Reaction Of Br Research Articles

Does bromine atom-free radical (Br·) undergo a chemical reaction before dimerization to dibromine (Br2)? An electrochemical investigation

The important question, does bromine atom free radical, Br·, undergo a chemical reaction with a substrate before it dimerizes to dibromine Br2, is investigated through electrochemical oxidation of bromide ion Br−. Cyclic voltammetry and spectrophotometry were used as electro-oxidation and monitoring devices. Simulation of cyclic voltammogram was used to obtain information about the mechanism of electro-oxidation process as well as reaction with substrates. It is shown that the primary electro-oxidation product (PEOP) of Br−, Br· (bromine atom-free radical), reacts with chemicals like chloroform, glycine, cytosine, etc. It can also dimerize, for example in acetonitrile. Reactions of PEOP Br· with substrate chloroform, acetone, DNA components adenine, cytosine, thymine and amino acid glycine were studied and their kinetics are reported here.

Influence of collision energy and vibrational excitation on the dynamics for the H+HBr→H2+Br reaction

Quasi-classical trajectory (QCT) calculations of H + HBr → H2+Br reaction have been performed on a recently proposed ab initio potential energy surface. The reaction probability and integral cross section are found to be in fairly good agreement with the available quantum mechanical (QM) results on this surface. The behavior of reactivity is well consistent with properties of exothermic reaction. Once the energy of vibrational excited HBr is larger than the barrier height, the integral cross sections for the reaction diverge at very low collision energies close to the threshold, similarly to capture reaction. In addition, differential cross sections show that scattering of the product H2 shift from backward to forward directions as the collision energy and vibrational quantum number increase. All the theoretical findings are reasonably explained by the properties of the surface, as well as reactive mechanisms. A detailed dynamical study of reaction, H+HBr (v=0-3,j=0) → H2+Br, with QCT method has been performed. Both present QCT and available QM results for the HBr (v=0, j=0) have been found to be fairly consistent with each other. The reaction switches from activated to capture-type with increase of the quantum number v.

Kinetics of the Bromine Reaction at Carbon-Polymer Composite Electrodes in the Presence of Complexing Agents

The zinc-bromine hybrid redox flow battery (RFB) is one of the few battery systems that have seen implementation for the medium to large scale energy storage. One of the issues identified with this flow battery is that of bromine crossover from the positive electrode to the zinc electrode, leading to loss of current efficiency. One of the ways in which this problem can be overcome is through the use of organic-based complexing agents which tie up the bromine as a thin surface layer and an immiscible liquid phase which is pumped from the electrode surface into an external reservoir. In this paper, we examine in detail the kinetics and chemistry of formation of these complexed bromine ion pairs and establish the concentrations of the various complexing agents required to achieve full complexation of the electrogenerated bromine species. The initial phases of the study used small chain (2,3 and 4-carbon) symmetrical tetraalkylammonium bromide salts to investigate the bromide oxidation and the stages of film formation on a variety of carbon-composite electrodes commonly employed in RFB systems. These data were then compared to the classical responses obtained on Pt and glassy carbon electrodes. What is clearly evident from the study is that the electrochemical reversibly of the Br-/Br2 reaction is attenuated on the carbon-composite surfaces in the presence of the complexing agents and leads to a second oxidation process at more positive potentials (Figure 1). The nature of this second oxidation and of the surface species formed is investigated using in-situ Raman spectroscopy and its electrochemical characteristics examined using electrochemical impedance spectroscopy and potentiodynamic polarisation techniques. Substantial changes in the electrochemical responses are recorded when film formation occurs and this event also led to a large increase in the measured impedance, requiring the use of an additional low frequency time constant element in the Randles equivalent circuit employed to fit the impedance spectrum acquired. The magnitudes and trends observed in the impedance parameters at the different concentration and complexing species employed will be discussed in the presentation. Since in an operational Zn-Br2 RFB system during charge, the oxidation reaction will be occurring on composite carbon surface covered with the Brx - complex, how this film impacts on the kinetics of the charge and discharge reaction will impact on the voltage efficiency. The understanding of the behaviour of these simpler complexing agents will thus enable the electrochemistry of more complex species, e.g. 1-methyl-1-ethylpyrrolidinium bromide, to be better understood and so lead to improvements in the efficiency of the process occurring at the positive electrode of the zinc-bromine hybrid RFB. Figure 1

The Reaction between Bromine and the Water Dimer and the Highly Exothermic Reverse Reaction.

