EPR kinetic study of the reactions of F and Br atoms with H2CO

The absolute rate constants of the reactions F + H₂CO → HF + HCO (1) and Br + H₂CO → HBr + HCO (2) have been measured using the discharge flow reactor-EPR method. Under pseudo-first-order conditions (¦H₂CO¦?¦F¦or¦Br¦), the following values were obtained at 298 K: k₁ = (6.6 ± 1.1) × 10^?11 and k₂ = (1.6± 0.3) × 10^?12, Units are cm³ molecule^?1s^?1. The stratospheric implication of these data is discussed and the value obtained for k makes reaction (2) a possible sink for Br atoms in the stratosphere.     

Kinetics of the reactions of Cl atoms with CH3Br and CH2Br2

The rate coefficients for the reactions of Cl atoms with CH₃Br, (k₁) and CH₂Br₂, (k₂) were measured as functions of temperature by generating Cl atoms via 308 nm laser photolysis of Cl₂ and measuring their temporal profiles via resonance fluorescence detection. The measured rate coefficients were: k₁ = (1.55 ± 0.18) × 10^?11 exp{(?1070 ± 50)/T} and k₂ = (6.37 ± 0.55) × 10^?12 exp{(?810 ± 50)/T} cm³ molecule^?1 s^?1. The possible interference of the reaction of CH₂Br product with Cl₂ in the measurement of k₁ was assessed from the temporal profiles of Cl at high concentrations of Cl₂ at 298 K. The rate coefficient at 298 K for the CH₂Br + Cl₂ reaction was derived to be (5.36 ± 0.56) × 10^?13 cm³ molecule^?1 s^?1. Based on the values of k₁ and k₂, it is deduced that global atmospheric lifetimes for CH₃Br and CH₂Br₂ are unlikely to be affected by loss via reaction with Cl atoms. In the marine boundary layer, the loss via reaction (1) may be significant if the Cl concentrations are high. If found to be true, the contribution from oceans to the overall CH₃Br budget may be less than what is currently assumed. © 1994 John Wiley & Sons, Inc.     

EPR kinetic study of the reactions of CF3Br with H atoms and OH radicals

Reactions of CF₃Br with H atoms and OH radicals have been studied at room temperature at 1–2 torr pressures in a discharge flow reactor coupled to an EPR spectrometer. The rate constant of the reaction H + CF₃Br → CF₃ + HBr (1) was found to be k₁ = (3.27 ± 0.34) × 10^?14 cm³/molec·sec. For the reaction of OH with CF₃Br (8) an upper limit of 1 × 10^?15 cm³/molec·sec was determined for k₈. When H atoms were in excess compared to NO₂, used to produce OH radicals, a noticeable reactivity of OH was observed as a result of the reaction OH + HBr → H₂O + Br, HBr being produced from reaction (1).     

Hydrogen abstraction by fluorine atoms: F + HX and F + DX (X = I,Br, Cl)

Absolute reaction rates for F + HX and F + DX (X = I, Br, Cl) have been obtained by monitoring the rise time of HF (DF) vibrational fluorescence following multiphoton dissociation of SF₆ in mixtures of HX (DX) and argon. The cross sections for reaction are, in units of 10^?16 cm², 4.37, 5.26, and 1.16 for HI, HBr, and HCl, respectively. The isotope effects k_HX/k_DX, are 1.29 ± 0.14, 1.29 ± 0.18, and 1.38 ± 0.29, respectively.     

57Fe Mössbauer effect study of a presumed singlet-triplet transition in [Fe(P4)X]BPh4 complexes,X = Br,I

The ⁵⁷Fe Mössbauer effect in the solid compounds [Fe(P₄)X]BPh₄ (X = Br, I; P₄ = hexaphenyl-1,4,7,10-tetraphosphadecane) has been studied between 4.2 and 298 K. A single temperature dependent doublet is observed. For the iodo complex, ΔE_Q = 2.25 mm s^?1, δ^IS = +0.13 mm s^?1, V_tzz 0, η ≈ 0.8 at 4.2 K, whereas ΔE_Q = 1.81 mm s^?1, δ^IS = +0.20 mm s^?1 at 298 K. The results are consistent with a singlet-triplet transition with k > 10⁹ s^?1.     

