Effects of solvent fluctuations on the rate of the thermal Z/E isomerization of N-benzylideneanilines in a highly viscous liquid hydrocarbon

The effects of the solvent viscosity on the thermal Z/E isomerization of three substituted N-benzylideneanilines were studied in a nonpolar aprotic solvent, 2,4-dicyclohexyl-2-methylpentane. By increasing the pressure to several hundred megapascals, the viscosity of the reaction system was raised high enough to retard the isomerization. The viscosity dependence of the observed rate constant was analyzed by assuming a two-step mechanism based on the two-dimensional reaction-coordinate model proposed by one of the present authors. The rate constant of this mechanism is given by 1/(k(TST)(-1)+k(f)(-1)) where k(TST) represents the rate constant expected from the transition state theory, while k(f) (>0) represents the part controlled by a solvent rearrangement during thermal fluctuations. The k(f) values were inversely proportional to a fractional power of the viscosity, in agreement with the theory. It was also found that, compared at the same temperature and viscosity, the k(f) values in the present solvent are larger than those in a polar aprotic solvent, glycerol triacetate, and in a polar protic solvent, 2-methyl-2,4-pentanediol, reported earlier.