Frictional effects on barrier crossing in solution: Comparison with the Kramers’ equation

In our efforts to examine the validity of the Kramers’ equation, the rate constants of the excited state isomerization of 1,1′-binaphthyl in n-alkane solvents were measured at room temperature using picosecond spectroscopy. These data, and data measured previously in n-alcohols, were compared with Kramers’ model using two forms for the friction. When a hydrodynamic model for the friction was used, good agreement was found for the alcohol data only. When the isomerization friction is assumed to scale linearly with the friction for overall reorientational motion, we find excellent agreement for both the alcohol and alkane solvents. In addition, the friction in alkanes is found to be considerably larger than that of alcohols of comparable viscosity. This provides a direct indication that the molecular aspects of the solute–solvent interaction play a role in the barrier crossing process.