Radical reactions in organic crystals under pressure: Experiment and theory

Abstract

The kinetics of radical transformations in gamma-irradiated dicarboxylic acid single crystals at a high pressure was studied by EPR spectroscopy. The activation parameters of the detachment of the hydrogen and reaction coordinates were determined. The dependence of the rate constant for the reaction on the pressure and crystal physical parameters was studied. The compression of crystals decelerated the detachment of hydrogen (the activation volumes were positive), and the rate-determining stage of the process was free volume fluctuations responsible for the approach of reagent active centers to each other and their arrangement and orientation along the reaction coordinate on the potential energy surface. The kinetic parameters of the reaction are closely related to the crystal characteristics such as the molar volume, compressibility, and thermal expansion coefficient. A theory describing this relation is suggested. Only part of the free volume was shown to be used for “reaction” fluctuations. The types of reagent motions responsible for the molecular organization of the transition state were identified.