Proton Exchange Between 3,6-Di-Tert-Butyl-2-Hydroxyphenoxyl and Dicarboxylic Acids Studied by Electron Spin Resonance Spectroscopy and Density Functional Theory Calculations.

Abstract

The reaction of intermolecular proton exchange between the spin probe 3,6-di-tert-butyl-2-hydroxyphenoxyl and some dicarboxylic acids, such as oxalic, maleic, succinic, and adipic, is studied. The experimental spectra of the radical with acids are recorded using dynamic electron spin resonance (ESR) spectroscopy. The studies are carried out at different temperatures in toluene. The rate constants of intermolecular proton exchange are determined by modeling the ESR spectra of the radical using equations based on the modified Bloch equations and the four-site jump model. It follows from the kinetic data that with an increase in the acid's carbon chain, the reaction rate slightly decreases, and the value of the activation barrier of the reaction increases. Analysis of the kinetic data using the Bell-Limbach model made it possible to calculate the components of the reaction's energy barrier. Quantum chemical calculations of the studied systems using the density functional theory B3LYP and the 6-31+G(d,p) basis set show that the proton exchange mechanism is a cooperative two-proton exchange. Calculations using the XMCQDPT/SA(4)-CASSCF(1,4)/6-31+G(d,p) method also show that excited states do not affect this process. This methodology can be used to determine the rate constants of intra- and intermolecular processes involving various OH- and NH-acids.