Ab initio DFT study of the hydrogen bridges in hexafluoro-acetylacetone, trifluoro-acetylacetone and some 3-substituted derivatives

All the theoretically possible keto and enol conformations of hexafluoro-ac etylacetone (HFAA) as well as those of 2-trifluoro- (2TFAA) and 4-trifluoro -acetylacetone (4TFAA), have been investigated at ab initio level by using the 6-31G** basis. The correlation energy was evaluated by means of the Bec ke functional following the Density Functional Theory. It was found that th e most stable conformers are always those stabilized by hydrogen bridges an d characterized by C-s symmetry, as the parent acetylacetone. 4TFAA is abou t 4 kJ/mol favored with respect to 2TFAA. The withdrawing effect of the tri fluoromethyl group(s) produces a decrease in the hydrogen bond strength (E- HB) With respect to that of acetylacetone; in any case such strength is enh anced when 3-substituent groups are able to increase the conjugation of the system or to cause strong steric effects. In several open conformations th e formation of a F... H-O bridge is observed, whose energy was estimated as 10 kJ/mol, at the most. (C) 2000 Elsevier Science B.V. All rights reserved .