The Role of Solvation in the Kinetics and the Mechanism of Hydroperoxide Radicals Addition to π‐Bonds of 1,2‐Diphenylethylene and 1,4‐Diphenylbutadiene‐1,3

Abstract

A combination of microcalorimetry, the rotating sector method, and ESR at 323 K in the environment of 10 solvents of different polarities was used to measure rate constants of addition of hydroperoxide radicals (HO • 2 ) to π bonds of trans-1,2-diphenylethylene and trans,trans-1,4-diphenylbutadiene-1,3 (k 2 ) and disproportionation rate constants of these radicals (k 3 ). With increasing dielectric constant of the medium, k 2 values increase from 69 to 410 M −1 · s −1 , and k 3 values almost do not change and are in the range of (1.0 ± 0.2) × 10 8 M −1 · s −1 . A linear dependence of logarithm values of rate constants from the dielectric constant of the medium in the coordinates of the Kirkwood-Onsager equation was found that allows to make a conclusion about the effect of nonspecific solvation in the studied systems. The quantum-chemical analysis (NWChem, DFT B3LYP/6-311G**) of the detailed mechanism for HO • 2 addition shows that the influence of the medium polarity reflects the superposition of the effects of nonspecific and specific solvation. The scale of the polar effect will depend on how different solvation energies of the transition and the initial reaction complexes. If a value of the solvation energy of the transition complex is larger than the solvation energy of the initial reaction complex, then the reaction rate should increase with an increase of the solvent's polarity and decrease otherwise. C