Showing papers on "Halogen published in 2019"

The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis.

Abstract: Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p Ka = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

Halogen-bonded cocrystallization with phosphorus, arsenic and antimony acceptors.

Abstract: The formation of non-covalent directional interactions, such as hydrogen or halogen bonds, is a central concept of materials design, which hinges on using small compact atoms of the 2nd period, notably nitrogen and oxygen, as acceptors. Heavier atoms are much less prominent in that context, and mostly limited to sulfur. Here, we report the experimental observation and theoretical study of halogen bonds to phosphorus, arsenic and antimony in the solid state. Combining 1,3,5-trifluoro-2,4,6-triiodobenzene with triphenylphosphine, -arsine, and -stibine provides cocrystals based on I···P, I···As and I···Sb halogen bonds. The demonstration that increasingly metallic pnictogens form halogen bonds sufficiently strong to enable cocrystal formation is an advance in supramolecular chemistry which opens up opportunities in materials science, as shown by colossal thermal expansion of the cocrystal involving I···Sb halogen bonds.

Halogen bonding as a supramolecular dynamics catalyst

Abstract: Dynamic processes have many implications in functional molecules, including catalysts, enzymes, host-guest complexes, and molecular machines. Here, we demonstrate via deuterium NMR relaxation experiments how halogen bonding directly impacts the dynamics in solid 2,3,5,6-tetramethylpyrazine cocrystals, catalyzing the methyl group rotation. On average, we observe a reduction of 56% in the rotational activation energy of the methyl groups in the halogen bonded cocrystals, contrasting the reduction of 36% in the hydrogen bonded cocrystals, with respect to pure 2,3,5,6-tetramethylpyrazine. Density functional theory calculations attribute this superior catalytic ability of the halogen bond to the simultaneous destabilization of the staggered conformation and stabilization of the gauche conformation, overall reducing the rotational energy barrier. Furthermore, the calculations suggest that the catalytic ability of the halogen bond may be tuneable, with stronger halogen bond donors acting as superior dynamics catalysts. Thus, halogen bonding may play a role in both assembly and promoting dynamical processes.

Generation of Halomethyl Radicals by Halogen Atom Abstraction and Their Addition Reactions with Alkenes

Abstract: α-Aminoradicals undergo halogen atom abstraction to form halomethyl radicals in reactions initiated by the combination of tert-butyl hydroperoxide, aliphatic trialkylamine, halocarbon, and copper(I) iodide. The formation of the α-aminoradical circumvents preferential hydrogen atom transfer in favor of halogen atom transfer, thereby releasing the halomethyl radical for addition to alkenes. The resulting radical addition products add the tert-butylperoxy group to form α-peroxy-β,β-dichloropropylbenzene products that are convertible to their corresponding β,β-dichloro-alcohols and to novel pyridine derivatives. Computational analysis clearly explains the deviation from traditional HAT of chloroform and also establishes formal oxidative addition/reductive elimination as the lowest energy pathway.

Electronic structures and elastic properties of a family of metal-free perovskites

Abstract: The electronic structures and elastic properties of three isostructural, metal-free perovskite materials, (C4N2H12)(NH4X3)·H2O (PIP-X, X = Br, Cl, I), were examined using density functional theory (DFT) calculations and high-pressure synchrotron X-ray diffraction experiments. The calculated band structures and density of states demonstrate that all the compounds possess large direct bandgaps of 5.34 eV for PIP-Cl, 4.67 eV for PIP-Br, and 4.13 eV for PIP-I. With the bromide and iodide, the valence band maximum and conduction band minimum mainly arise from the 3p- and 3s-states of the halogens, whereas the conduction band minimum of the chloride is dominated by the s-states of the nitrogen from the ammonium. Such an inverse dependence of bandgaps on the halogen radius originates from the increased band dispersions because of reduced halogen electronegativity. In addition, the full elastic constants of these compounds were calculated using DFT which enables the systematic mapping of their Young's moduli, shear moduli and Poisson's ratios. The N–H⋯X bond strength governed by the halogen radius is primarily responsible for the discrete modulus properties in these compounds. Notably, these metal-free perovskites constructed using hydrogen bonds exhibit comparable rigidity with their hybrid organic–inorganic counterparts assembled using coordination bonds. Furthermore, the high-pressure synchrotron powder X-ray diffraction experiments were performed on PIP-Br, which not only validated the DFT results but also revealed its comparable stiffness to methylammonium lead bromine (CH3NH3PbBr3) under hydrostatic stress.

