Hydrogen Atom Transfer from Benzyl Alcohols to -Oxyl Radicals. Reactivity Parameters.

A model for predicting the rate constants of hydrogen atom transfer (HAT) from the α-C-H bond of -substituted benzyl alcohols to -oxyl radicals was proposed. To quantify the factors governing the reactivity of both -oxyl radicals and benzyl alcohols, multivariate regression analysis was performed using various combinations of reactivity parameters. The analysis was based on a 2D array of 35 HAT reactions, the rate constants of which span 4 orders of magnitude. The proposed polyparameter equation approximates the experimental rate constants of reactions with high accuracy using three independent parameters: Brown and Okamoto's substituent constants σ in alcohol molecules and the spin population on O and N atoms in the N-O fragment of -oxyl radicals [calculated by DFT/B3LYP/6-31G(d,p)]. The rate constants of HAT reactions from -substituted benzyl alcohols to a series of aryl-substituted phthalimide--oxyl radicals containing either electron-withdrawing or electron-donating substituents (4-Cl, 4-HOOC, 4-CHO), quinolinimide--oxyl, benzotriazole--oxyl, and violuric acid radicals were experimentally determined at 30 °C in acetonitrile.