| 1. |
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| 2. |
- Shi, Tiesheng, et al.
(författare)
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Equilibrium and High-Pressure Kinetic Study of Formation and Proton-Assisted Aquation of Monodentate Acetate, Propionate, and Glycolate Complexes of Palladium(II) in Aqueous Solution
- 1996
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 1996:35, s. 735-740
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Tidskriftsartikel (refereegranskat)abstract
- SynopsisReversible formation and proton-assisted aquation of 1:1 monodentate carboxylate complexes of palladium(II) takes place via a transition state stabilized by hydrogen bonding in an activation process strongly dominated by bond making. Volume profiles have been established through high-pressure kinetic measurements.AbstractKinetics and equilibria for reversible formation of 1:1 monodentate complexes between Pd(H2O)42+ and acetic, propionic, and glycolic acid (RCOOH) according to the equation Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1) have been studied as a function of temperature and pressure in an aqueous medium with 0.60 ≤ [H+] ≤ 1.00 M, [RCOOH]/[H+] ≤ 1.0, and ionic strength 1.00 M. Stability constants β1 for the formation of acetate, propionate, and glycolate complexes are (2.19 ± 0.09) × 104, (2.10 ± 0.24) × 104, and (6.4 ± 0.1) × 103 M-1, repectively, at 25 °C and for a 1.00 M perchlorate medium. The rate expression, kobsd = k1[RCOOH] + k-1[H+], indicates that reaction between palladium and carboxylate anions is negligible under the experimental conditions used. Values for k1/M-1 s-1, k-1/M-1 s-1, ΔH1⧧/kJ mol-1, ΔH-1⧧/kJ mol-1, ΔS1⧧/J K-1 mol-1, ΔS-1⧧/J K-1 mol-1, ΔV1⧧/cm3 mol-1, and ΔV-1⧧/cm3 mol-1 at 25.0 °C are 19.3 ± 0.6, 32.8 ± 0.2, 52.5 ± 0.4, 57.5 ± 0.4, −44 ± 1, −23 ± 1, −8.1 ± 0.3, and −1.7 ± 0.2 for acetic acid; 12.0 ± 1.1, 26.4 ± 0.3, 55.3 ± 0.6, 56.8 ± 0.3, −32 ± 2, −34 ± 1, −8.9 ± 0.8, and −1.7 ± 0.2 for propionic acid; and 21.1 ± 0.3, 13.7 ± 0.1, 53.6 ± 0.4, 59.2 ± 0.6, −40 ± 1, −25 ± 2, −3.4 ± 0.2, and −2.3 ± 0.2 for glycolic acid. There is no relation between formation rate constants and pKa for these carboxylic acids. The nucleophilic properties of the carboxylic acids toward palladium(II) are similar to those of water molecules. A trigonal bipyramidal transition state stabilized by hydrogen bonding between the entering carboxylic acid and the leaving aqua ligand or (equivalent) between the entering oxonium ion and the leaving carboxylate ligand is suggested. Negative entropies, volumes of activation, and volume profiles support a strong contribution from bond making in the activation process.
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| 3. |
- Shi, Tiesheng, et al.
