The electron density in flavones I. Baicalein

The experimental charge density distribution of 5,6,7-trihydroxyflavone (1), has been determined from high-resolution X-ray diffraction data collected at 100 K. Additionally, high level single-point gas-phase calculations as well as periodic calculations have been carried out for (1). To our surprise, the topological analysis of the experimental electron density revealed a cage critical point in the geometric center of the tri-hydroxy substituted aromatic ring. This feature was independently confirmed by analysis of another study of 1. However, both high-level gas-phase and periodic ab initio calculations failed to reproduce this feature. The degree of bias on the electron density by using the Hansen-Coppens multipole model was tested by a multipole refinement of generated theoretical structure factors. This clearly showed that the cage critical point is not an artefact resulting from the use of the multipole model. Compound 1 contains a large number of weak, intermolecular hydrogen bonds and these are analysed using the Atoms in Molecules (AIM) approach, which leads to quantitative measures for hydrogen bond strength. The experimentally derived lattice energy of -467 kJ mol(-1) shows a rather strongly held crystal lattice.