Dynamic solvent effects on the thermal cyclization of a hexadienone formed from a diphenylnaphthopyran: An example of a system with distinctly separate medium and chemical contributions to the overall reaction coordinate

Abstract

Pressure effects on the rate of thermal fading of a colored hexadienone formed from 3,3-diphenyl-3H-naphtho[2,1-b]pyran were measured at various temperatures in three highly viscous solvents, i.e., 2,4-dicylohexyl-2-methylpentane, glycerol triacetate, and 2-methylpentane-2,4-diol, as well as in their chemically similar nonviscous counterparts, such as methylcyclohexane, methyl acetate, and ethanol. Both experimental data and quantum-mechanical calculations are in agreement with formation of a cyclic activated complex. In the viscous solvents, high viscosities resulted in retardations of solvent thermal fluctuations which, in turn, slowed down the cyclization. The analysis of experimental results unequivocally demonstrated that the dynamic solvent effects in the present systems have to be described by a two-dimensional model that includes separately defined chemical and medium coordinates. Predictions of an alternative one-dimensional model with a diffusion-controlled reaction coordinate are not supported by the experimental data.