(författare)
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Linear Free Energy Relationships for Complex Formation Reactions between Carboxylic Acids and Palladium(II). Equilibrium and High-Pressure Kinetics Study
- 1997
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 36:4, s. 528-536
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Tidskriftsartikel (refereegranskat)abstract
- SynopsisLinear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.AbstractAbstract ImageKinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow spectrophotometry. Kinetics traces for reactions between the metal ion and formic, butyric, lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acids can be described by single exponentials, which are assigned to the formation of monodentate complexes: Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1). Equilibrium constants K1 for lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acid reactions calculated from spectrophotometric equilibrium measurements and from kinetics (K1 = k1/k-1) are in good agreement. There is a linear correlation between the stability constants β1 of the carboxylato complexes and the first dissociation constants Ka1 of the carboxylic acids as expressed by log β1 = (0.48 ± 0.03)pKa1 + (2.1 ± 0.1). The formation rate constants k1 are insensitive to the basicity and steric properties of the carboxylic acids at 25 °C, due to an excellent isokinetic relationship between ΔH1⧧ and ΔS1⧧ with an isokinetic temperature of 292 K, suggesting also that all of the carboxylic acids react via the same mechanism. Rate constants k-1 are correlated with pKa1 of the entering carboxylic acids according to log k-1 = (0.47 ± 0.06)pKa1 − (0.7 ± 0.2), indicating that the weaker the carboxylic acid, the larger k-1. These facts together with the observation that a weak carboxylic acid is prone to form a strong complex as shown by the correlation between log β1 and pKa1 are interpreted in terms of a proton-assisted reaction mechanism which is further supported by the ionic strength dependence of the rate constant k-1, consistent with a direct attack of an oxonium ion on Pd(H2O)3OOCR+ for the reverse reaction. High-pressure stopped-flow measurements at 25 °C give activation volumes ΔV1⧧ = −4.9 ± 0.2 cm3 mol-1, ΔV-1⧧ = −2.7 ± 0.5 cm3 mol-1 for malonic acid, and ΔV1⧧ = −3.5 ± 0.2 cm3 mol-1, ΔV-1⧧ = −1.9 ± 0.4 cm3 mol-1 for citric acid, respectively. Existing data so far for Pd(H2O)42+ complex formation reactions obey a linear correlation between ΔV1⧧ and partial molar reaction volumes ΔV1° according to ΔV1⧧ = (0.92 ± 0.04)ΔV1° − (2.2 ± 0.2) cm3 mol-1, the slope of 0.92 indicating that bond-making between palladium and the entering ligands largely dominates the formation of the transition state. There are no linear correlations between ΔV1⧧ and partial molar volumes VL of the entering ligands, as has been claimed in previous literature for related reactions, or between ΔV1⧧ and ΔS1⧧.
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| 4. |
- Shi, Tiesheng, et al.
(författare)
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Structure-Reactivity Correlations for Complex Formation Reactions between Square-Planar Metal Centers and Thioethers
- 1996
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 35:20, s. 5941-5947
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Tidskriftsartikel (refereegranskat)abstract
- SynopsisReactivity trends of thioethers toward square-planar complexes can be generally interpreted by use of an equation of the form: log k = γ + α∑σ* + βθ, where k denotes second-order rate constants, ∑σ* the sum of Taft constants for the thioether substituents, and θ the thioether cone angles. γ represents an intrinsic parameter for each substrate complex, while α is an electronic and β a steric parameter. Large variations in both electronic and steric properties of the entering ligands indicate that there is no duality behavior in the reactions of thioethers with square-planar metal centers.AbstractAbstract ImageKinetics for complex formation between Pd(H2O)42+ and thioethers of largely varying electronic and steric properties, viz. MeSCH2COOH, (n-Pr)2S, EtSCH2CH2OH, S(CH2CH2CH2OH)2, S(CH2CH2OH)2, EtSCH2COOH, S(CH2COOH)2, S(CH2CH2COOH)2, (i-Pr)2S, (s-Bu)2S, (t-Bu)2S, and protonated thiomorpholine, S(C2H4)2NH2+, has been studied by use of stopped-flow spectrophotometry in an acidic aqueous medium. Second-order rate constants k1298 are 1.61 × 104, 8.0 × 104, 3.79 × 104, 3.69 × 104, 2.21 × 104, 1.84 × 104, 1.91 × 103, 1.34 × 104, 1.52 × 104, 7.75 × 103, 900, and 5.2 × 103 M-1 s-1, respectively. The reactivity toward Pd(H2O)42+ of all thioethers studied so far can be described as a function of their σ-donor properties as expressed by the sum of the Taft constants, ∑σ*, and their steric requirements as defined by cone angles, θ, by use of the equation: log k1 = (9.9 ± 0.3) − (0.67 ± 0.05)∑σ* − (0.059 ± 0.003)θ. Similarly, second-order rate constants k298 reported previously for reactions between thioethers and Pd(dien)H2O2+ and Pt(dien)Br+ are described by log k298 = (10.5 ± 0.6) − 0.67∑σ* − (0.081 ± 0.006)θ and log k298 = (4.6 ± 0.6) − 0.72∑σ* − (0.080 ± 0.006)θ, respectively. Hence, the reactivity trends of thioethers toward square-planar complexes can be given a general interpretation in terms of intrinsic, electronic, and steric parameters, by use of log k = γ + α∑σ* + βθ. Large variations in both electronic and steric properties of the entering ligands indicate that there is no “duality behavior” in the reactions of thioethers with square-planar metal centers, as claimed in previous literature. No steric threshold is observed for these sterically unhindered systems. There is a rough compensation effect between ΔH1⧧ and ΔS1⧧, i.e. a smaller ΔH1⧧ is usually accompanied by a larger negative ΔS1⧧, indicating that all thioethers react via the same mechanism. It appears that the much lower reactivity observed for the highly branched (t-Bu)2S is primarily caused by a high activation enthalpy. Volumes of activation have been determined for a series of thioethers with a constant cone angle, viz. EtSCH2COOH, S(CH2COOH)2, and S(CH2CH2COOH)2 through high-pressure stopped-flow measurements. Values of ΔV1⧧ are −7.9 ± 0.5, −8.1 ± 0.4, and −7.6 ± 0.3 cm3 mol-1, respectively. These values together with that for Et2S (−8.7 ± 0.1 cm3 mol-1) determined previously shows that variation of electronic properties, steric factors being kept constant, change the reactivity markedly, but have no observable influence on the activation volumes. Stability constants β1 for five palladium thioether complexes derived as the ratio between rate constants for forward and reverse reactions vary between (1.2 ± 0.3) × 104 and (3.2 ± 0.7) × 104 M-1.
