AM1 INVESTIGATION OF THE MOLECULAR STRUCTURE OF INTRAMOLECULARLY HYDROGEN BONDED ENOL FORMS OF SOME FLUORINATED BENZOCYCLIC beta-DIKETONES

AM1 SCF-MO calculations with full geometry optimization were performed to determine the structure and energetics of the various keto and enol tautomers of some fluorinated benzocyclic β -diketones ; 2- trifluoroacetyl indanone (1), 2-trifluoroacetyl tetralone (2) and 2-trifluoroacetyl benzosuberone (3) comprising five-, six- and seven-membered ring systems, respectively. The presence of two stable rapidly interconverting end cyclic and exocyclic enol forms with unsymmetrical hydrogen bonded chelate ring is established. The exocyclic enol form is favoured in the five-membered ring system (1), whereas the end cyclic enol form is favoured in the six- and seven-membered ring systems (2 and 3). The intramolecularly hydrogen bonded chelate ring is predicted to be planar in the five-membered ring system, nearly planar in the six-membered ring system but deviates from planarity by up to 12o in the Seven-membered ring system. The two carbonyl groups in the keto form of these compounds (1-3) are appreciably non-planar with a torsion angles of 101o, 63o, and 65o, respectively. The relative strength of the intermolecular hydrogen bond and the degree of delocalization of the chelated enol ring in these systems are discussed.