The entrance complex, transition state, and exit complex for the bromine atom plus water dimer reaction Br + (H2O)2 → HBr + (H2O)OH and its reverse reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ-PP. Based on the CCSD(T)/cc-pVQZ-PP results, the reaction is endothermic by 31.7 kcal/mol. The entrance complex Br⋯(H2O)2 is found to lie 6.5 kcal/mol below the separated reactants. The classical barrier lies 28.3 kcal/mol above the reactants. The exit complex HBr⋯(H2O)OH is bound by 6.0 kcal/mol relative to the separated products. Compared with the corresponding water monomer reaction Br + H2 O → HBr + OH, the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by 3.0, 3.8, and 3.7 kcal/mol, respectively. Both zero-point vibrational energies and spin-orbit coupling effects make significant changes to the above classical energetics. Including both effects, the predicted energies relation to separated Br + (H2O)2 are -3.0 kcal/mol [Br···(H2O)2 ], 28.2 kcal/mol [transition state], 26.4 kcal/mol [HBr···(H2O)OH], and 30.5 kcal/mol [separated HBr + (H2O)OH]. The potential energy surface for the Br + (H2O)2 reaction is related to that for the valence isoelectronic Cl + (H2O)2 system but radically different from the F + (H2O)2 system.

Development of High Surface Area Carbon Electrodes for the Bromine Reactions in H2-Br2 fuel Cells

The H2-Br2 system is considered as one of the viable options for large scale electrical energy storage due to faster H2 and Br2 reaction kinetics, low cost, and high round-trip efficiency. Since the bromine reactions in this fuel cell system do not need any precious metal catalysts, plain porous carbon paper can be used as an electrode material. However, these porous carbon electrodes have low surface area and hence multiple electrode layers are employed to improve the active surface area. While the active surface area is enhanced by using multiple electrode layers, it could lead to mass transport limited performance at higher current densities. To overcome this problem, porous carbon electrodes with multi-wall carbon nanotubes grown directly on their surface were developed. The cross section of a carbon nanotube based electrode is shown in Figure 1. In this presentation, we will discuss the synthesis and characterization of carbon nanotube based electrodes as well as their durability and actual performance in a H2-Br2 fuel cell. Also, the advantages of these carbon nanotube based electrodes over conventional porous carbon electrodes will be highlighted. Acknowledgements This work was funded by NSF through grant number EFRI-1038234 and DOE/ARPA-E under award number DE-AR0000262. References Haley Kreutzer, Venkata Yarlagadda, and Trung Van Nguyen, J. Electrochem Soc., 159(7), F331 (2012).Venkata Yarlagadda and Trung Van Nguyen, J. Electrochem. Soc., 160(6), F535 (2013).Cheng Wang, Mahesh Waje, Xin Wang, Jason M. Tang, Robert C. Haddon, and Yushan Yan, Nano Letters, 4(2), 345 (2004).Chunyu Du, Baorong Wang, and Xinqun Cheng, J. Power Sources, 187, 505 (2009).Venkata Yarlagadda and Trung Nguyen, ECS Transactions, 58(36), 25 (2014). Figure 1

Stereodynamical Origin of Anti-Arrhenius Kinetics: Negative Activation Energy and Roaming for a Four-Atom Reaction.

The OH + HBr → H2O + Br reaction, prototypical of halogen-atom liberating processes relevant to mechanisms for atmospheric ozone destruction, attracted frequent attention of experimental chemical kinetics: the nature of the unusual reactivity drop from low to high temperatures eluded a variety of theoretical efforts, ranking this one among the most studied four-atom reactions. Here, inspired by oriented molecular-beams experiments, we develop a first-principles stereodynamical approach. Thermalized sets of trajectories, evolving on a multidimensional potential energy surface quantum mechanically generated on-the-fly, provide a map of most visited regions at each temperature. Visualizations of rearrangements of bonds along trajectories and of the role of specific angles of reactants' mutual approach elucidate the mechanistic change from the low kinetic energy regime (where incident reactants reorient to find the propitious alignment leading to reaction) to high temperature (where speed hinders adjustment of directionality and roaming delays reactivity).

Theoretical study of the reaction kinetics of atomic bromine with tetrahydropyran.

