Reactions Of HBr Research Articles

Insight into generalized local softness based on ABEEM‐σπ method: Electrophilic additions to alkenes

AbstractAccording to the local hard‐soft acid – base (HSAB) principle, the generalized local softness has been investigated based on the electrophilic reactions of H2O, HF, HCl and HBr to alkenes, and the effect of the softness on the chemical reactivity was studied in terms of the ab initio and atom‐bond electronegativity equalization method plus σπ (ABEEM‐σπ) method. Studies show that the finite difference approximation with ab initio method cannot always be used to predict the regioselectivity of the investigated reactions in terms of the local HSAB principle. But ABEEM‐σπ model could well describe chemical reactivity according to the local HSAB principle. A significant characteristic of the ABEEM‐σπ method is that the double bond is explicitly considered and partitioned. The square sums of differences of the local softness of the reaction center atoms, that is, Δs, or ΔSG with the generalized local softness and with the electronegativity, have been studied on the considered electrophilic additions. Studies indicate that ΔSG can be utilized not only to predict regioselectivity rules but also to rationalize the chemical reactivity of these addition reactions, and the generalized local softness is more suitable for the investigation of chemical reactivity between two chemical species.

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct.

Chemical activation experiments and computational methods have been used to study the unimolecular reactions of C2H5CH2Br and C2D5CHFBr with 90 and 93 kcal mol-1 of vibrational energy, respectively. The four-centered elimination reactions of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr, and 1,1-HF reactions are also observed from C2D5CHFBr. The main focus was to search for the role of the C2D5(F)C:HBr adduct in the 1,1-HBr elimination for comparison with carbene adducts in 1,1-HX(Y) elimination from RCHXY (X,Y = Cl and F) molecules. Models of transition states and molecules from electronic structure calculations were used in statistical calculations of the rate constants to assign threshold energies for each reaction based on the experimental rate constants. The threshold energy for 2,1-HBr elimination from 1-bromopropane is 50 kcal mol-1, which is in basic agreement with thermal activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permits discussion of the kinetic isotope effects and the effect of F atom substitution on the threshold energy for 2,1-HBr elimination. Although CD3CD═CDF from 1,1-HBr elimination of C2D5CHFBr followed by D atom migration is an experimentally observed product, dissociation of the C2D5(F)C:HBr adduct may be the rate-limiting step rather than crossing the barrier associated with the transition state for 1,1-HBr elimination. The calculated dissociation energies of C2H5(X)C:HF adducts are 9.9, 9.3, and 9.0 kcal mol-1 for X = F, Cl, and Br, and the values for C2H5(F)C:HX are 9.9, 6.4, and ∼4.9 kcal mol-1.

Flame Inhibition Chemistry: Rate Coefficients of the Reactions of HBr with CH3 and OH Radicals at High Temperatures Determined by Quasiclassical Trajectory Calculations

Reactions of HBr with radicals are involved in atmospheric chemistry and in the mechanism of operation of bromine-containing flame retardants. The rate coefficients for two such reactions, HBr + OH and HBr + CH3, are available from earlier experiments at near or below room temperature, relevant for atmospheric chemistry, and in this domain, the activation energy for both has been found to be negative. However, no experimental data are available at combustion temperatures. In this work, to provide reliable data needed for modeling the action of brominated flame suppressants, we used the quasiclassical trajectory (QCT) method in combination with high-level ab initio potential energy surfaces to evaluate the rate coefficients of the two title reactions at combustion temperatures. The QCT calculations have been validated by reproducing the experimental rate coefficients at room temperature. At temperatures between 600 and 3200 K, the QCT rate coefficients display positive activation energies. We recommend the...

