Hydrogen Bonding: A Coulombic σ-Hole Interaction

Molecular electrostatic potentials, in conjunction with polarization, provide the key to understanding hydrogen bonding. As required by the Hellmann-Feynman theorem, hydrogen bonding is a Coulombic interaction between (a) a positive electrostatic potential associated with a region of lower electronic density on the hydrogen (a sigma-hole), and (b) a negative site on the hydrogen-bond acceptor. The charge distributions of both the hydrogen-bond donor and the acceptor reflect the polarizing effects of each other's electric fields. The greater the polarization, the stronger the interaction. This interpretation of hydrogen bonding applies to all of the different categories into which it has been subdivided; they are fundamentally similar. We show that if polarization is minor and the hydrogen bonds relatively weak, then their interaction energies correlate well with the product of the most positive electrostatic potential on the hydrogen and the most negative one on the negative site. It is argued that the partial covalent character that is often attributed to hydrogen bonds simply reflects a greater degree of polarization.