A detailed theoretical analysis of the reaction of atomic bromine with tetrahydropyran (THP, C5H10O) was performed using several ab initio methods and statistical rate theory calculations. Initial geometries of all species involved in the potential energy surface of the title reaction were obtained at the B3LYP/cc-pVTZ level of theory. These molecular geometries were reoptimized using three different meta-generalized gradient approximation (meta-GGA) functionals. Single-point energies of the stationary points were obtained by employing the coupled-cluster with single and double excitations (CCSD) and fourth-order Møller-Plesset (MP4 SDQ) levels of theory. The computed CCSD and MP4(SDQ) energies for optimized structures at various DFT functionals were found to be consistent within 2 kJ mol(-1). For a more accurate energetic description, single-point calculations at the CCSD(T)/CBS level of theory were performed for the minimum structures and transition states optimized at the B3LYP/cc-pVTZ level of theory. Similar to other ether + Br reactions, it was found that the tetrahydropyran + Br reaction proceeds in an overall endothermic addition-elimination mechanism via a number of intermediates. However, the reactivity of various ethers with atomic bromine was found to vary substantially. In contrast with the 1,4-dioxane + Br reaction, the chair form of the addition complex (c-C5H10O-Br) for THP + Br does not need to undergo ring inversion to form a boat conformer (b-C4H8O2-Br) before the intramolecular H-shift can occur to eventually release HBr. Instead, a direct, yet more favorable route was mapped out on the potential energy surface of the THP + Br reaction. The rate coefficients for all relevant steps involved in the reaction mechanism were computed using the energetics of coupled cluster calculations. On the basis of the results of the CCSD(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall rate coefficients can be expressed as kov.,calc.(T) = 4.60 × 10(-10) exp[-20.4 kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1) for the temperature range of 273-393 K. The calculated values are found to be in excellent agreement with the experimental data published previously.

Synthesis, Characterization, and Oxidation of New POCNimine-Type Pincer Complexes of Nickel

This report describes the synthesis and characterization of new pincer-type complexes of divalent and trivalent nickel. Refluxing toluene mixtures of NiBr2(CH3CN)n and the unsymmetrical pincer-type ligands Ph-POCHNimine-R gave (Ph-POCNimine-R)Ni(II)Br in good yields (Ph-POCHNimine-R = 1-(Ph2PO),3-(CHNR)C6H4; R = Bn, 1; Ph, 2; t-Bu, 3; Cy, 4). The C–H nickelation step involved in these syntheses is facilitated in the presence of NEt3. Treating compounds 1–4 with Br2, I2, N-bromosuccinimide (NBS) and N-chlorosuccinimide appears to bring about the desired one-electron oxidation of the Ni(II) center in these complexes, but the putative trivalent species decompose over time and could not be isolated. One of the decomposition products obtained in the Br2 reaction was identified as the zwitterionic compound [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)benzylidene)benzenaminium}], 5. To identify the factors that are important for isolation of stable trivalent derivatives, we prepared the divalent precursors (i...

Fundamental change in the nature of chemical bonding by isotopic substitution.

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br(4)HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

Some observations regarding the behavior of boron subphthalocyanines in polar aprotic solvents

In this paper, we describe some observations from the attempted reaction of Br - BsubPc with phenol in DMF and DMAc. During these efforts, we found that Br - BsubPc reacts with the respective solvents to produce axially substituted formate- BsubPc and acetate- BsubPc . When no phenol is present the reaction proceeds to completion in a relatively short period of time. However, when phenol was present in DMF the reaction produced a mixture of formate- BsubPc , phenoxy- BsubPc , and HO - BsubPc . Similar results were found for the less-reactive Cl - BsubPc and MsO - BsubPc . Aside from these observations, it was found that simple heating in wet acetone of Br - BsubPc quantitatively produced HO - BsubPc after a facile workup. This method of producing HO - BsubPc removes the high temperatures, long reaction times, and harmful chemicals required in other synthetic methods. These results are relevant to anyone considering the reaction of BsubPcs in polar aprotic solvents.

Functional ZnO/polymer core-shell nanowires fabricated by oxidative chemical vapour deposition

Functional ZnO-nanowire/polymer core-shell heterostructures were realized using oxidative chemical vapour deposition (oCVD). This dry and versatile technique allows uniform coating of semiconductor nanowires with polymers and simultaneous doping control of the shell. Here, 100 nm thick, p-doped shells of poly(3,4-ethylenedioxythiophene) (PEDOT) were deposited around n-conductive ZnO nanowires. Energy-dispersive x-ray spectroscopy confirms the incorporation of Br dopants into the PEDOT shell, and the resulting p-conductivity of the polymer shell is demonstrated by electrical measurements on nanowire arrays. Photoluminescence spectroscopy points to reactions of Br with the ZnO surface but proves that the nanowires show only little degradation of their optical properties.

