On the Role of Viscosity in the Eyring Equation

Abstract Transition‐state theory allows for the characterization of kinetic processes in terms of enthalpy and entropy of activation by using the Eyring equation. However, for reactions in solution, it fails to take the change of viscosity of solvents with temperature into account. A second‐generation unidirectional rotary molecular motor was used as a probe to study the effects of temperature‐dependent viscosity changes upon unimolecular thermal isomerization processes. By combining the free‐volume model with transition‐state theory, a modified version of the Eyring equation was derived, in which the rate is expressed in terms of both temperature and viscosity.