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

Abstract: A study of the strong N-X⋅⋅⋅- O-N+ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N-halosaccharins and pyridine N-oxides (PyNO). DFT calculations were used to investigate the X⋅⋅⋅O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X⋅⋅⋅O XBs. The XB interaction energies vary from -47.5 to -120.3 kJ mol-1 ; the strongest N-I⋅⋅⋅- O-N+ XBs approaching those of 3-center-4-electron [N-I-N]+ halogen-bonded systems (ca. 160 kJ mol-1 ). 1 H NMR association constants (KXB ) determined in CDCl3 and [D6 ]acetone vary from 2.0×100 to >108 m-1 and correlate well with the calculated donor×acceptor complexation enthalpies found between -38.4 and -77.5 kJ mol-1 . In X-ray crystal structures, the N-iodosaccharin-PyNO complexes manifest short interaction ratios (RXB ) between 0.65-0.67 for the N-I⋅⋅⋅- O-N+ halogen bond.

Catalysis Based on C−I⋅⋅⋅π Halogen Bonds: Electrophilic Activation of 2‐Alkenylindoles by Cationic Halogen‐Bond Donors for [4+2] Cycloadditions

Abstract: Homo- and cross-[4+2] cycloadditions of 2-alkenylindoles, catalyzed by cationic halogen-bond donors, were developed. Under mild reaction conditions, 3-indolyl-substituted tetrahydrocarbazole derivatives were obtained in good to excellent yields. Experimental and quantum calculation studies revealed that the electrophilic activation of 2-alkenylindoles was achieved by C-I⋅⋅⋅π halogen bonds.

A new type of halogen bond involving multivalent astatine: an ab initio study.

Abstract: Theoretical studies on the dimers formed by CO with the halides of multivalent astatine as a Lewis-acid center are carried out to examine the typical characteristics of supervalent halogen bonds. Calculations at the MP2/aug-cc-pVTZ level reveal that the multiple nucleophilic sites of multivalent halide monomers can promote the formation of various types of halogen bonds, among which the most stable ones are At–halogen bond complexes with multivalent astatine as a Lewis acid center, followed by the π–halogen bond dimers, and the weakest ones are the X–halogen bonds. Compared with multivalent Cl-, Br-, and I-centers, At, as the heaviest halogen, exhibits the highest halogen-bond donating ability. We found that the electrostatic term and the dispersion term play an important role in the overall attractive interaction energy, and the smallest attraction term for all complexes is the polarization term (ΔEpol). Moreover, the tri and pentavalent halides analyzed here possess very “flexible” tautomerism in which the transformation occurs during the formation of the dimers. AIM theory and NBO analysis are also employed here.

Halogen bonding of the aldehyde oxygen atom in cocrystals of aromatic aldehydes and 1,4-diiodotetrafluorobenzene

Abstract: Novel halogen bonded cocrystals with 1,4-diiodotetrafluorobenzene and aromatic aldehydes have been synthesized. We present the halogen bond acceptor potential of the aldehyde group oxygen atom in competition with the hydroxy, methoxy and pyridine groups.

A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H2O (X = F, Cl, Br)

Abstract: Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol−1, −5.56 kcal mol−1, and −4.97 kcal mol−1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol−1 and −3.03 kcal mol−1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size.

Halogen Substituent Effects on Concentration-Controlled Self-Assembly of Fluorenone Derivatives: Halogen Bond versus Hydrogen Bond

Abstract: Self-assembled behaviors of three fluorenone derivatives substituted by different halogen atoms, 2-(pentadecyloxy)-6-bromo-fluorenone (Br-FC15), 2-(pentadecyloxy)-6-chloro-fluorenone (Cl-FC15), and...

