Chalcogen Bonding: Experimental and Theoretical Determinations from Electron Density Analysis. Geometrical Preferences Driven by Electrophilic–Nucleophilic Interactions

Chalcogen bonding has been investigated in terms of the electron density distribution ρ( r ) around chalcogen atoms. The evolution of ρ( r ) along the series of chalcogen atoms is shown based on ab initio calculations on chalcogenophthalic anhydrides C 8 O 2 H 4 Chal (Chal = O, S, Se, and Te), where the Chal atom is in its sp 3 hybridization. From a detailed analysis of the experimental and theoretical electron density and the L ( r ) = −∇ 2 ρ( r ) function in the crystal phase of C 8 O 2 H 4 Se, we characterize directionality and strength of chalcogen bonding (Se···O and Se···Se) and hydrogen bonding (Se···H) interactions. In addition, several isolated dimers and a trimer of C 8 O 2 H 4 Se have been also studied at the X-ray geometry in order to compare interaction energies with those estimated from the measured electron density. Similarly to halogen atoms in halogen bonding interactions, the anisotropic distribution of ρ( r ) around the Chal atoms was found to be at the origin of chalcogen bonding. Therefore, the concepts, developed earlier for halogen bonding, are extended here to chalcogen bonding interactions. From the results of this work, the L ( r ) function proves to be more precise than the σ-hole concept to identify electrophilic sites of Se-atoms in sp 3 hybridization.