Studies of reorientational relaxation of pyridine in the liquid state and in CCl4 solutions by depolarized Rayleigh scattering

The depolarized Rayleigh scattering spectra of pyridine in CCl 4 have been measured as a function of pyridine concentration at several temperatures. The Rayleigh relaxation times obtained for pyridine are found to be independent of the pyridine concentration. This result indicates that the orientational pair correlation effect is negligibly small in neat liquid pyridine, and the Rayleigh relaxation time represents the single particle reorientation time. The Rayleigh relaxation time of the pyridine-CCl 4 system is found to vary with η/T with a significant intercept, in contrast to the prediction of the Stokes-Einstein-Perrin equation. Despite the finite intercept, slip boundary conditions for the hydrodynamic equations are applicable for describing the reorientation of pyridine in CCl 4 . Comparison of the present light scattering result with that obtained from the dielectric relaxation measurement suggests that the orientation of the axis perpendicular to the plane of molecule takes place in large angular jumps, in contrast to the small angle rotational diffusion model put forth previously. In view of the absence of orientational pair correlation, large angle reorientation as well as a finite non-zero intercept in the τ Ray versus η/T plot the dynamics of reorientation of pyridine in the liquid state is very analogous to that of benzene.