IS SLOW THERMAL-ISOMERIZATION IN VISCOUS SOLVENTS UNDERSTANDABLE WITH THE IDEA OF FREQUENCY-DEPENDENT FRICTION

Thermal Z|E isomerization of substituted azobenzenes and N-benzylideneanilines takes place slowly after fast photoinduced E|Z isomerization. Its rate constant kobs is smaller than about 10 3S-1 because of a high reaction barrier of about 50 kJ/mol. The pressure dependence of kobs measured in solvents as glycerol triacetate can well be understood in the framework of the transition state theory (TST) at low pressures. At high pressures, however, kobs begins to steeply decrease as the pressure increases, to be more exact, as the solvent viscosity η increases with the pressure, and the reaction enters the non-TST regime. Since the η induced decrease of kobs at high pressures is slower than η-1, it cannot be described by the Kramers theory which regards the reaction as the barrier surmounting by Brownian motions regulated by frequency independent friction. Next, it was adjusted to the Grote-Hynes theory incorporating the idea of frequency dependent friction. The situation of kobs mentioned earlier enabled us to derive, without adjustable parameters, the correlation time τSC among random forces for friction due to solvent microscopic motions in the generalized Langevin equation on which the theory is based. At η̃107 Pa s, we obtained TSC̃1 ms. It is too long to justify the theory, since such a long-time correlation cannot be realized among random forces exerting on the isomerizing moiety with an angstrom dimension. It will also be shown that τSC must be so long unphysically as to be at least much longer than 1 ps even if kobs at low pressures is adjusted to the theory. © 1995 American Institute of Physics.