Kinetic study of the reactions Br + IBr→I + Br2 and I + Br2→Br + IBr

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBr→I+Br₂ (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k₁=(2.7±0.4)×10⁻¹¹ cm³ molecule⁻¹s⁻¹. The rate constant of the reverse reaction: I+Br₂→Br+IBr (−1) has been obtained from the Br₂ consumption rate (with an excess of I atoms) and the IBr formation rate: k₋₁=(1.65±0.2)×10⁻¹³ cm³molecule⁻¹s⁻¹. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k₁ and k₋₁ and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br₂, is: K₁=k₁/k₋₁=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH₂₉₈°=−(3.6±0.1) kcal mol⁻¹ and the heat of formation of the IBr molecule, ΔH_f,298°(IBr)=(9.8±0.1) kcal mol⁻¹. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998     

Rate constants for the reactions of Br atoms with a series of alkanes,alkenes, and alkynes in the presence of O2

Rate constants of Br atom reactions have been determined using a relative kinetic method in a 20 l reaction chamber at total pressures between 25 and 760 torr in N₂ + O₂ diluent over the temperature range 293–355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O₂ present in the reaction system. Values of (6.8 ± 1.4) × 10^?15, (3.6 ± 0.7) × 10^?14, (1.5 ± 0.3) × 10^?12, (1.6 ± 0.3) × 10^?13, (2.7 ± 0.5) × 10^?12, (3.4 ± 0.7) × 10^?12, and (7.5 ± 1.5) × 10^?12 (units: cm³ s^?1) have been obtained as rate constants for the reactions of Br with 2,2,4-trimethylpentane, acetylene, propyne, ethene, propene, 1-butene, and trans-2-butene, respectively, in 760 torr of synthetic air at 298 K with respect to acetaldehyde as reference, k = 3.6 × 10^?12 cm³ s^?1. Formyl bromide and glyoxal were observed as primary products in the reaction of Br with acetylene in air which further react to form CO, HBr, HOBr, and H₂O₂. Bromoacetaldehyde was observed as an primary product in the reaction of Br with ethene. Other observed products included CO, CO₂, HBr, HOBr, BrCHO, bromoethanol, and probably bromoacetic acid.     

Kinetic and equilibrium study of Br atom reactions with trimethylsilane using the very low pressure reactor technique

The very low pressure reactor technique was used for measurement of the forward (k₁) and reverse (k_?1) rate constants for the Br atom reaction with trimethylsilane: Br + (CH₃)₃SiH ? HBr + (CH₃)₃S1. From the kinetic data and entropy estimation the bond dissociation energy for SiH in trimethylsilane is 90.1 = 1.1 kcal/mole. The Arrhenius parameters for k₁ and k_?1 were obtained.     

Production of electronically excited atoms. H + Br2 → HBr + Hb(*), H + HBr → H2 + Br(*)

By measurement of infrared chemiluminescence we have obtained for the branching ratio of the room temperature reaction H + Br₂ (1), k*₁/k₁ = 0.015 ± 0.004 and for H + HBr (2), k*₂/k₂ ? 0.013. For H + Br₂ → HBr(υ^· ? 6) + Br (1), the detailed rate constant k(υ^* = 6) = 0.014 ± 0.003 relative to k(υ^· = 4) = 100.     

Kinetics of nucleophilic attack upon coordinated organic moieties.: XXIII. Attack by cyclohexylamine upon the diene ligands in [MX2(1,5-cyclooctadiene)] (M = Pd,Pt; X = Cl,Br) complexes

Kinetic and spectroscopic studies of the reactions of cyclohexylamine with the complexes [MX₂(1,5-cyclooctadiene)] (I) (M = Pd; X = Cl, Br; M = Pt, X = Br) in acetone reveal the rate law, k_obs = K₁k₂[amine]², for the rapid sequence

For X = Br, the palladium(II) complex is ca. 70 times more reactive than its platinum(II) analogue. This is the first quantitative comparison reported to date for nucleophilic attack upon olefins coordinated to Pd^II and Pt^II centres. The reactivity order Pd^II ⪢ Pt^II may arise from the higher ionization potential of Pd²⁺ compared to Pt²⁺, which makes Pd^II a less effective back-π-bonder. Replacing the bromo ligands in [PdBr₂(1,5-COD)] by chloro ligands lowers the rate of formation of III by a factor of 8.     

Energy distribution among reaction products. XIV. F + CH4, F + CH3X(X  Cl,Br, I), F + CHnCl4−n(n = 1−3)

The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction:

For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?^′_ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH₃Cl to CH₃Br to CH₃I, viz <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.58 (1b), <?^′_ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.23 (2a), <?^′_ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?^′_ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH₂Cl, CHCl₂, CCl₃, due to increase in the number of secondary encounters between HF and the departing radical.     

