Importance of steric effect on the hydrogen bond strength of malondialdehyde and acetylacetone 3-substituted derivatives. An ab initio study

The molecular geometries of various 3-substituted malondialdehyde and acetylacetone derivatives, in their chelate, open and H-centred conformations, were optimized and the hydrogen bond energies evaluated, in order to estimate the effect of 3-substitution steric hindrance on the hydrogen bond strength. Calculations were performed at the ab initio 3-21G and 6-31G** level, with and without correlation energy inclusion, using the Moller-Plesset approach (where possible) and the B3LYP functional. The frequencies of the O-H stretching mode were evaluated, too. The obtained results indicate that the C-s chelate conformations are the most stable structures. The O ... O distances are rather longer than the values typical of the H-centred conformers, at least in gas phase. The strengthening of the hydrogen bridge, on passing from the parent to the 3-substituted derivatives, is not so relevant as expected on the ground of literature data, the maximum increase being about 21 kJ mol(-1) (in 3-t-butyl-acetylacetone). An increase of the steric effect to stress the O ... O distance caused the breaking of the hydrogen bridge instead of strengthening, so indicating that the literature X-ray geometries (in particular the O ... O distances) are governed, at least partially, by crystalline forces. (C) 1997 by John Wiley & Sons, Ltd.