A complete conformational analysis of the azo-enol, oxo-azo, and hydrazo forms of 3-phenylazo-2,4-pentanedione (PAPD), a b-dicarbonyl compound, was carried out by ab initio calculations, at the density functional theory (DFT) level. In addition to nine stable azo-enol conformers, three hydrazo and two oxo-azo were also obtained. The considerably higher energy of the oxo-azo and azo-enol compared to that of the most stable hydrazo conformer makes the presence of oxo-azo and azo-enol form, at least in the gas phase, unlikely. Harmonic vibrational frequencies of this stable hydrazo form and its deuterated analog were also calculated and compared with the experimental data. According to the theoretical calculations, the intramolecularly hydrogen bonded proton in the most stable tautomer moves in an asymmetric double minimum potential with a hydrogen bond stronger than that of 4-amino-3-penten-2-one (APO). The calculated O...N distance for the most stable conformer of PAPD is about 0.07–0.08 ÅA 0 shorter than that of APO.

Keywords