Kinetics of the reactions Br + NO2 + M and I + NO2+ M

The reactions Br + NO₂ + M → BrNO₂ + M (1) and I + NO₂ + M → INO₂ + M (2) have been studied at low pressure (0.6-2.2 torr) at room temperature and with helium as the third body by the discharge-flow technique with EPR and mass spectrometric analysis of the species. The following third order rate constants were found k₁₍₀₎ = (3.7 ± 0.7) × 10^?31 and k₂₍₀₎ = (0.95 ± 0.35) × 10^?31 (units are cm⁶ molecule^?2 s^?1). The secondary reactions X + XNO₂ → X₂ + NO₂ (X = Br, I) have been studied by mass spectrometry and their rate constants have been estimated from product analysis and computer modeling.     

Rate constant for the reaction Br + O3 → BrO + O2 from 248 to 418 K: Kinetics and mechanism

The rate constant for the Br + O₃ → BrO + O₂ reaction was measured by the discharge flow technique, employing resonance fluorescence detection of Br. Over the temperature range 248 to 418 K, in 1 to 3 torr of He, decays of Br in excess O₃ yield the value k₁ = (3.28 ± 0.40) × 10^?11 e^[?944±30]/T cm³ molecule^?1 s^?1. Cited uncertainties are at the 95% confidence level and include an estimate of the systematic errors. The rate constants for the reactions of O₃ with Br, Cl, F, OH, O, and N correlate with the electron affinities of the radicals suggesting that the reactions proceed through early transition states dominated by transfer of electron density from the highest occupied molecular orbital of ozone to the singly occupied radical MO. The implications of this new measurement of k₁ for stratospheric chemistry are discussed.     

Rate coefficients for the reactions CH3 + Br2 (224–358 K), CH3CO + Br2 (228 and 298 K), and Cl + Br2 (228 and 298 K)

Rate coefficients for the reactions of CH₃ + Br₂ (k₂), CH₃CO + Br₂ (k₃), and Cl + Br₂ (k₅) were measured using the laser‐pulsed photolysis method combined with detection of the product Br atoms using resonance fluorescence. For the reactions involving organic radicals, the rate coefficients were observed to increase with decreasing temperature and within the temperature range explored, were adequately described by Arrhenius‐like expressions: k₂ (224–358 K) = 1.83 × 10^?11 exp(252/T) and k₃ (228–298 K) = 2.92 × 10^?11 exp(361/T) cm³ molecule^?1 s^?1. The total, temperature‐independent uncertainty for each reaction (including possible systematic errors in Br₂ concentration measurement) was estimated as ～7% for k₂ and 10% for k₃. Accurate data on k₅ was obtained at 298 K, with a value of 1.88 × 10^?10 cm³ molecule^?1 s^?1 obtained (with an associated error of 6%). A limited data set at 228 K suggests that k₅ is, within experimental uncertainty, independent of temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 575–585, 2010     

Catalytic effects of cationic nanoparticle (CTABr/NaX/H<Subscript>2</Subscript>O,X = Cl,Br) for the piperidinolysis of phenyl salicylate ions

The expression of pseudo-second-order rate constants (k _X) for cationic nanoparticle (CN) [CTABr/NaX/H₂O, X = Br, Cl, CTABr = cetyltrimethylammonium bromide] catalyzed piperidinolysis-ionized phenyl salicylate (PSa^–), at constant [CTABr]_T, 0.1 M piperidine (Pip), and 35°C, were calculated from the relationship: k _obs = (k ₀ + k _X[NaX])/(1 + K ^X/S[NaX]), in which k ₀, k _X, and K ^X/S are constant kinetic parameters and k _obs represents the pseudo-first-order rate constant for Pip reaction with phenyl salicylate ion in the presence of CN. The source of the large catalytic effect of CN catalyst was shown to be due to the transfer of PSa^– from pseudo-phase of the CNs to the bulk aqueous phase through X^–/PSa^– ion exchange at the surface of the CNs.     

