Kinetics Study on Reaction between Dihydroartemisinic Acid and Singlet Oxygen: An Essential Step to Photochemical Synthesis of Artemisinin

Artemisinin is an excellent antimalarial drug widely used in clinical medicine. However, due to the limitation of natural source of artemisinin, the chemical synthesis of artemisinin has achieved substantial attention. Dihydroartemisinic acid is a key precursor for the synthesis of artemisinin. The reaction of dihydroartemisinic acid with singlet oxygen to form peroxide is a pivotal step in the photochemical preparation of artemisinin. Nevertheless, the reaction kinetics of dihydroartemisinic acid with singlet oxygen has not been investigated previously. Herein, we report the rate constants of the reaction between dihydroartemisinic acid and singlet oxygen. By directly detecting the luminescence decay kinetics of singlet oxygen at 1270 nm at room temperature, the reaction rate constants of singlet oxygen and dihydroartemisinic acid in different solvents are obtained to be 1.81\begin{document}$\times$\end{document}10\begin{document}$^5$\end{document} (mol/L)^-1·s^-1 in CCl\begin{document}$_4$\end{document}, 5.69\begin{document}$\times$\end{document}10\begin{document}$^5$\end{document} (mol/L)^-1·s^-1 in CH\begin{document}$_3$\end{document}CN, and 3.27\begin{document}$\times$\end{document}10\begin{document}$^6$\end{document} (mol/L)^-1·s^-1 in DMSO, respectively. It is found that the reaction rate constants of dihydroartemisinic acid with singlet oxygen increase as polarity of the solvent increases among the three solvents. These results provide fundamental knowledge to optimize experiment conditions of photochemical synthesis of artemisinin for improving the yields of artemisinin.     

The reactions of methyl radicals with atomic and molecular oxygen

The reaction of methyl radicals with atomic and molecular oxygen was studied with a photoionization mass spectrometer. The methyl radicals were generated by reacting oxygen atoms with ethylene in a fast-flow tube reactor. The rate constant for the reaction of methyl radicals with oxygen atoms was (1.0 ± 0.2) × 10^?10 cm³/molec · sec with no significant variation with temperature over the range of 259–341°K. The reaction of methyl radicals with molecular oxygen involves both a two-body reaction, having a rate constant \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {k_{{\rm 3a}} = (10^{- 12.54 \pm 0.35})\exp [(- 940 \pm 250)T^{- 1}]} & {{\rm cm}^{\rm 3} /{\rm molec} \cdot {\rm sec}} \end{array}$\end{document} and a three-body recombination having a negative temperature dependence. The methyl peroxy radical could be observed at its steady-state concentration. The rate constants determined at low pressures are compatible with the values determined at higher pressures by flash photolysis. Formaldehyde appears to be a major product of the two-body reaction of CH₃ with O₂, and also of the reaction of CH₃O₂ with oxygen atoms.     

Reactions of V-Substituted Polyoxometalates with L-Cysteine

Three polyoxometalates (POMs), (X=P or As) and , in their oxidized and reduced forms, were selected for direct reaction or electrocatalytic reaction with L-cysteine, because they have the most negative formal potentials among those POMs active for the desired reaction. The good linearity of the UV–Visible calibration curve obtained for the reaction of α₂-[P₂V^VW₁₇O₆₂]⁷⁻ with L-cysteine indicates both a simple 1:1 stoichiometry for the process and the possibility to select a wavelength domain in which the one-electron reduced forms of this POM is the only strongly absorbing species in the mixture. Another general result among the three selected POMs is the existence, in each example, of a sharp isosbestic point during the recording of individual spectrakinetics using a photodiode array system. The kinetics could be fitted accuretely to a mono-exponential rate equation and the rate constants were determined. Electrocatalysis of the oxidation of L-cysteine was carried out in the presence of α₂-[H₄P V^IVW₁₇O₆₂]⁹⁻ as an example. The rate constant measured by chronocoulometry for this system compares favourably with that extracted from stopped flow experiments.Dedicated in honor of Professor Michael T. Pope on the occasion of his retirement.     

Study on the intermediate of photooxidative degradation of phenylenevinylene oligomers

The results of the investigation on photooxidative degradation of five phenylenevinylene oligomers are as follows. The sequence of the photodegradation rate in self-sensitized reaction is the same as that in the biacetyl-sensitized reaction and the rate of photooxidative degradation increases upon the incorporation of electron-donating groups and decreases upon the incorporation of electron-accepting groups on the oligomers. The self-sensitized reaction rate of oligomers increases as the solvent changing from benzene to deuterated benzene and decreases with the addition of the singlet oxygen (¹O₂) scavenger. ¹O₂ is shown to be the main reactive intermediate in self-sensitized photolysis of phenylenevinylene oligomers, which was directly confirmed by ESR spin trapping experiments.     

