Are all short O-H•••O contacts hydrogen bonds?: A quantitative look at the nature of O-H•••O intermolecular hydrogen bonds

The hydrogen-bonded nature of short O-H-... O contacts has been investigated by comparing the components of the intermolecular interaction energy in the O-H ... O contacts present in the H2O ... H2O, H2O (OH-) and (OH-) (OH-) model dimers, and then in the more chemically significant H2C2O4... H2C2O4, H2C2O4... (HC2O4-) and (OH-)...(OH-) dimers, oriented as in the crystal of KHC2O4. The energy components were competed by means of intermolecular perturbation theory (IMPT). The inter-anion contacts are found to be repulsive due to the dominant repulsive character of the electrostatic term, the strongest terms in all dimers analyzed in this work. The same term is attractive in the neutral O-H ... O and even stronger in the charge assisted O-H ... O- contacts, whose hydrogen-bonded character is well known.