Coupling by charge transfer: role in bond stabilization for open-shell systems and ionic molecules and in harpooning and proton attachment processes

A variety of phenomena of apparently different nature can be compacted and described within a unifying picture by taking into account the role of the charge transfer interaction. Relevant information on this interaction is obtained by the analysis of bond stabilization in halides, oxides, sulphides and ionic dimers of rare gases. Most of this information comes from recent molecular beam experiments: when combined with the analysis of processes occurring at crossings between covalent and ionic states in alkali halides it leads to the characterization of the dependence of the charge transfer matrix element on basic physical properties of the interacting partners. The magnitude of the coupling matrix element is correlated to polarizabilities and charges. Its exponential decreasing with intermolecular distance is given in terms of ionization potentials and electron affinities, in the spirit of the study by Grice and Herschbach (Molec. Phys., 1973, 27, 159) on the long-range configuration interaction of ionic and covalent states. A proper representation is obtained both for the transition from van der Waals to chemical bonds and for the behaviour of different families of compounds, such as those of alkali halides and of rare-gas protonated systems.