Anchoring the potential energy surface for the Br + H2O → HBr + OH reaction

The forward and reverse reactions Br + H2O → HBr + OH are important in atmospheric and environmental chemistry. Five stationary points on the potential energy surface for the Br + H2O → HBr + OH reaction, including the entrance complex, transition state, and exit complex, have been studied using the CCSD(T) method with correlation-consistent basis sets up to cc-pV5Z-PP. Contrary to the valence isoelectronic F + H2O system, the Br + H2O reaction is endothermic (by 31.8 kcal/mol after zero-point vibrational, relativistic, and spin–orbit corrections), consistent with the experimental reaction enthalpy. The CCSD(T)/cc-pV5Z-PP method predicts that the reverse reaction HBr + HO → Br + H2O has a complex but no classical barrier. When zero-point vibrational energies are added, the transition state lies 0.25 kcal/mol above the separated products. This is consistent with the negative temperature dependence for the rate constant observed in experiments. The entrance complex is predicted to lie 2.6 kcal/mol below separated Br + H2O. The exit complex is predicted to lie 1.8 kcal/mol below separated HBr + OH.

Study of the Ne((3)P2) + CH3F electron-transfer reaction below 1 K.

Relatively little is known about the dynamics of electron-transfer reactions at low collision energy. We present a study of Penning ionization of ground-state methyl fluoride molecules by electronically excited neon atoms in the 13 μeV–4.8 meV (150 mK–56 K) collision energy range, using a neutral–neutral merged beam setup. Relative cross sections have been measured for three Ne((3)P2) + CH3F reaction channels by counting the number of CH3F(+), CH2F(+), and CH3(+) product ions as a function of relative velocity between the neon and methyl fluoride molecular beams. Experimental cross sections markedly deviate from the Langevin capture model at collision energies above 20 K. The branching ratios are constant. In other words, the chemical shape of the CH3F molecule, as seen by the Ne((3)P2) atom, appears not to change as the collision energy is varied, in contrast to related Ne((3)PJ) + CH3X (X = Cl and Br) reactions at higher collision energies.

High Surface Area Carbon Electrodes for the Bromine Reactions in H2-Br2 Fuel Cells

The H2-Br2 system is considered as one of the viable options for large scale electrical energy storage due to faster H2 and Br2 reaction kinetics, low cost, and high round-trip efficiency. In order to improve the performance of the H2-Br2 fuel cell, carbon electrodes with high active surface area are needed. A preliminary study was conducted to synthesize and characterize the carbon nanotube based electrodes. Based on the results obtained from SEM, TEM, and EDX analysis, multi-wall carbon nanotube growth is confirmed. Also, the results from the multi-step chronoamperometry study have shown that the active areas of carbon nanotube electrodes are significantly higher than a plain carbon electrode.

Thermal decomposition of tetrabromosilane and deposition of crystalline silicon

Pyrolysis of tetrabromosilane (SiBr4) was carried out by passing vaporized SiBr4 with hydrogen over a substrate heated to less than 1000°C. The substrate was then placed in a reaction furnace equipped with a flow system and the exhaust gas containing the reactants and products was analyzed versus temperature. SiBr4 began to decompose at 800°C simultaneously with the evolution of tribromosilane (SiHBr3), which was followed by the deposition of silicon in a crystalline form. The conversion of SiBr4 increased with increasing reaction temperature. No reaction occurred in the absence of H2. An experiment with tetrachlorosilane (SiCl4) under the similar conditions led to a decrease in SiCl4 concentration at temperatures greater than 900°C. This demonstrates that decomposition of SiBr4 is energetically favorable compared to that of SiCl4, as predicted from the Ellingham diagrams for the reactions of Br- and Cl-containing species. Thus, Si production from bromosilanes is an alternative to the process using SiHCl3, the so-called Siemens method, which is the main technology for Si production currently.

