Hypohalite ion catalysis of the disproportionation of chlorine dioxide

The disproportionation of chlorine dioxide in basic solution to give ClO2- and ClO3- is catalyzed by OBr- and OCl-. The reactions have a first-order dependence in both [ClO2] and [OX-] (X = Br, Cl) when the ClO2- concentrations are low. However, the reactions become second-order in [ClO2] with the addition of excess ClO2-, and the observed rates become inversely proportional to [ClO2-]. In the proposed mechanisms, electron transfer from OX- to ClO2(k1OBr- = 2.05 +/- 0.03 M(-1) x s(-1) for OBr(-)/ClO2 and k1OCl-= 0.91 +/- 0.04 M(-1) x s(-1) for OCl-/ClO2) occurs in the first step to give OX and ClO2-. This reversible step (k1OBr-/k(-1)OBr = 1.3 x 10(-7) for OBr-/ClO2, / = 5.1 x 10(-10) for OCl-/ClO2) accounts for the observed suppression by ClO2-. The second step is the reaction between two free radicals (XO and ClO2) to form XOClO2. These rate constants are = 1.0 x 10(8) M(-1) x s(-1) for OBr/ClO2 and = 7 x 10(9) M(-1) x s(-1) for OCl/ClO2. The XOClO2 adduct hydrolyzes rapidly in the basic solution to give ClO3- and to regenerate OX-. The activation parameters for the first step are DeltaH1(++) = 55 +/- 1 kJ x mol(-1), DeltaS1(++) = - 49 +/- 2 J x mol(-1) x K(-1) for the OBr-/ClO2 reaction and DeltaH1(++) = 61 +/- 3 kJ x mol(-1), DeltaS1(++) = - 43 +/- 2 J x mol(-1) x K(-1) for the OCl-/ClO2 reaction.     

Kinetics and mechanism of the oxidation of amaranth with hypochlorite

The reaction mechanism of the oxidation of Amaranth dye (2-hydroxy-1-(4-sulfonato-1-naphthylazo) naphthalene-3,6-disulfonate) with hypochlorite under varied pH conditions was elucidated by a kinetic approach. Under excess concentration of oxidant, the reaction followed pseudo-first-order kinetics with respect to Amaranth, and the oxidation was found to occur through two competitive reactions, initiated by hypochlorite and hypochlorous acid. The reaction order with respect to both OCl(-) ion and HOCl was unity. While the latter reaction was fast, the significance of the oxidation paths depended on the relative concentration of the two oxidizing species, which was dictated by the reaction pH. The role of the H(+) ion in the reaction was established. For the hypochlorite ion and hypochlorous acid facilitated reactions, the second-order rate coefficients were 1.9 and 23.2 M(-1) s(-1), respectively. The energy parameters were E(a) = 33.7 kJ mol(-1), ΔH(?) = 31.2 kJ mol(-1) and ΔS(?) = -190.6 J K(-1) mol(-1) for the OCl(-) ion-driven oxidation, and E(a) = 26.9 kJ mol(-1), ΔH(?) = 24.3 kJ mol(-1) and ΔS(?) = -222.8 J K(-1) mol(-1) for the reaction with HOCl-initiated oxidation. The major oxidation products for both the pathways were 3,4-dihydroxy naphthalene-2,7-disulfonic sodium salt (P(1)), dichloro-1,4-naphthoquione (P(2)) and naphtha(2,3)oxirene-2, 3-dione (P(3)). On the basis of the primary salt effect and other kinetic data, the rate law for the overall reaction and probable reaction mechanism was elucidated. The proposed mechanism was validated by simulations using Simkine-2.     

Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 degrees C. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) M(-1) s(-1)) between BrO(2)(-) and SO(3)(2-) to form a steady-state intermediate, (O(2)BrSO(3))(3-). General acids assist the removal of an oxide ion from (O(2)BrSO(3))(3-) to form OBrSO(3)(-), which hydrolyzes rapidly to give OBr(-) and SO(4)(2-). Subsequent fast reactions between HOBr/OBr(-) and SO(3)(2-) give Br(-) and SO(4)(2-) as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO(2)(-) and SO(3)H(-)/SO(2) react to form (OClOSO(3)H)(2)(-) and (OClOSO(2))(-) intermediates which decompose to form OCl(-) and SO(4)(2-). Subsequent fast reactions between HOCl/OCl(-) and S(IV) give Cl- and SO(4)(2-) as final products. SO(2) is 6 orders of magnitude more reactive than SO(3)H-, where k(5)(SO(2)/ClO(2)(-)) = 6.26 x 10(6) M(-1) s(-1) and k(6)(SO(3)H(-)/ClO(2)(-)) = 5.5 M(-1) s(-1). Direct reaction between ClO(2)(-) and SO(3)(2-) is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO(2)(-) and SO(3)(2-), while an O-S bond forms with ClO(2)(-) and SO(3)H-.     

S-oxygenation of thiocarbamides I: Oxidation of phenylthiourea by chlorite in acidic media

The oxidation of 1-phenyl-2-thiourea (PTU) by chlorite was studied in aqueous acidic media. The reaction is extremely complex with reaction dynamics strongly influenced by the pH of reaction medium. In excess chlorite concentrations the reaction stoichiometry involves the complete desulfurization of PTU to yield a urea residue and sulfate: 2ClO2- + PhN(H)CSNH2 + H2O --> SO4(2-) + PhN(H)CONH2 + 2Cl- + 2H+. In excess PTU, mixtures of sulfinic and sulfonic acids are formed. The reaction was followed spectrophotometrically by observing the formation of chlorine dioxide which is formed from the reaction of the reactive intermediate HOCl and chlorite: 2ClO2- + HOCl + H+ --> 2ClO2(aq) + Cl- + H2O. The complexity of the ClO2- - PTU reaction arises from the fact that the reaction of ClO2 with PTU is slow enough to allow the accumulation of ClO2 in the presence of PTU. Hence the formation of ClO2 was observed to be oligooscillatory with transient formation of ClO2 even in conditions of excess oxidant. The reaction showed complex acid dependence with acid catalysis in pH conditions higher than pKa of HClO2 and acid retardation in pH conditions of less than 2.0. The rate of oxidation of PTU was given by -d[PTU]/dt = k1[ClO2-][PTU] + k2[HClO2][PTU] with the rate law: -d[PTU]/dt = [Cl(III)](T)[PTU]0/K(a1) + [H+] [k1K(a1) + k2[H+]]; where [Cl(III)]T is the sum of chlorite and chlorous acid and K(a1) is the acid dissociation constant for chlorous acid. The following bimolecular rate constants were evaluated; k1 = 31.5+/-2.3 M(-1) s(-1) and k2 = 114+/-7 M(-1) s(-1). The direct reaction of ClO2 with PTU was autocatalytic in low acid concentrations with a stoichiometric ratio of 8:5; 8ClO2 + 5PhN(H)CSNH2 + 9H2O --> 5SO4(2-) + 5PhN(H)CONH2 + 8Cl- + 18H+. The proposed mechanism implicates HOCl as a major intermediate whose autocatalytic production determined the observed global dynamics of the reaction. A comprehensive 29-reaction scheme is evoked to describe the complex reaction dynamics.     

Ab initio/density functional theory and multichannel RRKM study for the ClO + CH2O reaction

