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

Density functional theory and ab initio calculations have been carried out to investigate the stability of different tautomers of 2-hydroxy and 2,3-di hydroxypyrazine in the gas phase as well as in different media. Intramolecu lar 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 st ability. The data obtained indicate 2-hydroxypyrazine to be more stable tha n its 2-keto tautomer by about 3 kcal/mol in the gas phase, while in soluti on the stability order is reversed. For 2,3-dihydroxypyrazine, the intramol ecular hydrogen bonded hydroxyoxo and the alpha-diketo tautomers are the mo st 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-hydroxypyr azine and 2,3-dihydroxypyrazine are in a good agreement with experimental f oundations. (C) 1999 Elsevier Science B.V. All rights reserved.