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| 5. |
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| 6. |
- Berglund, Johan, et al.
(författare)
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Kinetics and Mechanism for Manganese-Catalyzed Oxidation of Sulfur(IV) in Aqueous Solution
- 1993
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 1993:32, s. 4527-4538
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics for manganese-catalyzed autoxidation of sulfur(IV) (SO2.nH2O, HSO3-, SO32-) in aqueous solution has been studied spectrophotometrically at 25-degrees-C, 1 < pH < 4, [S(IV)] almost-equal-to 2.3 X 10(-5) M, 1 X 10(-6) M less-than-or-equal-to [Mn(II)] less-than-or-equal-to 1.5 X 10(-3) M, [Mn(III)] less-than-or-equal-to 4 X 10(-8) M, [O2] almost 2.5 X 10(-4) M, and low ionic strength. In the absence of added manganese(III), the kinetic traces display an induction period followed by a reaction first-order in sulfur(IV). Addition of a small amount of manganese(III) increases the rate significantly and suppresses the induction period. At pH 2.4, the first-order rate constant is k(obsd) = k[Mn(II)](1 + B[Mn(III)]0)/(A + [Mn(II)]), where A and B are constants and [Mn(III)]0 is the concentration of initially added manganese(III). The experiments are interpreted in terms of a free-radical chain mechanism. The first step, with rate constant k8 = (1.3 +/- 0.6) X 10(6) m-1 s-1, is a reaction between manganese(III) and a manganese(II) hydrogen sulfite complex with stability constant beta1 = (3 +/- 1) X 10(4) M-1: Mn(III) + MnHSO3+ -->(k8) 2Mn(II) + SO3- + H+. The catalytic activity of Mn(III/II) may be explained by formation of an oxo- (or hydroxo-) bridged mixed-valence precursor complex Mn(III)-O-Mn(II)-HSO3, in which bridged electron transfer produces the SO3 radical. When [Fe(III)] < 10(-6) M is added to the reacting system, the oxidation rate becomes much faster than the sum of the individual contributions from the manganese and iron catalyses; i.e., a synergistic effect is displayed. Initiation of the manganese-catalyzed oxidation in the absence of initially added manganese(III) is shown to be due to a trace impurity of ca. (1-2) x 10(-8) M iron(III). The SO3- radical is generated by the oxidation of sulfur(IV) by iron(III). In subsequent steps, manganese(II) is oxidized to manganese(III) by SO5- formed by oxidation of SO3 with dissolved oxygen. Computer simulation of the overall kinetics shows that an iron(III) concentration of 2 x 10(-8) M is indeed sufficient to initiate the manganese-catalyzed oxidation and explains the autocatalysis. At pH 4.0, the first-order rate constant is k(obsd) = (k[Mn(II)] + k[Mn(II)]2)/(A + [Mn(II)]). The quadratic manganese(II) term indicates formation of a sulfito-bridged manganese(II) complex, MnSO3Mn2+, which can also be attacked by manganese(III), forming SO3- radicals. Several discrepancies in previous literature may be explained by the reaction mechanism derived.
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| 7. |
- Berglund, Johan, et al.
