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-acetylacetone (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 Becke functional following the Density Functional Theory. It was found that the most stable conformers are always those stabilized by hydrogen bridges and characterized by C-s symmetry, as the parent acetylacetone. 4TFAA is about 4 kJ/mol favored with respect to 2TFAA. The withdrawing effect of the trifluoromethyl group(s) produces a decrease in the hydrogen bond strength (E-HB) With respect to that of acetylacetone; in any case such strength is enhanced when 3-substituent groups are able to increase the conjugation of the system or to cause strong steric effects. In several open conformations the 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.