Inhibition of hydrogen oxidation by HBr and Br 2

The high-temperature bromine chemistry was updated and the inhibition mechanisms involving HBr and Br 2 were re-examined. The thermochemistry of the bromine species was obtained using the Active Thermochemical Tables (ATcT) approach, resulting in improved data for, among others, Br, HBr, HOBr and BrO. Ab initio calculations were used to obtain rate coefficients for selected reactions of HBr and HOBr, and the hydrogen/bromine/oxygen reaction mechanism was updated. The resulting model was validated against selected experimental data from the literature and used to analyze the effect of HBr and Br 2 on laminar, premixed hydrogen flames. Our work shows that hydrogen bromide and molecular bromine act differently as inhibitors in flames. For HBr, the reaction HBr + H ⇌ H 2 + Br (R2) is rapidly equilibrated, depleting HBr in favor of atomic Br, which is the major bromine species throughout the reaction zone. The chain-breaking steps are then H + Br + M → HBr + M (R1), Br + HO 2 → HBr + O 2 (R7), and Br + Br + M → Br 2 + M (R8). In Br 2-doped flames, the reaction Br 2 + H ⇌ HBr + Br (R9) is far from equilibration and serves to deplete H in the reaction zone by competing with H + O 2 → O + OH. The inhibition is augmented by recombination of Br (R8). If the inlet Br 2 mole fraction exceeds about 20%, reactions (R8) and (R2) are both reversed, now acting to promote chain branching and increase the flame speed. According to the present model, cycles involving HOBr are not important for generation or removal of chain carriers in these flames.

Rotational dependence of the proton-transfer reaction HBr+ + CO2 → HOCO+ + Br. II. Comparison of HBr+ (2Π3/2) and HBr+ (2Π1/2)

The effects of reactant ion rotational excitation on the exothermic proton-transfer reactions of HBr(+)((2)Π(1/2)) and DBr(+)((2)Π(1/2)), respectively, with CO(2) were studied in a guided ion beam apparatus. Cross sections are presented for collision energies in the center of mass system E(c.m.) in the range of 0.23 to 1.90 eV. The HBr(+)/DBr(+) ions were prepared in a state-selective manner by resonance enhanced multiphoton ionization. The mean rotational energy was varied from 3.4 to 46.8 meV for HBr(+)((2)Π(1/2)) and from 1.8 to 40.9 meV for DBr(+)((2)Π(1/2)). Both reactions studied are inhibited by collision energy, as expected for exothermic reactions. For all collision energies considered, the cross section decreases with increasing rotational energy of the ion, but the degree of the rotational dependence differs depending on the collision energy. For E(c.m.) = 0.31 eV, the cross sections of the deuteron transfer are significantly larger than those of the proton transfer. For higher E(c.m.) they differ very little. The current results for the exothermic proton transfer are systematically compared to previously published data for the endothermic proton transfer starting from HBr(+)((2)Π(3/2)) [L. Paetow et al., J. Chem. Phys. 132, 174305 (2010)]. Additional new data regarding the latter reaction are presented to further confirm the conclusions. The dependences on rotational excitation found cannot be explained by the corresponding change in the total energy of the system. For both the endothermic and the exothermic reaction, the cross section is maximized for the smallest rotational energy, at least well above the threshold.

Reactions of HBr+ ions in the 2Π i , v + quantum states with H2 and HBr molecules

The mechanisms and kinetics of low-temperature ion-molecular reactions between the Br+, HBr+, and DBr+ions and the HBr, DBr, H2, and D2 molecules were studied. The HBr+ (i,v+) and DBr+(i,v+) ions were prepared in separate spin-orbit (i) and vibrational (v+) states by resonance multiphoton ionization in a free flow of halogen halides (HBr and/or DBr) with hydrogen, deuterium, or inert gases (Ar, He). The effectiveness of various reaction channels, including the exchange of charges, H and D atoms, and H+andD+ ions, was studied. The quantitative data on the kinetics of these reactions were obtained for separate quantum states of the ions. The resonance ionization of one of the two ion isotopomers H79Br+(D79Br+) or H81Br+(D81Br+) was used to study and compare the effectiveness of various ion-molecular reaction channels.

