Special Issue: Intramolecular Hydrogen Bonding 2017

Even after more than a century of study [1–6], scrutiny, and detailed examination, the H-bond continues [7–12] to evoke a level of fascination that surpasses many other phenomena. Perhaps it is the ability of the simple H atom, with but a single electron, to act as a glue that maintains contact between much more complicated species. Or it might be its geometry, which prefers to hold the bridging proton on a direct line between the two heavy atoms. Not to be ignored are the spectral features of the H-bond: the large red shift of the stretching frequency of the covalent A–H bond, coupled with its intensification, or the downfield shift of the proton’s NMR signal. Yet study of this bond is far from complete, with one surprise after another continuing to emerge. As it turns out, the aforementioned red shift, for example, long considered as the trademark of this bond, is not so characteristic after all. H-bonds that shift in the opposite direction, to the blue, have been observed [13–16] in a variety of systems. The long held belief that only very electronegative atoms like F, O, and N can participate in these bonds has been shattered, as one atom after another, S and Cl and even metals to name just a few, have been added [17–20] to the rapidly growing list