Comparison of and investigation into the size effects on the rotational dynamics of two spherical molecules: CCl4 and C60.

13C spin-lattice relaxation measurements were performed on CCl4 and C60, two spherical molecules, to investigate the role of molecular size on reorientational dynamics. Measurements were taken at common temperatures and in a common environment; deuterated chlorobenzene-d5. Our data indicate that both CCl4 and C6) reorient in the small-step diffusion limit. Correlation times for CCl4 were found to be smaller in CBZ than in the neat and much shorter than predicted by viscosity arguments. We have attributed the higher rotational freedom to higher "free volume" or lower solute-solvent contact probability, in CBZ than in the neat. Experimental tau C values for CCl4 and C60 in CBZ were found to be much different than those predicted by hydrodynamics. Pure inertia differences did not explain the disparities in the correlation times. However, a very reasonable explanation for our experimental findings was found by considering the solvent-to-probe molecular volume ratio and its effect on rotational diffusion. We applied four hydrodynamic-based theories to our data and found that the model by Gierer-Wirtz best duplicated our experimental observations. The success of this model suggests that the relative size of solute and solvent molecules, as well as events at the microscopic level, rather than bulk properties, are excellent descriptors of the factors affecting rotational diffusion.