Surfactant Control of Gas Uptake: Effect of Butanol Films on HCl and HBr Entry into Supercooled Sulfuric Acid

The entry of HCl into 60-68 wt % D(2)SO(4) and HBr into 68 wt % acid containing 0-0.18 M 1-butanol was monitored by measuring the fractions of impinging HCl and HBr molecules that desorb as DCl and DBr after undergoing H --> D exchange within the deuterated acid. The addition of 0.18 M butanol to the acid creates butyl films that reach approximately 80% surface coverage at 213 K. Surprisingly, this butyl film does not impede exchange but instead enhances it: the HCl --> DCl exchange fractions increase from 0.52 to 0.74 for 60 wt % D(2)SO(4) and from 0.14 to 0.27 for 68 wt % D(2)SO(4). HBr --> DBr exchange increases even more sharply, rising from 0.22 to 0.65 for 68 wt % D(2)SO(4). We demonstrate that this enhanced exchange corresponds to enhanced uptake into the butyl-coated acid for HBr and infer this equivalence for HCl. In contrast, the entry probability of the basic molecule CF(3)CH(2)OH exceeds 0.85 at all acid concentrations and is only slightly diminished by the butyl film. The OD groups of surface butanol molecules may assist entry by providing extra interfacial protonation sites for HCl and HBr dissociation. The experiments suggest that short-chain surfactants in sulfuric acid aerosols do not hinder heterogeneous reactions of HCl or HBr with other solute species.

Volcanic sources of tropospheric ozone‐depleting trace gases

Recent investigations report mixing ratios of BrO gas reaching 1 ppb in plume 4–7 km downwind of the summit of Soufrière Hills volcano, Montserrat. The detection of BrO in volcanic plumes is potentially important evidence of halogen‐catalyzed tropospheric ozone destruction and suggests that volcanoes either directly emit BrO or emit bromine species that are rapidly converted to reactive bromine in volcanic plumes. Species distribution models constrained by volcanic gas and condensate analytical data for arc, rift, and hot spot volcanoes demonstrate that shallow magma degassing generates volcanic gases with ppb to ppm levels of reactive radicals Br, Cl, H, and HO but no significant BrO or ClO. The conversion of volcanic Br and Cl by reaction with ozone in gas‐phase catalytic reaction cycles during plume transport can lead to secondary BrO and ClO at mixing ratios of a ppt and higher, possibly approaching a ppb for especially halogen‐rich volcanic gases, several kilometers downwind of degassing sources. In general, however, ppb BrO and ClO levels in volcanic plumes several kilometers downwind probably require near‐vent, high‐temperature reaction of magmatic gases with air and/or in‐plume heterogeneous chemical processes involving aerosols during plume transport. These processes oxidize bromine and chlorine in HBr and HCl, giving rise to increased levels of reactive Br, Cl, and HO, in addition to NOx. They can also produce significant amounts of the photochemically active precursors Br2, BrCl, and Cl2, which are photolyzable to reactive Br and Cl. The precursors may build up in nighttime volcanic plumes and accelerate ozone destruction and BrO and ClO production after sunrise. Downwind conversion of reactive Br, Cl, H, and HO from volcanic gas sources by catalytic reaction chains to HOBr and HOCl may trigger heterogeneous chemical processes, which are probably essential for sustaining ozone destruction in volcanic plumes. Gas‐phase reactions of HBr and HCl with HO in volcanic plumes might also boost BrO and ClO downwind. Thus in‐plume processes that generate reactive halogens may contribute significantly to ozone destruction in volcanic plumes. Enhanced levels of HOx and NOx in volcanic emissions may trigger nonhalogen catalytic cycles depleting ozone in tropospheric volcanic plumes.

Common Precursor Mechanism for the Heterogeneous Reaction of D2O, HCl, HBr, and HOBr with Water Ice in the Range 170−230 K: Mass Accommodation Coefficients on Ice

A chemical kinetic model is presented which fits the uptake kinetics of both steady-state and time-resolved heterogeneous reactions of D2O, HCl, HBr, and HOBr with ice substrates that have been studied in a Knudsen flow reactor. The salient feature of the model is the existence of two precursors to adsorption that explain the observed negative temperature dependence, as well as the absence of saturation, of the rate of uptake at the experimental conditions met in the Knudsen flow-reactor studies. From the temperature dependence of the fitted rate constants, enthalpy diagrams have been obtained in which the loosely bound precursor state has a binding energy on ice of 7.6−8.0, 6.4, 3.9, and 6.9 kcal/mol for D2O, HCl, HBr, and HOBr, respectively. The mass accommodation coefficients α at 200 K were 0.43 (D2O, B = bulk ice), 0.34 (D2O, C = condensed ice), 0.67 (HOBr, B-type ice), 0.36 (HCl, B-type ice), and 0.38 (HBr, B-type ice) and show a negative temperature dependence as well. The chemical kinetic model describes the transition between submonolayer Langmuir adsorption at low to multilayer adsorption at high doses or partial pressures.