(författare)
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Kinetics and Mechanism for Oxidation of Dissolved Sulfur Dioxide by Tetracyanodichloroplatinate(IV) in Acidic Aqueous Solution and for Formation and O-S Linkage Isomerization of Sulfitotetracyanochloroplatinate(IV)
- 1994
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 1994:33, s. 3346-3353
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation of SO2.nH2O/HSO3-/SO32- by Pt(CN)4Cl22- has been studied at 25-degrees-C and 1.0 M ionic strength in acidic aqueous solution by use of stopped-flow spectrophotometry. The stoichiometry of the reaction is 1:1 according to Pt(CN)4Cl22- + HSO3 + H2O --> Pt(CN)42- + 2Cl- + HSO4- + 2H+. It is first-order with respect to both [Pt(IV)] and [S(IV)]. The pH-dependence in the region 0.3 < pH < 4.5 is complex and indicates that Pt(CN)4Cl22- is reduced by both HSO3- and SO32- in two parallel reactions with second-order rate constants 1.34-0.5 M-1 s-1 and (4.5 +/- 0.1) x 10(5) M-1 s-1, respectively. The suggested mechanism involves attack by the incoming nucleophiles on coordinated chloride, followed by inner-sphere two-electron transfer to the metal center and formation of Pt(CN)42-and ClSO3H and ClSO3-. Chlorosulfuric acid and chlorosulfate(VI) hydrolyze in rapid subsequent reactions to HSO4-. The kinetics for reaction between Pt(CN)4ClOH2- and HSO3- has also been studied by use of variable-temperature and high-pressure stopped-flow spectrophotometry in an aqueous medium with 0.3 < pH < 2 and as a function of ionic strength 0.325 < I < 1.0 M. This is a three-step process. An oxygen-bonded sulfito complex, Pt(CN)4Cl(OSO2)3-, is formed rapidly by addition of HSO3- to the hydroxo ligand of Pt(CN)4ClOH2-, without breakage of the Pt-O bond. In the second step, this complex undergoes a slower, rate-determining intramolecular linkage isomerization to the thermodynamically more stable S-bonded isomer Pt(CN)4Cl(SO3)3-, which is reduced rapidly to Pt(CN)42- and HSO4- in an inner-sphere two-electron process. Rate constants at 25-degrees-C, DELTAH(double-dagger), DELTAS(double-dagger), and DELTAV(double-dagger) are for the uptake reaction (1.68 +/- 0.05) X 10(-5) M-1 s-1, 44 +/- 3 kJ mol-1, -7.5 +/- 0.4 J K-1 mol-1, and -9.4 +/- 0.4 cm3 mol-1 and for the O-S isomerization 4.5 +/- 0.2 s-1, 74.4 +/- 0.5 kJ mol-1, 16.6 +/- 0.1 J K-1 mol-1, and 5.3 +/- 0.4 cm3 mol-1. The positive entropy and volume of activation for the linkage isomerization indicate that this process takes place through an intramolecular rearrangement via a loosely bound transition state, in which the Pt-O bond is weakened before a new Pt-S bond is formed. In all experiments, the concentration of dissolved molecular oxygen remains constant. This is in agreement with the proposed mechanism, according to which electron transfer is completed within the solvent cage and no radical species are released into the bulk solution, starting an autoxidation of S(IV).
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| 8. |
- Berglund, Johan, et al.
