The Nature of Halogen⋅⋅⋅Halogen Synthons: Crystallographic and Theoretical Studies

Insights into the strength and nature of carbene···halogen bond interactions: a theoretical perspective

Halogen-bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays critical roles in fields as diverse as molecular biology, drug design and material engineering. In this work, we have examined the strength and origin of halogen bonds between carbene CH₂ and XCCY molecules, where X = Cl, Br, I, and Y = H, F, COF, COOH, CF₃, NO₂, CN, NH₂, CH₃, OH. These calculations have been carried out using M06-2X, MP2 and CCSD(T) methods, through analyses of surface electrostatic potentials V S(r) and intermolecular interaction energies. Not surprisingly, the strength of the halogen bonds in the CH₂···XCCY complexes depend on the polarizability of the halogen X and the electron-withdrawing power of the Y group. It is revealed that for a given carbene···X interaction, the electrostatic term is slightly larger (i.e., more negative) than the dispersion term. Comparing the data for the chlorine, bromine and iodine substituted CH₂···XCCY systems, it can be seen that both the polarization and dispersion components of the interaction energy increase with increasing halogen size. One can see that increasing the size and positive nature of a halogen's σ-hole markedly enhances the electrostatic contribution of the halogen-bonding interaction.

Supramolecular assembly in metal complexes: Two structural cases

With the current advances in our understanding of molecular components in the solid-state world, the relation between energy and geometry remains controversial. In this study, we deliver a concise account for supramolecular chemistry, and in order to illustrate some of its concepts we describe some structural and theoretical analyses for two unique cases of our work. We elaborate on a supramolecular model of controlled “one-pot” host–guest metal-mediated self-assembly reaction inside iron coordination polymer grid architecture, and we review halogen bonding by specifically observing M–Cl···Cl–M intermolecular interactions using Cambridge Structural Database (CSD) hit analyses with theoretical calculations.

Halogen bonds in 2D supramolecular self-assembly of organic semiconductors

Weak interactions between bromine, sulphur, and hydrogen are shown to stabilize 2D supramolecular monolayers at the liquid-solid interface. Three different thiophene-based semiconducting organic molecules assemble into close-packed ultrathin ordered layers. A combination of scanning tunneling microscopy (STM) and density functional theory (DFT) elucidates the interactions within the monolayer. Electrostatic interactions are identified as the driving force for intermolecular Br···Br and Br···H bonding. We find that the SS interactions of the 2D supramolecular layers correlate with the hole mobilities of thin film transistors of the same materials.

Revealing non-covalent interactions in solids: NCI plots revisited

In this article, the NCI method [Johnson et al., J. Am. Chem. Soc., 2010, 132, 6498] for plotting and analysing non-covalent interactions (NCI) is extended to periodic (solid-state) electron densities and implemented in the critic program. The new code uses self-consistent electron densities from a variety of electronic structure methods (pseudopotentials/plane-wave, FP-LAPW, local orbitals, etc.), and it can also build the promolecular density from the crystal geometry alone. As an example of the new code, intermolecular interactions in several molecular crystals are presented and analyzed. The connection with QTAIM studies is established and a reinterpretation of the NCI domains is given regarding the current knowledge of the field. The connection between NCI domains and intermolecular vibrations is made apparent, as well as the ability of the method to reveal the locality of bonding.

Role of Fluorine in Weak Interactions in Co-crystals

The presence of the C–F bond (commonly referred to as organic fluorine) in a large number of pharmaceutically relevant compounds suggests that it may be used in the production of novel salts and co-crystals that have intermolecular interactions involving fluorine. There is an ongoing debate in this context as fluorine is characterized by its high electronegativity, relatively small size and very low polarizability. The propensity of hydrogen to participate in generating highly directional and energetically stable hydrogen bonds has been exploited in the design of co-crystals and salts of many pharmaceutical compounds. A direct extension of this property to fluorine, however, is not plausible and thus intermolecular interactions involving fluorine must be quantified. Recent results and new approaches designed to evaluate organic fluorine which provide useful inputs for the design of co-crystals and salts are discussed in this chapter.

Polymorphic porous supramolecular networks mediated by halogen bonds on Ag(111)

Intermolecular structures of porous two-dimensional supramolecular networks are studied using scanning tunnelling microscopy combined with density functional theory calculations. The local configurations of halogen bonds in polymorphic porous supramolecular networks are directly visualized in support of previous bulk crystal studies.

1-Bromo-2,3,5,6-tetra-fluoro-4-nitro-benzene

In the title compound, C(6)BrF(4)NO(2), the nitro group is twisted by 41.7 (3)° with reference to the arene ring mean plane. The main inter-actions stabilizing the crystal structure include O⋯Br contacts [3.150 (2) and 3.201 (2) Å], while F⋯F inter-actions are minor [2.863 (3)-2.908 (3) Å].