Molecular conformations and intramolecular hydrogen bonding of 3-formylmalondialdehyde and 3-formylacetylacetone. An ab initio study

All the possible keto and enol conformations of 3-formylmalondialdehyde I and 3-formylacetylacetone II were fully optimized at the ab initio MP2/6-31G** level. The most stable forms, which are the chelate ones, were further studied also with the density functional theory, using the Becke's three parameters hybrid method and the LYP (Lee, Yang and Parr) functional in order to test the effect of correlation energy, coming from different approaches, on the stability order. Although the stability difference (Delta E) is remarkably affected by the approach adopted for its evaluation, all results indicate that both compounds, in gas phase, exist as a mixture of their two chelate tautomers, E-1 (having the formyl C=O group and the C=C double bond of the chelate ring in cis position) and E-3 (formyl C=O and C=C in trans position). The hydrogen bond energy is slightly higher than in the parent compounds malondialdehyde and acetylacetone, in line with the calculated frequencies of the O-H stretching mode and with the shortening of the (OO)-O-... distance. (C) 1997 by John Wiley & Sons, Ltd.