Metal–Halogen Bonding Seen through the Eyes of Vibrational Spectroscopy

Abstract: Incorporation of a metal center into halogen-bonded materials can efficiently fine-tune the strength of the halogen bonds and introduce new electronic functionalities. The metal atom can adopt two possible roles: serving as halogen acceptor or polarizing the halogen donor and acceptor groups. We investigated both scenarios for 23 metal–halogen dimers trans-M(Y2)(NC5H4X-3)2 with M = Pd(II), Pt(II); Y = F, Cl, Br; X = Cl, Br, I; and NC5H4X-3 = 3-halopyridine. As a new tool for the quantitative assessment of metal–halogen bonding, we introduced our local vibrational mode analysis, complemented by energy and electron density analyses and electrostatic potential studies at the density functional theory (DFT) and coupled-cluster single, double, and perturbative triple excitations (CCSD(T)) levels of theory. We could for the first time quantify the various attractive contacts and their contribution to the dimer stability and clarify the special role of halogen bonding in these systems. The largest contribution to the stability of the dimers is either due to halogen bonding or nonspecific interactions. Hydrogen bonding plays only a secondary role. The metal can only act as halogen acceptor when the monomer adopts a (quasi-)planar geometry. The best strategy to accomplish this is to substitute the halo-pyridine ring with a halo-diazole ring, which considerably strengthens halogen bonding. Our findings based on the local mode analysis provide a solid platform for fine-tuning of existing and for design of new metal–halogen-bonded materials.

Selective Emergence of the Halogen Bond in Ground and Excited States of Noble-Gas-Chlorine Systems.

Abstract: Molecular-beam scattering experiments and theoretical calculations prove the nature, strength, and selectivity of the halogen bonds (XB) in the interaction of halogen molecules with the series of noble gas (Ng) atoms. The XB, accompanied by charge transfer from the Ng to the halogen, is shown to take place in, and measurably stabilize, the collinear conformation of the adducts, which thus becomes (in contrast to what happens for other Ng-molecule systems) approximately as bound as the T-shaped form. It is also shown how and why XB is inhibited when the halogen molecule is in the 3 Π u excited state. A general potential formulation fitting the experimental observables, based on few physically essential parameters, is proposed to describe the interaction accurately and is validated by ab initio computations.

Halogen Bond-Assisted Electron-Catalyzed Atom Economic Iodination of Heteroarenes at Room Temperature.

Abstract: A halogen bond-assisted electron-catalyzed iodination of heteroarenes has been developed for the first time under atom economic condition at room temperature. The iodination is successful with just 0.55 equiv of iodine and 0.50 equiv of peroxide. The kinetic study indicates that the reaction is elusive in the absence of a halogen bond between the substrate and iodine. The formation of a halogen bond, its importance in lowering the activation barrier for this reaction, the presence of radical intermediates in a reaction mixture, and the regioselectivity of the reaction have been demonstrated with several control experiments, spectroscopic analysis, and quantum chemical calculations. Allowing the formation of the halogen bond may offer a new strategy to generate the reactive radical intermediates and to enable the otherwise elusive electron-catalyzed reactions under mild reaction conditions.

Experimental and Theoretical Evidence for Nitrogen–Fluorine Halogen Bonding in Silver-Initiated Radical Fluorinations

Abstract: We report experimental and computational evidence for nitrogen–fluorine halogen bonding in Ag(I)-initiated radical C–H fluorinations. Simple pyridines form [N–F–N]+ halogen bonds with Selectfluor t...

Halogen Bonding Interactions for Aromatic and Nonaromatic Explosive Detection

Abstract: Improved sensing strategies are needed for facile, accurate, and rapid detection of aromatic and nonaromatic explosives. Density functional theory was used to evaluate the relative binding interaction energies between halogen-containing sensor model molecules and nitro-containing explosives. Interaction energies ranged from -18 to -14 kJ/mol and highly directional halogen bonding interactions were observed with bond distances ranging between 3.0 and 3.4 A. In all geometry optimized structures, the sigma-hole of electropositive potential on the halogen aligned with a lone pair of electrons on the nitro-moiety of the explosive. The computational results predict that the strongest interactions will occur with iodine-based sensors as, of all the halogens studied, iodine is the largest, most polarizable halogen with the smallest electronegativity. Based on these promising proof-of-concept results, synthetically accessible sensors were designed using 1,4-dihalobenzene (X = Cl, Br, and I) with and without tetra-fluoro electron withdrawing groups attached to the benzene ring. These sensing molecules were embedded onto single walled carbon nanotubes that were mechanically abraded onto interdigitated array electrodes, and these were used to measure the responses to explosive model compounds cyclohexanone and dimethyl-dinitro-benzene in nitrogen gas. Amperometric current-time curves for selectors and control molecules, including concentration correlated signal enhancement, as well as response and recovery times, indicate selector responsiveness to these model compounds, with the largest response observed for iodo-substituted sensors.

