Systematic experimental charge density analysis of anion receptor complexes

The first systematic electronic resolution study of a series of urea-based anion receptor complexes is presented. The hydrogen bonding in these multi-component systems was fully characterised using Bader's Quantum Theory of Atoms In Molecules (QTAIM) with the strength of the various N–H⋯anion hydrogen bonds quantified and the individual contributions of different intermolecular forces to the overall receptor: anion interaction derived by comparison of the charge densities in the related complexes. The strength of the N–H⋯anion hydrogen bonds was correlated to the basicity of the anion and related to the structure of the receptors. The geometric criteria used to identify hydrogen bonding interactions in standard resolution X-ray diffraction studies were shown to be valid for stronger interactions. However, these geometric criteria are less reliable and lead to assumptions that are not necessarily upheld when applied to weaker intermolecular interactions. The presence of these could only be confirmed by charge density studies. The effect that changes to the receptor substitution pattern have on the entire supramolecular system is illustrated by the differences in the electrostatic potential distributions and atomic charges across the series. The application of systematic high resolution studies to rationalise a variety of host–guest systems has been demonstrated.