AB INITIO STUDY OF THE PROTOTROPIC TAUTOMERISMOF OXIPYRIDINES
In this paper, the tautomerism of oxypyridines was considered in terms of the preference of their prototropic forms, using a non-empirical quantum chemical method, taking into account the electronic correlation. The effects of electronic correlation were taken into account in the approximation of the electron density functional (DFT) of the Kohn-Sham using the exchange-correlation potential of Becke-Lee-Yang-Parr (b3lyp), as well as the theory of Möller-Plesset perturbations of the second order (MP2) in the standard valence-split Popl basis STO-6-311G** taking into account polarization p-functions on hydrogen atoms and d-functions on heavy atoms of the second period. Conducted research prototropic tautomerism of 2-, 3 - and 4-oxypyridins was made both in the gas phase and aqueous solution. To simulate the aquatic environment, the most common model of a polarized continuum was used. The electronic and geometry structure of tautomeric forms was established, the analysis of the total energy of the compounds was also made, and it was shawn that the position of the tautomeric equilibrium of keto- and oxy-forms was in General agreement with the experimental data. Both models of electronic correlation show a qualitatively similar picture of the tautomer transition energy, but the quantitative estimation of methods can differ quite significantly. On the basis of the analysis of the performed series of 2-, 3- and 4- hydroxypyridine calculations, it can be concluded that the calculations of possible tautomeric forms of oxypyridines, carried out taking into account the electronic correlation in the approximation of the electron density functional show a much better agreement with the experiment than the accounting of corrections in perturbation theory. It is also evident that the use of a polarized continuum model does not always show acceptable reliability for quantifying solvent effects. The transition to tautomeric forms resulting in a violation of the aroma of the pyridine cycle is also extremely unprofitable from the point of view of the energy process. Also, all isomers of hydroxypyridine rehybridization the carbon atoms associated with an oxygen atom, the calculations show that the transition from oxy-form to the corresponding ketone is characterized by significant energy consumption and, therefore, practically unrealizable.