Topological analysis of the charge density in short intramolecular O-H•••O hydrogen bonds.: Very low temperature X-ray and neutron diffraction study of benzoylacetone

A study of intramolecular hydrogen bonding in benzoylacetone (l-phenyl-l,3-butadione) has been carried out with 8.4(4)K X-ray data and 20(1)K neutron data. Analysis of the neutron data shows that the hydrogen, between the two oxygens in the keto-enol part of the molecular structure, is asymmetrically placed in a large flat potential well. The charge density obtained from X-ray and neutron data has been analyzed by using multipolar functions and topological methods, which gave evidence of extensive pi-delocalization in the keto-enol group. The multipole populations show that there are large formal charges on the oxygens and the enol hydrogen, which impart polar character to the hydrogen bond. This effect is also evident in the Laplacian and in the electrostatic potential calculated from the X-ray data and it is found that the hydrogen position is stabilized by both electrostatic and covalent bonding contributions at each side of the hydrogen atom. The resonance assisted hydrogen bonding model has been refined to reflect the redistribution of charge.