A theoretical study on 2-hydroxypyrazine and 2,3-dihydroxypyrazine: tautomerism, intramolecular hydrogen bond, solvent effects

Abstract

Density functional theory and ab initio calculations have been carried out to investigate the stability of different tautomers of 2-hydroxy and 2,3-dihydroxypyrazine in the gas phase as well as in different media. Intramolecular hydrogen bond has been also considered. Effect of electron correlation on geometries is taken into account at MP2 and density functional approach at B3LYP level. Energy calculations at different levels up to CISD/6-31++G(d,p)//MP2/6-31+G* are reported for accurate estimation of conformational stability. The data obtained indicate 2-hydroxypyrazine to be more stable than its 2-keto tautomer by about 3 kcal/mol in the gas phase, while in solution the stability order is reversed. For 2,3-dihydroxypyrazine, the intramolecular hydrogen bonded hydroxyoxo and the α-diketo tautomers are the most stable species at all theoretical levels in the gas phase. The stability of hydroxyoxo and diketo tautomers is significantly increased in solution due to their higher dipole moments. The data obtained for both 2-hydroxypyrazine and 2,3-dihydroxypyrazine are in a good agreement with experimental foundations.