Keto-enol tautomerism in pyruvic acid -: Theoretical (HF, MP2 and DFT in the Vacuo) studies

Extended quantum-chemical calculations {HF, MP2, DFT(B3LYP) in vacuo} were performed for neutral pyruvic acid and its enol forms. Among various tautomers-rotamers considered, three keto (Tce, Tte and Cte) and six enol structures (E1-E6) are found to be thermodynamically stable. The stability order for the keto and enol isomers: Tce > Tte > Cte > E1 > E2 > E3, E4, E5 > E6 is the same at each level of computations. The keto Tce structure has the lowest Gibbs free energy (G). The G value of the most stable enol E I structure is larger than those of the three keto structures by a few kcal mol(-1).