Reaction Kinetics between Thiobases and Singlet Oxygen Studied by Direct Detection of the 1O2 Luminescence Decay

Thiobase derivatives have received important investigations due to their wide usage as phototherapeutic agents and their potential carcinogenic side effects as immunosuppressants. The substitution of oxygen atom by the sulfur atom makes the ultraviolet absorption of thiobases redshifted and absorbs UVA light (>300 nm), resulting in unusual high quantum yield of triplet state to generate the singlet oxygen (¹O₂) through photosensitization. As a type of reactive oxygen species, ¹O₂ is highly reactive toward thiobases. Herein, we report the measurements of reaction rate constants between di erent thiobases and ¹O₂ in different solvents through the direct detection of ¹O₂ luminescence decay kinetics at 1270 nm. The rate constants of thiouracils with ¹O₂ are five times smaller than that of thioguanine with ¹O₂, which suggests that thiopurines are more reactive than thiopyrimidines and thus less suitable to be a photosensitive drug on the application of photodynamic therapy. Additionally, the rate constants of thiobases and ¹O₂ were found to be obviously influenced by the solvent polarity. With the increase of solvent polarity, the rate constants of thiobases and ¹O₂ decrease.     

The gas-phase pyrolysis of alkyl nitrites. III. Isopropyl nitrite

The rate of decomposition of isopropyl nitrite (IPN) has been studied in a static system over the temperature range of 130–160°C. For low concentrations of IPN (1–5 × 10^?5M), but with a high total pressure of CF₄ (～0.9 atm) and small extents of reaction (～1%), the first-order rates of acetaldehyde (AcH) formation are a direct measure of reaction (1), since k₃ » k₂(NO): \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$ {\rm IPN}\begin{array}{rcl} 1 \\ {\rightleftarrows} \\ 2 \\ \end{array}i - \Pr \mathop {\rm O}\limits^. + {\rm NO},i - \Pr \mathop {\rm O}\limits^. \stackrel{3}{\longrightarrow} {\rm AcH} + {\rm Me}. $\end{document} Addition of large amounts of NO (～0.9 atm) in place of CF₄ almost completely suppressed AcH formation. Addition of large amounts of isobutane – t-BuH – (～0.9 atm) in place of CF₄ at 160°C resulted in decreasing the AcH by 25%. Thus 25% of \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^{\rm .} $\end{document} were trapped by the t-BuH (4): \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^. + t - {\rm BuH} \stackrel{4}{\longrightarrow} i - \Pr {\rm OH} + (t - {\rm Bu}). $\end{document} The result of adding either NO or t-BuH shows that reaction (1) is the only route for the production of AcH. The rate constant for reaction (1) is given by k₁ = 10^{16.2±0.4–41.0±0.8}/θ sec^?1. Since (E₁ + RT) and ΔH°₁ are identical, within experimental error, both may be equated with D(i-PrO-NO) = 41.6 ± 0.8 kcal/mol and E₂ = 0 ± 0.8 kcal/mol. The thermochemistry leads to the result that \documentclass{article}\pagestyle{empty}\begin{document}$ \Delta H_f^\circ (i - {\rm Pr}\mathop {\rm O}\limits^{\rm .} ) = - 11.9 \pm 0.8{\rm kcal}/{\rm mol}. $\end{document} From ΔS°₁ and A₁, k₂ is calculated to be 10^10.5±0.4M^?1·sec^?1. From an independent observation that k₆/k₂ = 0.19 ± 0.03 independent of temperature we find E₆ = 0 ± 1 kcal/mol and k₆ = 10^9.8+0.4M^?;1·sec^?1: \documentclass{article}\pagestyle{empty}\begin{document}$ i - \Pr \mathop {\rm O}\limits^. + {\rm NO} \stackrel{6}{\longrightarrow} {\rm M}_2 {\rm K} + {\rm HNO}. $\end{document} In addition to AcH, acetone (M₂K) and isopropyl alcohol (IPA) are produced in approximately equal amounts. The rate of M₂K formation is markedly affected by the ratio S/V of different reaction vessels. It is concluded that the M₂K arises as the result of a heterogeneous elimination of HNO from IPN. In a spherical reaction vessel the first-order rate of M₂K formation is given by k₅ = 10^9.4–27.0/θ sec^?1: \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm IPN} \stackrel{5}{\longrightarrow} {\rm M}_2 {\rm K} + {\rm HNO}. $\end{document} IPA is thought to arise via the hydrolysis of IPN, the water being formed from HNO. This elimination process explains previous erroneous results for IPN.     