Molecular Beam Scattering from Supercooled Sulfuric Acid: Collisions of HCl, HBr, and HNO3 with 70 wt D2SO4

Proton exchange, residence time, and gas uptake measurements are used to explore collisions and reactions of HCl, HBr, and HNO3 with 70 wt % D2SO4 at 213 K. These studies help to provide a detailed...

PREPARATION AND PROPERTIES OF DIAZENIDO, HYDRAZIDO(2-), AND HYDRIDO-HYDRAZIDO(2-) COMPLEXES OF TUNGSTEN. X-RAY PHOTOELECTRON SPECTROSCOPY OF DINITROGEN AND HYDRAZIDO(2-) COMPLEXES OF MOLYBDENUM AND TUNGSTEN

Abstract Reactions of HBr with trans-[W(N2)2(dppe)PPh2Me)2] (1) (dppe = Ph2CH2CH2PPh2) result in protonation of coordinated N2 but no formation of ammonia or hydrazine. The tungsten-containing product depends upon the reaction conditions: (i) in MeOH, the product formed is [WBr(NNH2) (dppe)(PPh2Me)2]HBr2 (2) which converts to the hydride, [WBr2(H)(NNH2(dppe)(PPh2Me)](Br(3), with loss of phosphine in THF or CH2Cl2, (ii) in THF or CH2Cl2, the hydride (3) is formed directly. Reaction of 2 with Na2CO3 in MeOH results in the loss of HBr and the formation of the diazenido complex [WBr(NNH)(dppe)(PPh2Me)2] which reacts further with Na2CO3 in benzene under N2 to lose HBr and form a mixture of 1 and trans-[W(N2)(dppe)2]. The reaction of 1 with aqueous HF forms [WF(NNH2)(dppe)(PPh2Me)2]BF4. The X-ray photoelectron spectra of trans-[M(N2)2 (dppe)2], [MBr(NNH2)(dppe)2Br (M = Mo, W), [WCl(NNH2)(dppe)2]Cl, [WCl(N)(dppe)2]Cl and [WCl(NH) (dppe)2] are reported. In all of these complexes, nitrogen is in a highly reduced form.

Kinetics of the reactions of HBr with O3 and HO2: The yield of HBr from HO2 + BrO

An upper limit on the yield of HBr from reaction (R1) (HO2 + BrO → products) has been determined by measuring an upper limit for the rate coefficient of the reverse reaction (R1′) (HBr + O3 → HO2 + BrO). The limits measured at 300 and 441 K were extrapolated to low temperatures to determine that the yield of HBr from reaction (R1) is negligible throughout the stratosphere (<0.01% of k1). An upper limit for the rate coefficient of the reaction of HO2 with HBr was also determined to be very low ≤3×10−17 cm3 molecule−1 s−1 at 300 K and ≤3×10−16 cm3 molecule−1 s−1 at 400 K. The implications of these results to stratospheric chemistry are discussed.

Selected ion flow drift tube studies of the reaction of HBr + with various neutral molecules

The reactions of HBr + with CF 4, NO, CO 2, CH 4, N 2O, SO 2, COS, H 2S and SF 6 have been studied in a SIFDT apparatus. For some reactions the variation of rate constant with centre of mass collision energy E cm has been investigated. The rate constant for the CH 4 reaction exhibits an unusual variation with E cm which is shown to be due to the different energy variations of the proton transfer, hydrogen abstraction and electron transfer channels. The reduced mobilities of HBr + and H 2Br + in helium have been determined up to E/N −120 Td.

An absolute proton affinity scale in the ∼130–140 kcal mol−1 range

The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable-temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol−1, utilizing the literature value of P.A.(Br)=131.8 kcal mol−1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol−1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from selected ion flow drift tube measurements. An upper limit to P.A.(SF6) of 127 kcal mol−1 has been inferred from the instability of SF6 H+ towards dissociation into SF+5 and HF.

