Systematic characterization of transition states for radical decompositions

Abstract

A one-parameter analytical potential energy function for β-bonds in free radicals is described, which accounts quantitatively for their observed stretching frequencies and the position and size of the activation barriers for their fission. It is shown that such a function can be used to characterize a priori the corresponding transition states by assuming that the structural and spectroscopic changes taking place along the minimum energy path track the development of pi bonding rather than following a simple exponential dependence on the reaction coordinate. This procedure, tested by comparing predicted A-factors and isotope effects with experimental data for alkyl radicals and ab-initio calculations on C_2H_5, fully encodes the basic features of radical decomposition reactions and provides a simple, realistic, and self-consistent technique for the estimation of their kinetic parameters.