Tautomeric and Conformational Properties of Methyl Acetoacetate, CH₃OC(O)−CH₂−C(O)CH₃:  Electron Diffraction and Quantum Chemical Study

The tautomeric properties of methyl acetoacetate, CH₃OC(O)−CH₂−C(O)CH₃, have been investigated in the gas phase by gas electron diffraction (GED), IR(matrix) spectroscopy, and quantum chemical calculations (the MP2 approximation with 6-31G** and 6-311G(2df) basis sets and the B3LYP method with 6-31G** and 6-31++G** basis sets). GED results in a mixture of 80(7)% enol tautomer and 20(7)% diketo form at 309(6) K. Only one enol form with the O−H bond adjacent to the methyl group, CH₃OC(O)−CHC(OH)CH₃, is present. The GED analysis cannot distinguish between the three diketo conformers which are predicted by quantum chemical calculations. IR(matrix) spectra confirm the presence of a mixture of enol and keto forms with the enol tautomer strongly prevailing. Quantum chemical calculations with the MP2 approximation predict a very small contribution of the enol tautomer, in contrast to the experiments. B3LYP calculations, however, reproduce the experimental tautomeric mixture very closely.