Beyond QTAIM: NCI Indexes as a Tool to Reveal Intermolecular Bonds in Molecular Aggregates

Abstract

This chapter dwells on a novel descriptor for the visualization of non-covalent interactions, NCI (for Non-Covalent Interactions), based on the Reduced electron Density Gradient (RDG) and enabling an easy-to-catch visualization of non-covalent interactions. In this chapter we will see how this index is based on the bosonic kinetic energy density, revealing a complete spectrum of chemical structures. It recovers the shell structure and Lewis picture of ELF (covalent bonds, lone pairs), but also interactions not recovered by ELF because they are not related to localization, such as ionic and non-covalent interactions, as recovered by QTAIM bond critical points (BCPs). These QTAIM interactions are visualized as isosurfaces, which also provide a very intuitive description of delocalized interactions, overcoming the pair interactions as defined by BCPs. Finally, we will also show that weak intramolecular interactions, both attractive and repulsive, can be visualized by NCI too, even in the absence of QTAIM features. Since NCI provides a complete picture of chemical structure, it becomes an ideal tool to analyze molecular aggregates. We will finish the chapter with several examples applied to molecular solids, showing the ability of NCI to quickly characterize delocalized interactions in benzene from X-ray densities. We will also show an approximation of NCI from promolecular densities applicable to protein structures (e.g. xyz or pdb files). Finally, we will show an example where NCI has helped in revealing a new type of bonding that enables explanation of new experimental results under pressure and unexpected macroscopic properties.