Abstract: The geometries of 3-X-2(1H)-pyridones(X=NO₂, NH₂, COOH) and their tautomers in the gas phase and DMSO solution(SCRF) have been obtained at HF/6-31G^**and MP₂/6-31G^**levels. The influences of intramolecular hydrogen bond on the tautomeric equilibrium have also been investigated. In the gas phase, the calculated results show that the enol form is more stable than the keto form for 3-NO₂-2(1H)-pyridone, and the keto?enol equilibrium shift right due to the formation of intramolecular hydrogen bond between 3-nitro and 2-hydroxyl group, while for the amino and carboxyl-derivatives, the keto forms are favored over the enol forms because of the stronger intermolecular hydrogen bonds in keto-tautomers. However, owing to the polarization effects of solvent, it can be predicted that the keto-tautomers are dominant for all of the title compounds in DMSO solution, which are in good agreement with the experimental results.

Key words: Substituent effect, Tautomerism, Intromolecular hydrogen bond, Self-consistent reaction field, Ab initio calculation