Kinetic parameters and geometry of the transition state of decarboxylation reactions of carboxyl and formyl radicals

Abstract

Experimental data on the degradation of carboxyl radicals RCO 2 . → R. + CO2 (where R is an alkyl or aryl) and formyl radicals ROC.O → R. + CO2 (where R is an alkyl or aryl) in the liquid phase were analyzed in terms of the method of crossing parabolas. The kinetic parameters that characterize this decomposition were calculated. Using the density functional theory, the transition state was calculated for the reaction of methyl radical addition to CO2 at the C and O atoms. A semiempirical algorithm was developed for the calculation of the geometric parameters of the transition state for decomposition reactions of carboxyl and formyl radicals and the reverse addition reactions of R. with CO2. The kinetic (activation energies and rate constants) and geometric (interatomic distances in the transition state) reaction parameters were calculated for 37 degradation reactions of various formyl and carboxyl radicals. The activation energies and geometric parameters of the transition state were calculated for the alternative addition reactions of R. to CO2:R. + CO2 → RCO 2 . (where R is an alkyl or aryl) and R. + CO2 → ROC.O (where R is an alkyl or aryl). A linear correlation between the interatomic distance r #(C...C) (or r #(C...O)) in the transition state and the reaction enthalpy ΔH e was found for the degradation reactions of carboxyl and formyl radicals (at br e = const). The enthalpies, activation energies, and interatomic distances in the transition state for the degradation of carboxyl and formyl radicals and their formation were compared.