Forty years of progress in the study of the hydrogen bond

The author looks back at developments over the last few decades concerning the H–bond. The list of atoms involved as proton donor and acceptor has broadened dramatically, including most electronegative atoms and even metals. The factors that control the transfer of the proton across the H–bond have been elucidated and show the importance of even minor changes in its geometry. Small stretches can shut down the transfer entirely, and certain bends can force a proton to transfer against a pK gradient. Along with the recognition that a CH..O interaction can represent a true H–bond, and one with strength comparable to more traditional H–bonds, has come an understanding of its contributions to protein structure and function. The replacement of the bridging of H by any of a litany of electronegative atoms leads to similarly strong interactions, with many features virtually indistinguishable from a true H–bond. These noncovalent interactions are typically referred to as halogen, chalcogen, pnicogen, and tetrel bonds, depending upon the identity of the substitute bridging atom.