The potential energy surface for the CH(2)O + ClO reaction was calculated at the QCISD(T)/6-311G(2d,2p)//B3LYP/6-311G(d,p) level of theory. The rate constants for the lower barrier reaction channels producing HOCl + HCO, H atom, OCH(2)OCl, cis-HC(O)OCl and trans-HC(O)OCl have been calculated by TST and multichannel RRKM theory. Over the temperature range of 200-2000 K, the overall rate constants were k(200-2000K) = 1.19 x 10(-13)T(0.79) exp(-3000.00/T). At 250 K, the calculated overall rate constant was 5.80 x 10(-17) cm(3) molecule(-1) s(-1), which was in good agreement with the experimental upper limit data. The calculated results demonstrated that the formation of HOCl + HCO was the dominant reaction channel and was exothermic by 9.7 kcal/mol with a barrier of 5.0 kcal/mol. When it retrograded to the reactants CH(2)O + ClO, an energy barrier of 14.7 kcal/mol is required. Furthermore, when HOCl decomposed into H + ClO, the energy required was 93.3 kcal/mol. These results suggest that the decomposition in both the forward and backward directions for HOCl would be difficult in the ground electronic state.     

Kinetics and mechanism of formation of the platinum-thallium bond: the [(CN)(5)Pt-Tl(CN)(3)](3)(-) complex

Formation kinetics of the metal-metal bonded [(CN)(5)PtTl(CN)(3)](3)(-) complex from Pt(CN)(4)(2)(-) and Tl(CN)(4)(-) has been studied in the pH range of 5-10, using standard mix-and-measure spectrophotometric technique at pH 5-8 and stopped-flow method at pH > 8. The overall order of the reaction, Pt(CN)(4)(2)(-) + Tl(CN)(4)(-) right harpoon over left harpoon [(CN)(5)PtTl(CN)(3)](3)(-), is 2 in the slightly acidic region and 3 in the alkaline region, which means first order for the two reactants in both cases and also for CN(-) at high pH. The two-term rate law corresponds to two different pathways via the Tl(CN)(3) and Tl(CN)(4)(-) complexes in acidic and alkaline solution, respectively. The two complexes are in fast equilibrium, and their actual concentration ratio is controlled by the concentration of free cyanide ion. The following expression was derived for the pseudo-first-order rate constant of the overall reaction: k(obs) = (k(1)(a)[Tl(CN)(4)(-) + (k(1)(a)/K(f)))(1/(1 + K(p)[H(+)]))[CN(-)](free) + k(1)(b)[Tl(CN)(4)(-)] + (k(1)(b)/K(f)), where k(1)(a) and k(1)(b) are the forward rate constants for the alkaline and slightly acidic paths, K(f) is the stability constant of [(CN)(5)PtTl(CN)(3)](3)(-), and K(p) is the protonation constant of cyanide ion. k(1)(a) = 143 +/- 13 M(-)(2) s(-)(1), k(1)(b) = 0.056 +/- 0.004 M(-)(1) s(-)(1), K(f) = 250 +/- 54 M(-)(1), and log K(p) = 9.15 +/- 0.05 (I = 1 M NaClO(4), T = 298 K). Two possible mechanisms were postulated for the overall reaction in both pH regions, which include a metal-metal bond formation step and the coordination of the axial cyanide ion to the platinum center. The alternative mechanisms are different in the sequence of these steps.     

Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan

The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.     

A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution

A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H(2)O)(2)]ClO(4) (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/- 0.06 at 25 degrees C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degrees C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M(-1) s(-1), DeltaH(#)(f(I)) = 62 +/- 3 kJ mol(-1), DeltaS(#)(f(I)) = -30 +/- 10 J K(-1) mol(-1), and DeltaV(#)(f(I)) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an I(a) mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M(-1) s(-1) at 25 degrees C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I(d) or D substitution mechanism on the basis of the reported kinetic data. At pH > or =4.5, the second substitution step also can be monitored (pH 5.5 and 25 degrees C: k(f(II)) = 21.1 +/- 0.5 M(-1) s(-1), DeltaH(#)(f(II)) = 60 +/- 2 kJ mol(-1), DeltaS(#)(f(II)) = -19 +/- 6 J K(-1) mol(-1), and DeltaV(#)(f(II)) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I(d) or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.     

