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. Contents PAGE 1. Introduction 1.1. Motivation and dedication 166 1.2. Scope and outline of the paper 167 2. Isotropic interactions and van der Waals forces 168 2.1. 2 S +1S atom– 168 2.2. Combination rules and correlation formulas 170 2.3. 2 S +1S Ion– 172 3. Anisotropic interactions and open-shell effects 175 3.1. General 175 3.2. 2 S +1P atom– 176 3.3. Charge transfer and bond stabilization 178 3.4. 2 S +1P ion – 180 3.5. Dications 181 4. Final remarks 182 4.1. Towards atom–molecule and molecule–molecule interactions 182 4.2. Prospects for future work 184 4.3. From van der Waals interactions to chemical bonds 185 Acknowledgements 185 Appendix A– Interatomic forces by molecular beam scattering 185 Appendix B–Basic contributions to the interatomic interactions and their dependence on physical properties f involved species 188 Appendix C–Electronic anisotropy and orbital alignment 192 References