A theoretical study of proton transfer dynamics in complexes of some substituted benzoic acids with 3,6-di-tert-butyl-2-hydroxyphenoxyl (DTBPO) radical is presented. To elucidate the transfer mechanism, reaction pathways for various complexes of benzoic acid derivatives with DTBPO were modeled. The calculations were performed by the DFT method at the UB3LYP/6-31G+(d, p) level of theory using QST3, IRC proce-dures, in vacuum and toluene medium (PCM solvation model). Geometric and kinetic parameters of com-plexes with o-, p-, and m-isomers of nitrobenzoic and chlorobenzoic acids were calculated. Theoretically es-timated activation barrier of 29-30 kJ/mol turned out to be approximately 30 % higher than the previously ob-tained experimental data. It was noted that in the case of o-isomers of aromatic acids the coplanarity of the transition state structure is violated, in contrast to the initial state. This leads to a change in the proton transfer mechanism. The dynamics of charge distribution, dipole moment and electrostatic potential in the studied complexes were analyzed. Based on the calculated data, it was shown that the studied intermolecular proton exchange process occurs throughout Hydrogen Atom Transfer (HAT), and not throughout Proton-Coupled Electron Transfer (PCET) mechanism.

Quantum chemical studies of the protolytic ability of some dicarboxylic acids are carried out. The geometric and kinetic parameters of the dimeric molecules of maleic, succinic, tartaric, oxalic, and adipic acids are in-vestigated. The dimerization energies of these substances are determined by considering the basis set super-position error (BSSE). The effect of the presence of a carbon skeleton, unsaturated bonds, and hydroxy sub-stituents on the dicarboxylic acids kinetic parameters is confirmed. The frontier molecular orbitals of the studied dimeric acids molecules are considered and the HOMO-LUMO energy gap is determined. The ob-tained values of the energy gaps show an increase in the stability of a number of cyclic compounds formed by the participation of two hydrogen bonds. The ability of the acids to form complexes with the 3,6-di-tert-butyl-2-hydroxyphenoxyl semiquinone radical is studied. The effect of the nature of the solvent on the activation barrier of the complexation reaction of the semiquinone radical — dicarboxylic acid system is analyzed using the CPCM and IEFPCM models. The dependence of the energy parameter on the solvent polarity is estab-lished using the examples of toluene, tetrahydrofuran, and nitrobenzene. The DFT method at the B3LYP lev-el, together with the 6-31+G (d, p) basis set, is used to optimize molecular structures. The calculations are carried out using the Gaussian 16 Revision A.03 WIN64.