Nonempirical modeling of protolytic processes in dimeric molecules of amino acids

Abstract

Quantum-chemical modeling of intermolecular proton exchange in dimers of aminoacetic acid was carried out using an unempirical unrestricted Hartree-Fock method using the 3–21G basis set. Three main structural isomers of dimeric molecules of aminoacetic acid have been quantum-chemically identified. For these isomers, the search for the structure of the transition state of the proton exchange reaction was carried out using the quadratic synchronous transit QST2 (Quadratic Synchronous Transit Approach) procedure. The symmetrical structure of the transition state for the dimer of aminoacetic acid is noted, in the formation of hydrogen bonds of which two carboxyl functional groups are involved. The kinetics and mechanism of intermolecular migration of a proton in the dimer of aminoacetic acid have been studied using the internal IRC (Intrinsic Reaction Coordinate method) method. Curves are obtained for the dependence of the total energy of the reaction system under consideration on the internal coordinate of the reaction. The activation energy of proton exchange in dimers of aminoacetic acid is estimated as the difference in the total energies of the transition and initial states of the reaction system. The minimum value of the activation energy (26 kJ/mol in the forward direction and 34 kJ/mol in the reverse direction) was obtained for the intermolecular proton exchange reaction in the dimer of aminoacetic acid, in the formation of the hydrogen bonds of which two carboxyl functional groups are involved. The maximum value of the activation energy (244 kJ/mol in the forward direction and 236 kJ/mol in the reverse direction) was obtained for the intermolecular proton exchange reaction in the dimer of aminoacetic acid, in the formation of hydrogen bonds of which two amino groups are involved.