Tautomeric Properties and Gas-Phase Structure of Acetylacetone

The tautomeric and structural properties of acetylacetone, CH₃C­(O)­CH₂C­(O)­CH₃, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). The analysis of GED intensities resulted in the presence of 100(3)% of the enol tautomer at 300(5) K and 64(5)% of the enol at 671(7) K. The enol tautomer possesses <i>C</i>_<i>s</i> symmetry with a planar ring and strongly asymmetric hydrogen bond. The diketo form possesses <i>C</i>₂ symmetry. The experimental geometric parameters of both tautomeric forms are reproduced very closely by B3LYP/aug-cc-pVTZ and MP2/cc-pVTZ methods