Hydrogen bond energetics from topological analysis of experimental electron densities: Recognising the importance of the promolecule

In a recent paper [E. Espinosa, E. Molins, C. Lecomte, Chem. Phys. Lett. 285 (1998) 170] the kinetic energy density G(rc) and potential energy density V(rc) at the bond critical point rc in X–H⋯O hydrogen bonds were deduced from topological analyses of experimental electron distributions extracted from accurate X-ray diffraction experiments. We show that the promolecule, a model of non-interacting overlapping spherical atomic electron densities, can explain many of the observed results as well as the exponential dependence on the H⋯O distance. This raises the question of whether present experimental results differ from a simple reference model for weak interactions.