DIRECT EXPOSURE OF MANNALIAN CELLS TO PURE EXOGENOUS SINGLET OXYGEN (ΔgO2

Mammalian cells attached to membrane filters or deposited on filters without attachment were exposed to gas-phase singlet oxygen (¹O₂) in the absence of any other reactants. Cells were exposed in a monolayer or less, in the absence of external medium, during steady-state ¹O₂ generation, ensuring that singlet oxygen impinged directly and equally on all cells simultaneously. The current methodology for cell exposure ensures that ¹O₂ is initially the only reactive species to which the cells are exposed. Results seen with this system can therefore be attributed solely and unambiguously to events initiated by ¹O₂. Further, all cells in the sample receive the same magnitude of exposure per surface area per time interval, which supports calculations of the amount of ¹O₂ required for irreversible cell damage, based on measured ¹O₂ flux and exposed cell surface area. Exposure to pure ¹O₂ irreversibly damaged a variety of cell types, including rat basophilic leukemia, human squamous carcinoma and Chinese hamster lung fibroblast cell lines, and murine primary hepatocytes. Cell survival curves following exposure to ¹O₂ followed apparent first-order kinetics. A large number of singlet oxygen collisions (? 10¹²-10¹³) were required to inactivate a cell, on average, indicating a low probability that singlet oxygen collision will reduce cell survival. Regardless of cell type or the survival endpoint measured, lethal toxicity required a fairly constant number of ¹O₂ collisions per cell. This poses a serious caveat in the assignment of causality in correlating ¹O₂-initiated cellular damage with mechanism of death, i.e. most damage observed will not be related to death. The importance of various toxic effects of ¹O₂, whether lethal or nonlethal, will depend on the magnitude of exposure and therefore on the context in which exposure occurs.     

以二氰蒽为敏化剂的聚异戊二烯敏化光降解

<正> 近年来,以二氰蒽(DGA)为敏化剂对烯类化合物进行光敏氧化研究颇受重视。Foote等曾对此作过系统工作,他们从热力学、闪光光解以及对氧化产物进行分析等方法确认其间存在着电子转移。最近Steichen等指出:二氰蒽也可做为单线态氧敏化剂在光氧化反应中发挥作用。可以认为二氰蒽作为敏化剂兼有电子转移和能量转移的能     

单分子水对H_2O_2+Cl气相反应影响的理论研究

在aug-cc-pVTZ基组下采用CCSD(T)和B3LYP方法,研究了H2O2+Cl反应,并考虑在大气中单个水分子对该反应的影响.结果表明,H2O2+Cl反应只存在一条生成产物为HO2+HCl的通道,其表观活化能为10.21kJ·mol-1.加入一分子水后,H2O2+Cl反应的产物并没有发生改变,但是所得势能面却比裸反应复杂得多,经历了RW1、RW2和RW3三条通道.水分子在通道RW1和RW2中对产物生成能垒的降低起显著的负催化作用,而在通道RW3中则起明显的正催化作用.利用经典过渡态理论(TST)并结合Wigner矫正模型计算了216.7-298.2 K温度范围内标题反应的速率常数.结果显示,298.2 K时通道R1的速率常数为1.60×10-13cm3·molecule-1·s-1,与所测实验值非常接近.此外,尽管通道RW3的速率常数kRW3比对应裸反应的速率常数kR1大了46.6-131倍,但该通道的有效速率常数k'RW3却比kR1小了10-14个数量级,表明在实际大气环境中水分子对H2O2+Cl反应几乎没有影响.     

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S₂O₈^2? or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 10⁸ m ^?1 s^?1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash‐photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2‐chlorobenzylalcohol and 2‐chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O₂(a¹Δ_g) generation (λ = 400 nm) for the VCE in D₂O, Φ_Δ = (1.3 ± 0.1) × 10^?3, was determined by measuring the O₂(a¹Δ_g) phosphorescence at 1270 nm. The value of the overall quenching constant of O₂(a¹Δ_g) by clomazone was found to be (5.7 ± 0.3) × 10⁷ m ^?1 s^?1 in D₂O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was k_r = (5.4 ± 0.1) × 10⁷ m ^?1 s^?1, which means that the quenching process is mainly reactive.     

