Universal Features of the Electron Density Distribution in Hydrogen-Bonding Regions: A Comprehensive Study Involving H center dot center dot center dot X (X = H, C, N, O, F, S, Cl, pi) Interactions

Topological analyses of the theoretically calculated electron densities for a large set of 163 hydrogen-bonded complexes show that H center dot center dot center dot X interactions can be classified in families according to X (X = atom or a orbital). Each family is characterised by a set of intrinsic dependencies between the topological and energetic properties of the electron density at the hydrogen-bond critical point, as well as between each of them and the bonding distance. Comparing different atom-acceptor families, these dependencies are classified as a function of the van der Waals radius r(X) or the electronegativity chi(X), which can be explained in terms of the molecular orbitals involved in the interaction. According to this ordering, the increase of chi(X) leads to a larger range of H center dot center dot center dot X distances for which the interaction is of pure closed-shell type. Same dependencies observed for H center dot center dot center dot O interactions experimentally characterised by means of high-resolution X-ray diffraction data show a good agreement with those obtained from theoretical calculations, in spite of a larger dispersion of values around the expected fitting functions in the experimental case. Theoretical dependencies can thus be applied to the analysis of the experimental electron density for detecting either unconventional hydrogen bonds or problems in the modelling of the experimental electron density.