Resonance assisted hydrogen bonding and dynamic mechanism for crystal disorder in the enolic form of acetylacetone: a theoretical analysis

We report a theoretical analysis of the electronic structure of the enolic form of acetylacetone with the B3PW91, MP2 and CCSD methods using the aug-cc-pVDZ basis set. Although both the GIAO-NICS values and the current density maps display a clear non-aromatic character for cis-2-enol of acetylacetone, C2A, and for the TS for the interconversion of two equivalent cis-2-enols of acetylacetone, TSA, an NBO analysis reveals an important stabilising conjugation stretching all over the skeleton of heavy atoms. The stabilization of TSA by resonance makes possible the dynamic process producing the crystal disorder observed in X-ray diffraction.