CALCULATION OF THE TAUTOMERIC PROPERTIES OF 3-FORMYL-TETRINIC ACID BY THE AB INITIO AND DFT METHODS

Abstract

We have carried out a nonempirical quantum‐chemical calculation with full optimization of the geometry of all theoretically possible tautomeric forms of 3‐formyl‐tetrinic acid (FTRA) according to the Moller–Plesset second‐order perturbation theory with the use of a 6–31G(d) double zeta basis set. The correlation corrections to the total energy of molecules for optimized geometric configurations were calculated in the 6‐31G(d,p) basis. All the possible tautomeric forms of FTRA and the harmonic vibrational frequencies were also calculated within the density functional theory (DFT) with the use of the Perdew–Burke–Ernzerhof (PBE) functional in a three‐exponent basis. It is shown that FTRA in vapors (in the perfect gas approximation) exists in the form of a mixture of three enol forms, among which two exo‐forms predominate. The frequencies and forms of normal vibrations for each cis‐enol tautomer in the region of vibrations of keto‐groups and double bonds differ widely, which permits identification of the tautomers present in the mixture. The possible mechanisms of enol‐enol transformations of 3‐acyltetrinic acids are discussed.