Hydrogen bond strengths revealed by topological analyses of experimentally observed electron densities

Interatomic interactions such as hydrogen bonds (HB) can be adequately described and classified by the topological properties of the electron density ρ(r) at the (3,−1) critical points rCP where the gradients of ρ(r) vanish. We have analysed the topological properties of ρ(r) at the intermolecular critical points of 83 experimentally observed HBs [X–H⋯O (X=C,N,O)], using accurate X-ray diffraction experiments. In spite of different models, methods and experimental conditions employed to obtain the topological properties of ρ(r), we show that, for closed-shell interactions, the kinetic energy density G(rCP) and the potential energy density V(rCP) at the critical point depend exponentially on the H⋯O distance. Moreover, theoretical calculations for several HB dissociation energies follow the same law as does V(rCP), with a simple change of scale.