Highly soluble fluorine containing Cu(I) AlkylPyrPhos TADF complexes

Abstract: Luminescent Cu(I) AlkylPyrPhos complexes with a butterfly-shaped Cu2I2 core and halogen containing ancillary ligands, with a special focus on fluorine, have been investigated in this study. These complexes show extremely high solubilities and a remarkable (photo)chemical stability in a series of solvents. A tunable emission resulting from thermally activated delayed fluorescence with high quantum yields was determined by luminescence and lifetime investigations in solvents and solids. Structures of the electronic ground states were analyzed by single crystal X-ray analysis. The structure of the lowest excited triplet state was determined by transient FTIR spectroscopy, in combination with quantum chemical calculations. With the obtained range of compounds we address the key requirement for the production of organic light emitting diodes based on solution processing.

pH-dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces

Abstract: . The mechanisms of molecular halogen production from frozen saline surfaces remain incompletely understood, limiting our ability to predict atmospheric oxidation and composition in polar regions. In this laboratory study, condensed-phase hydroxyl radicals (OH) were photochemically generated in frozen saltwater solutions that mimicked the ionic composition of ocean water. These hydroxyl radicals were found to oxidize Cl− , Br− , and I− , leading to the release of Cl2 , Br2 , I2 , and IBr. At moderately acidic pH (buffered between 4.5 and 4.8), irradiation of ice containing OH precursors (either of hydrogen peroxide or nitrite ion) produced elevated amounts of I2 . Subsequent addition of O3 produced additional I2 , as well as small amounts of Br2 . At lower pH (1.7–2.2) and in the presence of an OH precursor, rapid dark conversion of I− to I2 occurred from reactions with hydrogen peroxide or nitrite, followed by substantial photochemical production of Br2 upon irradiation. Exposure to O3 under these low pH conditions also increased production of Br2 and I2 ; this likely results from direct O3 reactions with halides, as well as the production of gas-phase HOBr and HOI that subsequently diffuse to frozen solution to react with Br− and I− . Photochemical production of Cl2 was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at pH = 1.8. Though condensed-phase OH was shown to produce Cl2 in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for Cl2 production in the Arctic atmosphere. The condensed-phase OH-mediated halogen production mechanisms demonstrated here are consistent with those proposed from recent Arctic field observations of molecular halogen production from snowpacks. These reactions, even if slow, may be important for providing seed halogens to the Arctic atmosphere. Our results suggest the observed molecular halogen products are dependent on the relative concentrations of halides at the ice surface, as we only observe what diffuses to the air–surface interface.

On the capability of metal–halogen groups to participate in halogen bonds

Abstract: A number of halogen (X) atoms were covalently attached to a metal (M) and the ability of the X atom to act as electron acceptor in a halogen bond to nucleophile NCH was assessed. Both Cl and Br were considered as halogen atom, with NH3 and CO as other ligands attached to the metal. Metals tested were Ti, Mn, and Zn in various combinations of oxidation state, coordination, and overall charge. In the majority of cases, the strong electron-releasing power of the metal imbues the halogen atom with a high negative partial charge and minimizes the development of a σ-hole. As such, the M atom is generally a stronger attractor for the incoming nucleophile than is the halogen. Nonetheless, there are cases where a halogen bond can form such as Ti(CO)4Br+, TiCl3+, and MnCl4+, each with a different coordination. A requisite of halogen bond formation is generally an overall positive charge, although neutral species can engage in such bonds, albeit much weaker.

