show significant deviation s from this model. The diffusion of these particles along the z direction i s distinctly subdiffusional with the mean square displacement growing as ap proximate to t(0.8). The randomization and anisotropy parameters for all fo ur sorbates are similar but differ significantly from the predictions of th e random walk model. The relative rates of diffusion along the straight and zigzag channels are more sensitive to the nature of the sorbate than the a nisotropy and randomization parameters. For all five sorbates, the subdiffu sional behavior along the z direction, as well as deviations from the predi ctions of the random walk model, are more pronounced at higher concentratio ns. The anisotropy in the short-time dynamics has been examined by studying the velocity autocorrelation functions and the instantaneous normal mode s pectra. For very short times of less than 0.5 ps, the velocity autocorrelat ion function and its directional analogues are virtually identical, but div ergences are seen by times of the order of 1 ps. The instantaneous normal m ode spectra show the expected correlation between the diffusion coefficient , the Einstein frequency, and the fraction of imaginary modes. There is no significant anisotropy in the INM spectra that is consistent with the behav ior of the velocity autocorrelation functions for short time scales.