The pseudo-enzymatic action of tri-o-thymotide clathrates applied to the determination of the absolute configuration of four halohydrins

The heterogeneous reactions of gaseous HCl and HBr, as external agents, with trans-2,3-dimethyloxirane and 2,2,3-trimethyloxirane as guest molecules in tri-o-thymotide clathrates are shown to allow the ready determination of the absolute configuration of the end-products owing to highly specific host-reactant interactions.

The reactions of ground and excited state sodium atoms with hydrogen halide molecules

The reactions of ground and excited state Na atoms with hydrogen halide (HX) molecules have been studied using the crossed molecular beams method. With both increasing translational and increasing electronic energy, the reactive cross sections increase in the reactions of HCl and HBr. From product angular and velocity distributions detailed center-of-mass information is derived. For the reactions of Na (3 2S 1 2 , 3 2P 1 2 , 4 2D 5 2 , 5 2S 1 2 ) with HCl, the product NaCl is back-scattered with respect to the incoming Na atom in the center-of-mass frame of reference. The reaction of each Na state studied with HCl is direct and proceeds via collinear and near-collinear Na-Cl-H approach geometries. For the Na (3 2P 3 2 ) and Na (4 2D 5 2 ) reactions with HCl the predominant transition state symmetry is 2Σ in a collinear (C ∞ν) Na-Cl-H geometry. This is consistent with the reaction proceeding via electron transfer from the Na atom to the halide atom. Absolute reactive cross sections for each state of Na studied with HCl were determined by comparison with both small and large angle elastic scattering. We were unable to observe Na atoms with over 4 eV of electronic energy react with HF up to collision energies of 13 kcal/mole.

ChemInform Abstract: Reactions of HCl(g), DCl(g), and HBr(g) with the Adduct Pyrazine‐Phosphorus(V) Chloride.

AbstractThe heterogeneous reaction between (I) and HCl at ‐63 °C indicates that initial protonation of the p‐nitrogen in the pyrazine ring takes place.

Raman spectra of Cl and Br at 12 K produced from the reaction of HCl or HBr with NO2

AbstractThe products of the reactions of HCl or HBr with nitrogen dioxide at 12 K have been examined using Raman spectroscopy. For the NO2+HCl reaction spectroscopic evidence for Cl, asym‐N2O2, N2O3, N2O4, ClNO2, ClNO3, NO, NO, and sym‐N2O2 was found. For the reaction with HBr only Br, was detected due to resonance Raman enhancement effects. These results are consistent with the previously proposed gas‐phase reaction scheme for HCl+NO2. However, the reaction proceeds further at low temperatures to produce Cl (or Br). The Raman frequencies for Cl were observed at 515(ν3), 462(ν1), and 236 cm−1(ν2). The Br resonance Raman spectra, which represent the first well documented spectra for this species, show strong fundamental bands at 281(ν1) and 227 and 238 cm−1 (doublet for ν2) as well as an overtone doublet at 471 and 487 cm−1.

Infrared spectra of the M+HBr2− and the M+HClBr− ion pairs and their deuterium analogs isolated in argon matrices at 15 K

The matrix reactions of alkali bromide salts and HBr and DBr have been investigated in argon matrices. In each case, an intense band was observed from the reaction product, which was assigned to the antisymmetric stretch, ν3, of the HBr2− and DBr2− anions in the M+HBr2− and M+DBr2− ion pairs. Comparisons to literature spectra, in conjunction with quartic deuterium shifts, indicate that the anion has a linear, centrosymmetric geometry in the isolated ion pair. A less intense band was also observed which has been assigned to the combination band ν1+ν3 for the HBr2− anion. Reaction of alkali bromide salts with HCl and alkali chloride salts with HBr have been shown to yield identical absorptions for the M+HClBr− ion pairs. The spectra suggest that both type I and type II HClBr− anions exist, and the resulting form may be determined by cation position.

Infrared laser specific reactions of boron compounds. CO 2 laser control of the exchange reactions B(CH 3) nBr m + HBr → B(CH 3) n-1 Br m + 1 + CH 4

Laser specific product formation has been achieved in the exchange reactions of HBr with boron trimethyl B(CH 3) 3 and the mixed boron methyl bromides B(CH 3) 2Br and B(CH 3)Br 2. The non-thermal nature of these reactions was shown by the line specific action of the CO 2 laser radiation and by an in-situ temperature probe.