Pulse radiolysis studies of intermolecular charge transfers involving tryptophan and three-electron-bonded intermediates derived from methionine

The oxidation processes of the radiation-generated, three-electron-bonded intermediates AcMet₂ [S??S]⁺ and AcMet [S??Br] were investigated by pulse radiolysis via their reactions with tryptophan (TrpH). These intermediates were derived from N-acetyl-methionine amide (N-AcMetNH₂) and N-acetyl-methionine methyl ester (N-AcMetOMe). The bimolecular rate constant k of the reaction between each intermediate and l-tryptophan (TrpH) was measured. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 3.4?×?10⁸ and 2.2?×?10⁸?dm³?mol^?1?s^?1 at pH?=?1 and 4.5, respectively. For N-AcMetOMe, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 4.0?×?10⁸ and 2.8?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. The rate constants for the intermolecular transformation of Met [S??Br] into TrpH⁺ or Trp were also estimated. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??Br] with TrpH were 2.6?×?10⁸ and 3.3?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. Related mechanisms were discussed.     

Kinetic study on the reaction of cis-dioxo-(N-(5-X-salicylidene)-2-aminobenzenethiolato) molybdenum(VI) with ethyldiphenylphosphine

The reactions of ethyldiphenylphosphine with a number of cis-dioxomolybdenum(VI) Schiff base coordination complexes are described. These molybdenum complexes incorporate tridentate Schiff base ligands obtained from the condensation of 5-X-salicylaldehyde (X = Cl, Br, H, CH₃O) with o-aminobenzenethiol. Oxomolybdenum(IV) Schiff base complexes were observed as products of the reaction of these Mo(VI) complexes with PEtPh₂. The kinetics for these reactions were followed spectrophotometrically and the applicable rate law is ? d[MoO₂L]/dt = k₁[MoO₂L][PEtPh₂]. The k₁'s were shown to vary systematically as the X-substituent on the ligand was changed. For MoO₂(5-X-SSP), the specific rate constants at 30°C span the range from 19.6 × 10^?4 M^?1 sec^?1 (X = Br) to 8.4 × 10^?4 M^?1 sec^?1 (X = CH₃O). It was also observed that a correlation exists between the cathodic reduction potentials (E_pc) and the k₁'s within the series. The rate of reaction of MoO₂(5-X-SSP) with PEtPh₂ was altered and systematically controlled through ligand design.     

Rate constant for the reaction of atomic bromine with ozone

The rate constant for the reaction Br + O₃ → BrO + O₂ has been measured over the temperature range 224 to 422 K in a discharge flow system using a mass spectrometer as a detector. Results, expressed in the form k₁ = (3.34 ± 0.40) × 10^?11 X exp[?(978 ± 36)/T] cm³ s^?1, are compared with previous measurements.     

Equilibrium,kinetics, and mechanism of the malonic acid–iodine reaction

The equilibrium constant for the reaction CH₂(COOH)₂ + I₃^? ? CHI(COOH)₂ + 2I^? + H⁺, measured spectrophotometrically at 25°C and ionic strength 1.00M (NaClO₄), is (2.79 ± 0.48) × 10^?4M². Stopped-flow kinetic measurements at 25°C and ionic strength 1.00M with [H⁺] = (2.09-95.0) × 10^?3M and [I^?] = (1.23-26.1) × 10^?3M indicate that the rate of the forward reaction is given by (k₁[I₂] + k₃[I₃^?]) [HOOCCH₂COO^?] + (k₂[I₂] + k₄[I₃^?]) [CH(COOH)₂] + k₅[H⁺] [I₃^?] [CH₂(COOH)₂]. The values of the rate constants k₁-k₅ are (1.21 ± 0.31) × 10², (2.41 ± 0.15) × 10¹, (1.16 ± 0.33) × 10¹, (8.7 ± 4.5) × 10^?1M^?1·sec^?1, and (3.20 ± 0.56) × 10¹M^?2·sec^?1, respectively. The rate of enolization of malonic acid, measured by the bromine scavenging technique, is given by k_en[CH₂(COOH)₂], with k_en = 2.0 × 10^?3 + 1.0 × 10^?2 [CH₂(COOH)₂]. An intramolecular mechanism, featuring a six-member cyclic transition state, is postulated to account for the results on the enolization of malonic acid. The reactions of the enol, enolate ion, and protonated enol with iodine and/or triodide ion are proposed to account for the various rate terms.     

Rate of the reaction I2 + F2 → products

The rate coefficient, k1, for the reaction I2+F2k1→ products has been measured at room temperature to be k1 = (1.9 = 0.4) × 10?15 cm3/molecule s. The macroscopic rate is compared to microscopic cross-section data obtained from molecular beam experiments and is found to be consistent with the bimolecular reaction I2 + F2→ I2F + F.DG|National Research Council/Resident Research Associate.