From Weak Interactions to Covalent Bonds: Weak, Moderate and Strong Hydrogen Bonds

Abstract

The strength of hydrogen bonds is situated between covalent bonds and van der Waals interactions. Various proposals have been presented to classify hydrogen bonds according to their strength; these are usually weak, moderate and strong interactions. This classification may be based on other parameters, such as geometric, energetic, spectroscopic or topologic ones (mainly on the Quantum Theory of Atoms in Molecules, QTAIM, characteristics). For weaker hydrogen bonds the electrostatic interaction is the most important attractive term, while for stronger hydrogen bonds these are the charge transfer and polarization terms. Certain classes of hydrogen bonds are often attributed to the strength of interaction: low-barrier hydrogen bonds (LBHBs) are usually strong interactions while hydrogen bonds with π- and σ- electrons acting as proton acceptors are usually weak interactions, although there are numerous exceptions.