Experimental benchmarks and phenomenology of interatomic forces: open-shell and electronic anisotropy effects

Abstract This article gives a perspective view of some representative experimental information available on interatomic forces. They play a role in gaseous properties, but modern quantitative information comes from spectroscopy and molecular beam scattering. This latter technique is emphasized here: recent experimental results and consideration of physical properties of interacting species is complementary to progress of modelling based on ab initio or other quantum chemical calculations. Interactions involved in closed-shell–closed-shell species are considered to be typical of the so-called ‘non-covalent’ forces, although additional effects of a ‘chemical’ nature are demonstrated to be non-negligible in some cases. The partition of the interaction into van der Waals (repulsion + dispersion) and possibly electrostatic and/or induction components is analysed. Interactions involving open-shell species offer a most interesting phenomenology, because electronic anisotropy often provides further strength to the bonds, which are usually weaker than ordinary chemical bonds. Again, the focus is on experimental information (especially on scattering of magnetically analysed open-shell atoms) and on the understanding that comes from the analysis of the ample phenomenology accumulated. Additional terms such as those of specific ‘covalent’ nature appear in the partition of the interaction, besides those already mentioned. The extension of this approach for describing molecular anisotropies is also outlined. TableDownload CSVDisplay Table Acknowledgements We express our thanks to all our coworkers who have contributed in the past to the developments described in this article and whose names are associated to ours in the bibliographic references. Support from the Italian Ministry for Education, University and Research (MIUR) through PRIN and FIRB contracts is acknowledged.