(författare)
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Kinetics and Mechanism for Reduction of Tetrachloroaurate(III), trans-dicyanodichloroaurate(III), and trans-dicyanodibromoaurate(III) by Sulfite and Hydrogen Sulfite
- 1995
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 34:2, s. 513-519
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Tidskriftsartikel (refereegranskat)abstract
- Reduction of [AuCl4]- and trans-[Au(CN)2X2]- (X = Cl, Br) by sulfur(IV) as SO2.nH2O, HSO3-, and SO3(2-) has been studied at 25-degrees-C in aqueous solution with ionic strength 1.0 M and 0 < pH < 2.3 by use of stopped-flow spectrophotometry. Redox takes place directly without initial substitution at the gold(III) centers with stoichiometry Au(III):S(IV) = 1:1 and with Au(I) complexes and HSO4- as products. A mechanism with two parallel redox reactions and HSO3- and SO3(2-) as reductants results in the following respective rate constants for reduction of [AuCl4]-, trans-[Au(CN)2Cl2]-, and trans-[Au(CN)2Br2]-: by HSO3-, 35 +/- 9, (1.5 +/- 0.2) x 10(2), and (1.7 +/- 0.2) x 10(3) M-1 s-1; by SO3(2-) (6.8 +/- 0.4) x 10(6), (1.6 +/- 0.1) x 10(7), and (1.8 +/- 0.1) x 10(8) M-1 s-1. Reduction is ca. 10(5) times faster with SO3(2-) than with HSO3-. A halide-bridged, two-electron transfer in a transition state where the sulfur of the reductant interacts with the halid ligand and which is further stabilized through direct interaction between the positive metal center and the negatively charged oxygen of the sulfite/hydrogen sulfite is proposed. Reduction of trans-[Au(CN)2Br2]- is ca. 10 times faster than reduction of trans-[Au(CN)2Cl2]-, in agreement with bromide being a more efficient bridging ligand for electron transfer. In the case of [AuCl4]- and trans-[Au(CN)2Cl2]-, there is also a parallel solvolytic pathway, with the acid hydrolysis of these complexes being rate-determining for the reduction, with rate constants (2.4 +/- 0.6) x 10(-2) and (5.6 +/- 1.4) x 10(-2) s-1, respectively. Intermediate formation of sulfite radicals and formation of dithionate as reaction product in the [AuCl4]- reaction, as claimed in recent literature, can most likely be ruled out.
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| 9. |
- Berglund, Johan, et al.
(författare)
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Kinetics and Mechanism for the Redox Reaction between Hexaaquathallium(III) and Sulfur Dioxide in Acidic Aqueous Solution
- 1994
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Ingår i: Journal of the Chemical Society. Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1472-7773 .- 0300-9246 .- 1364-5447. ; 1994:9, s. 1435-1439
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation of dissolved sulfur dioxide, SO2·nH2O, HSO3– and SO32–, by hexaaquathallium(III) has been studied in acidic aqueous solution with ionic strength 1.0 mol dm–3 at 25 °C. The stoichiometry of the reaction has been determined: [Tl(H2O)6]3++ SO2(aq)+ 2H2O →[Tl(H2O)6]++ HSO4–+ 3H+. The kinetics has been studied by use of stopped-flow spectrophotometry under pseudo-first-order conditions with either SIV or TlIII in excess. The reaction is first order in both [SIV] and [TlIII] and has a complex [H+] dependence. The rate decreases with increasing [H+] in the range 0.1 < [H+] < 1.0 mol dm–3. The results are interpreted in terms of a reaction mechanism where sulfito complexes are formed by reaction between [Tl(H2O)6]3+ or [Tl(H2O)5(OH)]2+ and HSO3–. These complexes decompose to the products [Tl(H2O)6]+ and HSO4– by intramolecular inner-sphere electron-transfer processes. The thallium(III) sulfito complexes are weak, as indicated by the absence of any transient absorbance peaks in the UV region. Potentiometric measurements of the oxygen concentration and spectrophotometric measurements showed that TlIII does not initiate a free-radical chain oxidation of SIV by dissolved oxygen.
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| 10. |
- Berglund, Johan, et al.
(författare)
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Manganese-Catalyzed Autoxidation of Dissolved Sulfur Dioxide in the Atmospheric Aqueous Phase
- 1995
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Ingår i: Atmospheric Environment. - 1352-2310. ; 29:12, s. 1379-1391
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Tidskriftsartikel (refereegranskat)abstract
- Autoxidation of SO2(aq) in the presence of manganese(II) is one of the important pathways for sulfuric acid formation in atmospheric clouds and fogs. Recent experimental results indicating that the catalyzed reaction takes place via a complex free-radical mechanism are discussed. Previous literature is reviewed in the light of this mechanism. Under atmospheric conditions of low total concentrations of manganese(II) ( < 2 × 10−5 M) and sulfur(IV) ( ≤ 10−5 M) and 2.5 < pH < 5, the rate law for conversion of SO2(aq) to SO3(aq) is reduced to d[S(IV)]/dt = k[Mn(II)][S(IV)], where [S(IV)] denotes the total concentration. A value of the overall rate constant k of 1.4 × 103 M−1s−1 is recommended for use in atmospheric model calculations.
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