Halogen bonding in the structures of pentaiodobenzoic acid and its salts

Abstract: Structural characterization of pentaiodobenzoic acid (PIBA) and its salts was performed for the first time. According to XRD and DFT calculation data, in all cases, halogen bonding-type interactions play a very important role in crystal packing formation.

Halogen and hydrogen bonding in the layered crystal structure of 2-iodoanilinium triiodide, C6H7I4N

Abstract: Abstract C6H7I4N, monoclinic, P21/m (no. 11), a = 9.2818(2) Å, b = 6.55289(16) Å, c = 11.0561(3) Å, β = 114.051(3)°, V = 614.08(3) Å3, Z = 2, Rgt(F) = 0.0180, wRref(F2) = 0.0367, T = 109(2) K.

Using Chlorine Atoms to Fine-Tune the Intermolecular Packing and Energy Levels of Efficient Nonfullerene Acceptors

Abstract: Replacing the halogen atoms on the end group was considered an efficient way to enhance the performance of organic solar cells (OSCs), such as tuning energy levels, and improving sunlight absorptio...

Cooperation and competition of hydrogen and halogen bonds in 2D self-assembled nanostructures based on bromine substituted coumarins

Abstract: Three coumarin derivatives (Co16, 6-Br-Co16 and 6,8-Br-Co16) with ester, ether, and carbonyl groups and different numbers of bromine substituents on the coumarin cores were synthesized. Their two-dimensional (2D) self-assemblies were probed by scanning tunneling microscopy (STM) at the liquid–solid interface. The Co16 molecules formed a zigzag lamellar pattern on the HOPG surface through intermolecular H⋯O hydrogen bonds. The 6-Br-Co16 molecule with one Br substituent was found to form a uniform zigzag linear pattern, owing to intermolecular Br⋯OC (ester groups) halogen bonds and H⋯Br hydrogen bonds. The dibromo-substituted 6,8-Br-Co16 molecule could form a dislocated linear pattern, in which the molecules are linked through the Br⋯OC (ester group) halogen bond, and Br⋯Br and H⋯Br bonds. The STM results reveal that the introduction of the Br atom actuates the formation of the Br⋯OC bond instead of the H⋯OC bond, which is the key dominant force to form the different 2D adlayers. At the same time, the cooperation of H⋯Br hydrogen bonds leads to the stability of the nanostructures. DFT calculations were performed to unravel the cooperation and competition mechanism for the formation of halogen bonds and hydrogen bonds in 2D molecular self-assembly.

The many flavours of halogen bonds - message from experimental electron density and Raman spectroscopy.

Abstract: Experimental electron-density studies based on high-resolution diffraction experiments allow halogen bonds between heavy halogens to be classified. The topological properties of the electron density in Cl⋯Cl contacts vary smoothly as a function of the inter­action distance. The situation is less straightforward for halogen bonds between iodine and small electronegative nucleophiles, such as nitro­gen or oxygen, where the electron density in the bond critical point does not simply increase for shorter distances. The number of successful charge–density studies involving iodine is small, but at least individual examples for three cases have been observed. (a) Very short halogen bonds between electron-rich nucleophiles and heavy halogen atoms resemble three-centre–four-electron bonds, with a rather symmetric heavy halogen and without an appreciable σ hole. (b) For a narrow inter­mediate range of halogen bonds, the asymmetric electronic situation for the heavy halogen with a pronounced σ hole leads to rather low electron density in the (3,−1) critical point of the halogen bond; the properties of this bond critical point cannot fully describe the nature of the associated inter­action. (c) For longer and presumably weaker contacts, the electron density in the halogen bond critical point is only to a minor extent reduced by the presence of the σ hole and hence may be higher than in the aforementioned case. In addition to the electron density and its derived properties, the halogen–carbon bond distance opposite to the σ hole and the Raman frequency for the associated vibration emerge as alternative criteria to gauge the halogen-bond strength. We find exceptionally long C—I distances for tetra­fluoro­diiodo­benzene molecules in cocrystals with short halogen bonds and a significant red shift for their Raman vibrations.

Single-Crystal NMR Characterization of Halogen Bonds.

Abstract: Oxygen-17-enriched triphenylphosphine oxide and three of its halogen-bonded cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene as halogen bond donors have be...