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

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/rho) at the corresponding topological critical points (CPs) of the L(r) = -del(2)rho(r) function. According to the topological and energetic properties at CPs of p(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 Delta(L/rho) = (L/rho)(CC) - (L/rho)(CD), we conclude that a strong dispersion contribution assists Hal3-synthons in this competition.