Role of the V(V)/ 1O2Complex in Oxidative Reactions in the H2O2/V(V)/AcOH System: Oxidation of Alkenes and Anthracenes

Anthracene and its alkyl derivatives undergo oxidation in the V^(V)/H₂O₂/AcOH system via a nonradical mechanism through the intermediate formation of the vanadium(V) complex with singlet dioxygen as a ligand. The ¹O₂molecule is transferred from this complex to an unsaturated substrate. The free singlet dioxygen ¹O₂(¹ _g ) is almost inactive toward anthracene in AcOH solution. Consequently, the vanadium(V) complex with singlet dioxygen is the only oxidant species active in the reaction. The ratio between the rate constant of the reaction of this complex with 2-ethylanthracene and the rate constant of its deactivation is an order of magnitude greater than the ratio between the rate constant of the reaction of dissolved free singlet dioxygen with the same substrate and the rate constant of its deactivation (physical quenching).     

Oxidation of Chiral 1,4-Cyclohexadienes: A Comparison of Chemically and Photochemically Generated Singlet Oxygen1O2(1Δg)

Chiral alkyl-substituted 2,5-cyclohexadiene-l-carboxyIic acids la-c have been oxidized in water and in methanol with singlet oxygen, ¹O₂ (¹Δ_g), generated either photochemically or chemically from the catalytic system hydrogen peroxide/sodium molybdate. These methods were compared in terms of chemo-, regio- and diastereoselec-tivities and the chemical (k_T) and physical (k_q) quenching rate constants of ¹O₂ were determined. The ratio of the cis and trans isomers of the hydroperoxides 2a-c is not influenced by the source of ¹O₂ but, on the other hand, it depends slightly on the solvent and greatly on the steric hindrance of the substituents linked to the chiral carbon. The results may be interpreted on the basis of the successive formation of an exciplex and a perepoxide that evolves either by giving the final allylic hydroperoxide or by dissociating into the starting substrate and singlet or triplet oxygen.     

A Modified Kinetic Model for the Peroxydisulfate-Iodide Reaction

One kinetic model for the oxidation of iodide ion by peroxydisulfate ion in aqueous solution is proposed. The reaction is regarded as \documentclass{article}\pagestyle{empty}\begin{document} {\rm S}_2 {\rm O}_8^{2 -} + {\rm I}^ - {\rm IS}_2 {\rm O}_8^{3 -} \end{document}, followed by the reaction \documentclass{article}\pagestyle{empty}\begin{document} {\rm IS}_2 {\rm O}_8^{3 -} + {\rm I}l_2 + 2{\rm SO}_4^{2 -} \end{document}. If the initial rates V are obtained from the formation of the iodine molecules, the reaction rate constant k₁ and the ratio k₂/k_-1 can be estimated by plotting the values of [S₂O₈^2?][I^?]/V against that of 1/[I^?]. The extrapolated value for k₁ is 2.20×10^?2 L/mol-sec and k₂/k_-1 is calculated to be 4.25×10² mol/L at 27°C in a solution with an ionic strength of 0.420.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

Oxidation of diazo compounds by triphenyl phosphite ozonide. Quenching of singlet oxygen by diazo compounds

The reaction of triphenyl phosphite ozonide with various types of diazo compounds results in their oxidation, which is accomplished by singlet oxygen (¹O₂) evolved during thermal decomposition of the ozonide. A decrease in the ionization potential of the substrate results in an increase in the overall rate constant of quenching of¹O₂. In the case of 9-diazofluorene, the main channel of¹O₂ quenching is physical quenching.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1567–1571, September, 1994.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-5231).     

VLPR study of the reaction Br + CH3CHO

The reaction \documentclass{article}\pagestyle{empty}\begin{document}${\rm Br} + {\rm CH}_3 {\rm CHO}\buildrel1\over\rightarrow{\rm HBr} + {\rm CH}_3 {\rm CO}$\end{document} has been studied by VLPR at 300 K. We find k₁ = 2.1 × 10¹² cm³/mol s in excellent agreement with independent measurements from photolysis studies. Combining this value with known thermodynamic data gives k_-1 = 1 × 10¹⁰ cm³/mol s. Observations of mass 42 expected from ketene suggest a rapid secondary reaction: in which step 2 is shown to be rate limiting under VLPR conditions and k₂ is estimated at 10^12.6 cm³/mol s from recent theoretical models for radical recombination. It is also shown that 0 ? E₁ ? 1.4 kcal/mol using theoretical models for calculation of A₁ and is probably closer to the lower limit. Reaction ?1 is negligible under conditions used.     

