NMR diffusion studies of spherical molecules : tetramethylsilane and buckyballs

The structural properties and hydrodynamic size of a molecule in solution at infinite dilution are connected to its diffusion coefficient through the Stokes-Einstein-Sutherland equation. In this study, buckyballs (C60) and mixtures of tetramethylsilane and CDCl3 which closely approximate spherical molecules in solution were investigated using nuclear magnetic resonance (NMR) self-diffusion experiments. It was found that the change in diffusion coefficient of mixtures of TMS and CDCl3 was only correlated to the viscosity of the solution. The C60 PGSE NMR data was then analysed using various models of obstruction. It was found that the decrease in C60 diffusion with increasing concentration can be explained on the basis of aggregation alone, and thus C60 self-obstruction must be negligible under these experimental conditions.