The hydrogen bond is a three-center, four-electron interaction of linkage X-H...Y in which a H atom covalently bonded to electronegative atom X forms an additional weak bond to a second electronegative atom Y. The extent to which the interaction with atom Y is of ionic or covalent character has been an issue of some debate ever since the H-bond was first subjected to quantum mechanical analysis in the 1920s. Weinhold and Klein recently reported the natural bond orbital (NBO) analysis of H-bonding in complexes formed from ions of like charge, [A-H...B]2±. They argued that charge transfer effects dominate in these complexes because the monomer interactions must be strongly destabilized by electrostatic (e2/R) repulsion. Interactions in these complexes were thus referred to as anti-electrostatic hydrogen bonds (AEHB). Frenking and Caramori responded that the simple e2/R Coulomb potential cannot accurately represent the electrostatic interactions in AEHB complexes and that the conclusions of Weinhold and Klein therefore reflected the bias of the NBO methods. We have undertaken a study to examine in more detail the extent of electrostatic interactions and charge transfer using the natural energy decomposition analysis (NEDA). We describe the anti-electrostatic H-bonds in three of Weinhold and Klein’s dimer dianions. These include the dimers of bicarbonate (HOCO2–), bioxalate (HOOCCO2–), and p-biphthalate (HOOCC6H4CO2–). Electrostatic forces in these AEHB complexes, which clearly account for the long-range repulsion between the monomers, become (perhaps counter-intuitively) attractive near equilibrium geometries. Treatments of electrostatic interactions that do not reasonably reflect the monomer charge distributions cannot reliably describe the attractive behavior at short range.

Nature of the anti-electrostatic hydrogen bond

251st National Meeting (2016)

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