Kinetics in highly viscous solutions: Dynamic solvent effects in 'slow' reactions

Abstract

In combination with viscous liquids, high pressure was effectively used to study dynamic solvent effects on the rate of geometrical isomerization about a carbon-nitrogen and a nitrogen-nitrogen double bond in the molecules at their electronic ground (S0) state. All of the reactions were slow enough to guarantee the validity of the transition state theory (TST) at atmospheric pressure and the TST-expected pressure effects were observed. However, strong pressure-induced retardations replaced such effects at high pressures. These retardations were concluded as an indication of a shift of the reaction from the equilibrium TST-valid region to the nonequilibrium TST-invalid region. In other words, slow thermal fluctuations of the solvent molecules at high pressures made it possible to observe dynamic solvent effects on unimolecular S0-state reactions. Separation of the static pressure effects from the dynamic solvent effects were performed by extrapolations of the rate constant in the TST-valid region to the TST-invalid one. This operation allowed us to make a direct comparison of the observed rate constant with that expected from TST. The results strongly suggest a necessity to separate the chemical and the medium coordinates in the reaction systems studied. © 1999 IUPAC.