Definitions and Concepts

Abstract

The modern concept of the hydrogen bond has its basis in the principle of the relative atomic electronegativities as put forward by Pauling in his Nature of the Chemical Bond [24]. As one proceeds from left to right of the Periodic Table, the electronegativity increases the electron density over that necessary to balance the nuclear charge and the electric potential is negative; the additional charge on the nuclei is increasingly less screened by the additional electron, even though the overall charge is neutral. This means that in the direction of an X-H bond, the proton is increasingly descreened as X proceeds from carbon to fluorine and its electric potential is increasingly positive. This concept is supported by the observation that the strongest bonds involve hydrogens attached to fluorine atoms and the weakest hydrogen bonds involve those attached to carbon atoms. It has long been known that with very electronegative donor and acceptor atoms, the hydrogen bond resembles a covalent bond, whereas, with weakly electronegative atoms, it is primarily electrostatic [42].