Thermal Rate Constants for the O(3P) + HBr and O(3P) + DBr Reactions: Transition-State Theory and Quantum Mechanical Calculations

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

Scattering resonance state of Br+HBr(v=0)→BrH(v′=0)+Br reaction explored by partial potential energy surface method

The partial potential energy surface(PPES) of Br+HBr(v=0)→BrH(v′=0)+Br was designed by coupling the vibration energy and the minimum energy of the corresponding reaction path, Vmep. All the calculations were performed at the theoritical level of QCISD(T)/6-311++G**//MP2/6-311++G**. Based on the analysis of PPES, the dynamic “Eyring Lake” mechanism gave birth to the scattering resonance state. The resonance energy was also obtained via PPES. Then a lifetime matrix of the resonance state was established by solving the translational wave-function via the numerical propagation method. Then the reaction resonance lifetime was calculated to be 125 fs. It is in good agreement with the experimental result.

Surfactant-Promoted Reactions of Cl2 and Br2 with Br– in Glycerol

Gas-liquid scattering experiments are used to explore reactions of gaseous Cl2 and Br2 with a 0.03 M solution of the surfactant tetrahexylammonium bromide (THABr) dissolved in glycerol. At thermal collision energies, 79 ± 2% of incident Cl2 molecules react with Br(-) to form Cl2Br(-) in the interfacial region. This reaction probability is three times greater than the reactivity of Cl2 with 3 M NaBr-glycerol, even though the interfacial Br(-) concentrations are similar in each solution. We attribute the high 79% uptake to the presence of surface THA(+) ions that stabilize the Cl2Br(-) intermediate as it is formed in the charged, hydrophobic pocket created by the hexyl chains. Cl2Br(-) generates the single exchange product BrCl in a 1% yield close to the surface, while the remaining 99% desorbs as the double exchange product Br2 over >0.1 s after diffusing deeply into the bulk. When NaCl is added to the surfactant solution in a 20:1 Cl(-)/Br(-) ratio, the Cl2 reaction probability drops from 79% to 46 ± 1%, indicating that Cl(-) in the interfacial region only partially blocks reaction with Br(-). In parallel, we observe that gaseous Br2 molecules dissolve in 0.03 M THABr for 10(4) times longer than in 3 M NaBr. We attribute this change to formation of stabilizing interfacial and bulk-phase THA(+)Br3(-) ion pairs, in analogy with the capture of Cl2 and formation of THA(+)Cl2Br(-) pairs. The THA(+) ion appears to be a powerful interfacial catalyst for promoting reaction of Cl2 and Br2 with Br(-) and for ferrying the resultant ions into solution.

Computational study on the mechanisms and reaction pathways of the CX3O2 + Br (X = F and Cl) reactions

The mechanisms for the reactions of CX3O2 (X=F and Cl) with atomic bromine were studied using quantum chemistry methods. The result indicates that the title reactions could take place on the singlet and triplet potential energy surfaces (PES). On singlet PES, the most favourable product is CX3OOBr (IM1) formed via a barrierless of CX3O2 association with Br. As for the CCl3OO+Br reaction, other products such as CCl3O+BrO, CCl2O+ClOBr, ClOCCl2OBr and CCl2O+Cl+OBr could make certain contributions in different conditions. On the triplet PES, substitution and direct abstraction mechanisms were located, but it could be negligible compared with that on the singlet PES. Time Dependent Density Functional Theory (TDDFT) calculation indicates that CX3OOBr could take photolysis easily under the sunlight.

Telluride clusters of molybdenum: Synthesis, structures, and NMR spectra

High-temperature reactions of Mo, chalcogen (S or Se), Te, and Br2 in molar ratio Mo: S/Se: Te: Br = 3: 1: 6: 4 were carried out. The reaction products were subjected to mechanochemical activation with K(Dtp) (Dtp = (EtO)2PS2) in a vibrational mill, resulting in the formation of new compounds [Mo3(μ3-Q)0.5(μ3-O)0.5(μ2-Te2)3(Dtp)3](Dtp) (Q = Se (I) and S (II)). The structure of compound I has been established by X-ray diffraction analysis. Solutions of compounds I and II contain mixtures of [Mo3(μ3-Q)(μ2-Te2)3(Dtp)3]+ and [Mo3(μ3-O)(μ2-Te2)3(Dtp)3]+, which is confirmed by mass spectrometry and 31P, 77Se, and 125Te NMR spectroscopy. Quantum-chemical calculations of the 125Te NMR chemical shifts were performed. The compounds are also characterized by IR spectroscopy, Raman spectroscopy, and elemental analysis. Structure I contains short nonvalent contacts between the sulfur atom of the out-of-sphere Dtp anion and the axial tellurium atoms of the cluster.