ANALYSIS OF MICROVISCOSITY AND REACTION COORDINATE CONCEPTS IN ISOMERIZATION DYNAMICS DESCRIBED BY KRAMERS THEORY

In this work we have studied the isomerization dynamics of a cyanine dye molecule in solution. The viscosity and temperature dependencies of the isomerization rate have been measured in the series of n-alcohols for three different sizes of the isomerizing group. From these measurements we conclude that the shear viscosity of the solvent is not a good measure of the microscopic friction experienced by the isomerizing groups. The friction is varying in a nonhydrodynamic manner with viscosity, which shows that the relative volume of the isomerizing group and solvent molecules (V(p)/V(s)) is a critical parameter determining the microscopic friction. When the microscopic friction is calculated using a model for molecular rotational relaxation proposed by Dote, Kievelson, and Schwartz [J. Phys. Chem. 85, 2169 (1981)], good fits to Kramers' equation is obtained. Similar models for microscopic rotational and translational friction combined with Kramers' equation also yield an apparent improvement over the hydrodynamic Kramers description. The measurements also show that the non-Kramers behavior of the reaction rates have a more complex origin than the (V(p)/V(s)) dependence of the microscopic friction, that possibly can be traced back to a more general failure of the hydrodynamic description of friction (frequency dependent friction), or to a temperature and solvent dependence of the potential surface parameters. The results also suggest that the detailed nature of the reaction coordinate plays an important role in determining the detailed viscosity dependence of the isomerization. Thus a reactive motion mainly experiencing rotational friction is much more sensitive to the molecular size and free-volume effects, than is the isomerization controlled by translational friction.