Using X-ray Derived Charge Densities to detect electron delocalization effects and Non-covalent Interactions

Being based on a quantum observable and measurable quantity, the Electron Density (ED) based descriptors retain the advantage of enabling a direct comparison of theoretical predictions with experimental results. We review here our most recent work aimed at evaluating whether two of such descriptors, the Source Function, SF, [1] and the Reduced Density Gradient (RDG), are able to unveil electron delocalization effects (EDEs) and non covalent interactions (NCI), respectively. Making use of ab-initio EDs, we recently proved that the SF clearly detects EDEs in a series of supposedly electron-conjugated compounds [2]. That study is here extended to molecular crystals (benzene, a substituted binaphtyl-2-ol, citrinin), whose ED is derived from X-ray diffraction data. Regardless of the ED origin, the SF appears a useful tool to study fine details of EDEs, and independently from any symmetry constraint (e.g. sigma/pi separation of the ED). A novel NCI descriptor, based on the RDG and enabling an easy-to-catch image of either the supposedly attractive (dispersive, hydrogen bonding) or allegedly repulsive (steric) interactions, was recently proposed [3]. We have applied [4] this same tool to experimentally-derived ED's of molecular solids (austdiol, benzene, famotidine), and discussed its performance in synergy with Bader's analysis. We have also explored [5] the amount and type of information that is lost when the IAM replaces the "true" ED in evaluating the RDG. References [1] R.F.W. Bader, C. Gatti, Chem. Phys. Lett. 287 (1998) 233-238. [2] E. Monza, C. Gatti, L. Lo Presti, E. Ortoleva, J. Phys. Chem. A 115 (2011) 12864-12878. [3] E.R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-Garcia JACS 132 (2010), 6498. [4] G. Saleh, C. Gatti, L. Lo Presti, J. Contreras-Garcia, submitted (2012) [5] G. Saleh, C. Gatti, L. Lo Presti, submitted (2012)