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 C8O2H4Chal (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) = −∇2ρ(r) function in the crystal phase of C8O2H4Se, 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 C8O2H4Se 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.

Trimers based on intermolecular halogen-bonding interactions (Hal 3 -synthons) have been studied in hexachlorobenzene, hexabromobenzene, pentachlorophenol, and pentabromophenol. Attention is paid to the comparison of Cl 3 -and Br 3 -synthons and to their competition with hydrogen bonds (HBs), based on the experimental and theoretical charge density analyses in crystal and gas phases. The main differences between Cl 3 -and Br 3 -synthons are established coming from the particular structure of the valence shell charge concentration region in Cl and Br atoms. Electrophilic−nucleophilic interactions take place within the intermolecular regions of Hal 3 -synthons by putting face-to-face charge depletion (CD) and charge concentration (CC) regions belonging to the valence shell of the halogen atoms. The electrostatic interaction follows the electrophilic and nucleophilic power of these regions and is monitored by the negative Laplacian values normalized to charge density unit (L/ρ) at the corresponding topological critical points (CPs) of the L(r) = −∇ 2 ρ(r) function. According to the topological and energetic properties at CPs of ρ(r) and L(r), it is observed that Hal 3 -synthons can successfully compete with intermolecular HBs in the analyzed structures. On the basis of the estimated interaction energy and the electrostatic descriptor Δ(L/ρ) = (L/ρ) CC − (L/ρ) CD , we conclude that a strong dispersion contribution assists Hal3-synthons in this competition.

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