The ionic hydrogen bond.
1.1. Historical Background Hydrogen bonds1,2 are one of the principal intermolecular forces. A special class are ionic hydrogen bonds (IHBs) that form between ions and molecules with bonds strengths of 5-35 kcal/mol, up to a third of the strength of covalent bonds. These strong interactions are critical, for example, in ionic clusters and nucleation, in electrolytes, ion solvation, and acid-base chemistry, in the structures of ionic crystals, surfaces, silicates, and clays, in surface adsorption, and in self-assembly in supramolecular chemistry and molecular crystals.3 IHBs are also important in bioenergetics including protein folding, enzyme active centers, formation of membranes and proton transport, and biomolecular recognition. With such wide-ranging roles, the fundamental properties of IHB interactions need to be understood. The energetics of IHB interactions cannot be isolated and quantified in the condensed phase. However, these interactions can be isolated and studied quantitatively in gas phase. These studies lead to a fundamental understanding of relations between IHB bond strengths and molecular structure, the solvation of ions, especially in the critical inner shells, and acid-based phenomena and bioenergetics. This review will present the basic insights that have been obtained in the past four decades. It will present a comprehensive review of the thermochemistry and its structural implications obtained from ab initio calculations. Relevant recent results from spectroscopy will be also illustrated. The advent of variable temperature high-pressure mass spectrometry (HPMS) introduced by Field and co-workers in the 1960s4 and pulsed high-pressure mass spectrometry (PHPMS) introduced by Kebarle in the late 1960s5 have been particularly significant. These workers realized that at pressures of several torrs and gas densities of 1016-1017 cm-3 in ion sources, ions can undergo 103-106 collisions with neutral molecules during typical residence times of 0.1-10 ms and establish thermal ion populations and * E-mail: m.mautner@solis1.com. 213 Chem. Rev. 2005, 105, 213−284