Making experiment and theory talking together: electron delocalization effects and non covalent interactions detection via the Source Function and the Reduced Density Gradient

Being based on a quantum observable and (easily) measurable quantity, the Electron Density (ED) based descriptors retain the great advantage of enabling a comparison of theoretical predictions with experimental results on the same grounds. We discuss, here, the capability of two of such descriptors, the Source Function (SF) [1] and the Reduced Density Gradient (RDG), to unveil electron delocalization effects and to detect non covalent interactions, respectively. The SF enables one to view chemical bonding and other chemical paradigms under a new perspective [1,2]. We recently addressed the question of whether the SF is or is not also capable to reveal electron delocalization effects (EDEs) in a series of supposedly electron-conjugated compounds, investigated through ab-initio methods [2]. The study is here extended to various molecular crystals (benzene, a substituted binaphtyl-2-ol, citrinin), whose ED and SF results were obtained from X-ray diffraction data. Regardless of the derivation of ?, the answer to the question above seems to be convincingly positive. The capability of revealing EDEs is independent from a ?/? separation of ?. Hence, the SF allows to recover EDEs even when ?/? separation is unfeasible (e.g. ED given on a grid) or when the departure from symmetry would however preclude it. A novel non covalent interaction (NCI) descriptor, based on the RDG and enabling an easy-to-catch pictorial visualization of either the supposedly attractive (dispersive, hydrogen bonding) or allegedly repulsive (steric) intermolecular interactions, was recently developed [3]. We apply this same tool for the first time [4] to experimentally-derived ED's of molecular solids (austdiol, benzene, famotidine) discussing its performance in synergy with Bader's analysis. [1] R.F.W. Bader, C. Gatti, "A Green's Function for the density" Chem. Phys. Lett. 287, 233-238 (1998). [2] C. Gatti "The Source Function descriptor as a tool to extract chemical information from theoretical and experimental electron densities" Struct. Bond. 1 $DOI: 10.1007/430_2010_31 2 (2011); E. Monza, C. Gatti, L. Lo Presti, E. Ortoleva "Revealing Electron Delocalization through the Source Function" J. Phys. Chem. A 115, 12864-12878 (2011). [3] E.R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-Garcia, A.J. Cohen, W. Yang "Revealing Noncovalent Interactions" J. Am. Chem. Soc, 132, 6498-6506 (2010). [4] G. Saleh, C. Gatti, L. Lo Presti, J. Contreras-Garcia, submitted (2012)