Hydrogen bonding is among the most fundamental interactions in biology and chemistry, providing an extra stabilization of 1-40 kcal/mol to the molecular systems involved. This wide range of stabilization energy underlines the need for a general and comprehensive theory that will explain the formation of hydrogen bonds. While a simple electrostatic model is adequate to describe the bonding patterns in the weak and moderate hydrogen bond regimes, strong hydrogen bonds, on the other hand, require a more complete theory due to the appearance of covalent interactions. In this study, conceptual DFT tools such as local hardness, eta(r) and local softness, s(r), have been used in order to get an alternative view on solving this hydrogen-bonding puzzle as described by Gilli et al. [J. Mol. Struct. 2000, 552, 1]. A series of both homonuclear and heteronuclear resonance-assisted hydrogen bonds of the types O-H...N, N-H...O, N-H...N, and O-H...O with strength varying from weak to very strong have been studied. First of all, DeltaPA and DeltapK(a) values were calculated and correlated to the hydrogen bond energy. Then the electrostatic effects were examined as hard-hard interactions accessible through molecular electrostatic potential, natural population analysis (NPA) charge, and local hardness calculations. Finally, secondary soft-soft interaction effects were entered into the picture described by the local softness values, providing insight into the covalent character of the strong hydrogen bonds.
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