"Two-Step" Mechanism in Single-Step Isomerizations. Kinetics in Highly Viscous Liquid Phase

The kinetic effects of pressure on the thermal Z/E isomerization of substituted azobenzenes and N-benzylideneanilines were studied in a viscous solvent, glycerol triacetate. In the lower pressure region (P ≦ 200 MPa (=2000 bar)), the pressure effects observed were in accordance with the transition-state theory (TST). However, at higher pressures, all of the reactions studied were retarded relatively strongly by an increase in pressure, which increases the solvent viscosity. The observations were analyzed by assuming the existence of two steps in the reaction. In the first step, molecular arrangement in the solvated state is transformed during solvent diffusive fluctuations to form the intermediate state M; this step was considered to be slowed by the increase in viscosity. In the second step, the energy-barrier crossing toward the product surface takes place, and this process was assumed to be little influenced by the viscosity. In this two-step mechanism, the rate constant in the steady state can be given by 1 /(k_TST^-1+ k_f^-1), where k_tstis the TST-expected rate constant and k_fthe fluctuation-limited one. The rate constants observed nicely fit this formula, where the k_fvalues thus obtained gave linear Arrhenius plots with a fractional-power-law dependence (k_f∝ η^-β, 0 < β < 1) on the viscosity η. Compared at the same viscosity, the k_fvalues were almost independent of the reaction temperature in most of the reactions studied, suggesting that the steric transformations of the reactant generally remain minimal in the first step.