Infrared Photodissociation Spectroscopic and Theoretical Study of the HC2nO+ (n=3-6) Cations

The carbon chain cations, HC_2nO⁺ (n=3-6) are produced via a pulsed laser vaporization supersonic expansion ion source in the gas phase. Their infrared spectra are measured via mass-selected infrared photodissociation spectroscopy of the CO “tagged” [HC_2nO·CO]⁺ cation complexes in 1600-3500 cm^-1 frequency range. The geometric and electronic structures of the [HC_2nO·CO]⁺ complexes and the core HC_2nO⁺ (n=3-6) cations are determined with the aid of density functional theory calculations. These HC_2nO⁺(n=3-6) ions are identified to be linear carbon chain derivatives terminally capped by hydrogen and oxygen. The triplet ground states are 10-15 kcal/mol lower in energy than the singlet states, indicating cumulene-like carbon chain structures.     

EPR and Spectrophotometric Studies of Free Radicals (O2°−, °OH, BPD-MA°−) and Singlet Oxygen (1O2) Generated by Irradiation of Benzoporphyrin Derivative Monoacid Ring A

Abstract— Benzoporphyrin derivative monoacid ring A (BPD-MA), a chlorin-type molecule, is a new photosensitizer currently in phase II clinical trials for the treatment by pho-todynamic therapy of cancerous lesions, psoriasis and pathologic neovascularization. The photochemistry (type I and/or II) of BPD-MA has been studied in homogeneous solution and in aqueous dispersions of unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) using electron paramagnetic resonance and spectrophotometric methods. When oxygen-saturated solutions of BPD-MA were illuminated with 690 nm light, singlet oxygen (¹O₂), superoxide anion radical (O₂^?), hydroxyl radical (OH) and hydrogen peroxide (H₂O₂) were formed. The BPD-MA generates ¹O₂ with a quantum yield of ca 0.81 in ethanolic solution. The quantum yield does not change upon incorporation of BPD-MA into liposomes of DPPC. The superoxide anion radical was generated by the BPD-MA anion radical (BPD-MA^?) via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The rate of production of 0₂ was also dependent on the concentration of BPD-MA used (3-100 μM). The quantum yield of O₂^?was found to be 0.011 and 0.025 in aqueous solution and DPPC liposomes, respectively. Moreover, O₂^_upon dis-proportionation can generate H₂O₂ and ultimately the highly reactive OH via the Fenton reaction. In anaerobic homogeneous solution, BPD-MA^?was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of BPD-MA. These findings suggest that the photodynamic action of BPD-MA may proceed via both type I and type II mechanisms.     

Kinetics of the reaction OH (v = 0) + O3 → HO2 + O2

The rate constant of the reaction OH (v = 0) + O₃

HO₂ + O₂ was measured over the temperature range from 220 to 450°K at total pressures between 2 and 5 torr using ultraviolet fluorescent scattering for the detection of OH radicals. An Arrhenius expression, k₁ = 1.3 × 10^?12 exp(?1900/RT) cm³/sec was obtained and the rate constant for the reaction HO₂ + O₃

OH + 2O₂ was inferred to be less than 0.1 k₁ over the entire temperature interval.     

The kinetics of the bromate–sulfite reaction system

The kinetics of the reaction between BrO₃⁻ and sulfite was studied by measuring the concentrations of [Br⁻] and [H⁺] both in buffered and in unbuffered solutions. A mechanism was applied for simulation of the experimental observations. Rate constants k₁=(0.027±0.004) M⁻¹s⁻¹ and k₂=(85±5) M⁻¹s⁻¹ were determined for the following reactions: \halign{\hfill $#$\hfill &\hfill\qquad\qquad #\cr 3\ \rm HSO_{3}\!^{-}+BrO_{3}\!^{-}\longrightarrow 3\ SO_{4}\!^{2-}+Br^{-}+3\ H^{+}& (1)\cr 3\ \rm H_{2}SO_{3}(\hbox{or}\ SO_{2.}\hbox{aq})+BrO_{3}\!^{-}\longrightarrow 3\ SO_{4}\!^{2-}+Br^{-}+6\ H^{+}& (2)\cr } Rate constant k₁ was obtained directly from the experimental results on unbuffered reactions, where Reaction (1) was predominant. Rate constant k₂ was obtained by computer fitting of [Br⁻] to the experimental values for buffered reactions, where the rate of Reaction (2) was about four times higher than that of Reaction (1). © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 869–874, 1998