Charge Density Analysis and Topological Properties of Hal₃-Synthons and Their Comparison with Competing Hydrogen Bonds

Abstract

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.

Self-assembly mechanism based on charge density topological interaction energies

“…As noted previously for halogen-based synthons [46], it becomes apparent that the eCDE values may be of use to categorize the packing motifs of importance to self-assembly. For example, synthons may be considered as primary units (secondary, tertiary, etc.)…”

“…The non-covalent interactions can be further characterized in terms of the estimated electronic kinetic and potential energies at the bond critical point, as demonstrated previously [46]. The ratio of these energies 〈| V| /G〉 provides an indicator for the flavor of the interaction; closed-shell (|V | /G < 1), pure shared-shell |V | /G > 2), and intermediate interactions between electronic states (1 < |V | /G < 2) [47].…”

“…The eCDE as a synthons descriptor has been previously defined and was developed in the context of the comparison between halogen-based synthons (Hal = Cl, Br), and between halogen and hydrogen bonds by Brezgunova and co-workers [46]. The eCDE for S1 amounts to 197.1 kJ mol −1 and represents the highest value among all aggregates observed in 1.…”

On atom–atom `short contact' bonding interactions in crystals

“…In the case of challenging experimental charge density determinations, such as those describing halogen and chalcogen heavy atoms involved in very weak interactions, the experimental properties of the topology of (r) in the intermolecular regions are systematically supported by theoretical calculations and vice versa. As an example, joint experimental and periodic theoretical charge density studies of Br-and Se-compounds involving very weak interactions (Brezgunova et al, 2012(Brezgunova et al, , 2013 indicate reproducible bond paths and (r) features in the intermolecular regions, as well as in the close vicinity of the heavy atoms.…”

“…Recent experimental studies using the topological analysis of the Laplacian of the electron density demonstrate that the orientation of atom-atom interactions (and therefore that of the corresponding intermolecular interactions) is indeed governed by electrophilic/nucleophilic interactions of regions of charge concentration (CC) with regions of charge depletion (CD) in the atomic valence shells of the atom-atom pair interactions (Bui et al, 2009). This complementarity, which has also been investigated in halogen, chalcogen and weak hydrogen bonding (Brezgunova et al, 2012(Brezgunova et al, , 2013, is such that the CC-CD directions are almost completely aligned with the given internuclear atom-atom directions, even for interactions with energies estimated as low as <5 kJ mol À1 . Bond paths and CC-CD interactions are clearly indicative of atom-atom interactions.…”