Tautomeric properties and gas-phase structure of 3-methyl-2,4-pentanedione

The tautomeric and structural properties of 3-methyl-2,4-pentanedione, CH3C(O)CH(CH3)C(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). Analysis of GED intensities resulted in the presence of 100(3)% enol tautomer at 274(7)K. Quantum chemical calculations predict enol concentrations from 8% at MP2/6-31G(d,p) level to 100% at B3LYP/aug-cc-pVTZ. This would show the failure of the MP2/6-31G(d,p) method. The enol ring possesses CS symmetry with a strongly asymmetric hydrogen bond. The experimental geometric parameters are reproduced very closely by the B3LYP/aug-cc-pVTZ method.