Oxidation of chlorine(III) by hypobromous acid: kinetics and mechanism

The kinetics and mechanism of the chlorine(III)-HOBr reaction were studied by the stopped-flow method under acidic conditions, pH 1.0-3.0, in 1.0 M NaClO(4) and at 25.0 degrees C. The overall redox process occurs in two consecutive steps via the formation of the BrClO(2) intermediate. The electron transfer reactions are coupled with bromine hydrolysis, the formation of the tribromide ion, and the protolytic equilibrium of chlorine(III). On the basis of simultaneous evaluation of the kinetic traces, the following rate constants were obtained for the redox steps: HClO(2) + HOBr right harpoon over left harpoon BrClO(2) + H(2)O, k(3) = (3.34 +/- 0.02) x 10(4) M(-1) s(-1), k(-3) = (3.5 +/- 1.3) x 10(3) s(-1); BrClO(2) + ClO(2)(-)<==>2ClO(2) + Br(-), k(4) = (2.9 +/- 1.0) x 10(7) M(-1) s(-1). The second step was practically irreversible under the conditions applied, and the value of k(-4) could not be determined. The equilibrium constant for the formation of BrClO(2), K(3) = 9.5 M(-1), was calculated from the kinetic results, and it was confirmed that this species is a very powerful oxidant. The redox potential was also estimated for the BrClO(2) + e(-) = Br(-) + ClO(2) reaction: epsilon(0) approximately 1.70 V.     

Kinetics and mechanism of the interaction of nitric oxide with pentacyanoferrate(II). Formation and dissociation of [Fe(CN)5NO]3 -

The interaction of NO with [Fe(CN)(5)H(2)O](3)(-) (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)(5)NO](3)(-) was studied by UV-vis spectrophotometry. The reaction product is the well characterized nitrosyl complex, described as a low-spin Fe(II) bound to the NO radical. The experiments were performed in the pH range 4-10, at different concentrations of NO, temperatures and pressures. The rate law was first-order in each of the reactants, with the specific complex-formation rate constant, k(f)( )()= 250 +/- 10 M(-)(1) s(-)(1) (25.4 degrees C, I = 0.1 M, pH 7.0), DeltaH(f)() = 70 +/- 1 kJ mol(-)(1), DeltaS(f)() = +34 +/- 4 J K(-)(1) mol(-)(1), and DeltaV(f)() = +17.4 +/- 0.3 cm(3) mol(-)(1). These values support a dissociative mechanism, with rate-controlling dissociation of coordinated water, and subsequent fast coordination of NO. The complex-formation process depends on pH, indicating that the initial product [Fe(CN)(5)NO](3)(-) is unstable, with a faster decomposition rate at lower pH. The decomposition process is associated with release of cyanide, further reaction of NO with [Fe(CN)(4)NO](2)(-), and formation of nitroprusside and other unknown products. The decomposition can be prevented by addition of free cyanide to the solutions, enabling a study of the dissociation process of NO from [Fe(CN)(5)NO](3)(-). Cyanide also acts as a scavenger for the [Fe(CN)(5)](3)(-) intermediate, giving [Fe(CN)(6)](4)(-) as a final product. From the first-order behavior, the dissociation rate constant was obtained as k(d) = (1.58 +/- 0.06) x 10(-)(5) s(-)(1) at 25.0 degrees C, I = 0.1 M, and pH 10.2. Activation parameters were found to be DeltaH(d)() = 106.4 +/- 0.8 kJ mol(-)(1), DeltaS(d)() = +20 +/- 2 J K(-)(1) mol(-)(1), and DeltaV(d)() = +7.1 +/- 0.2 cm(3) mol(-)(1), which are all in line with a dissociative mechanism. The low value of k(d) as compared to values for the release of other ligands L from [Fe(II)(CN)(5)L](n)()(-) suggests a moderate to strong sigma-pi interaction of NO with the iron(II) center. It is concluded that the release of NO from nitroprusside in biological media does not originate from [Fe(CN)(5)NO](3)(-) produced on reduction of nitroprusside but probably proceeds through the release of cyanide and further reactions of the [Fe(CN)(4)NO](2)(-) ion.     

Kinetic origin of the chelate effect. Base hydrolysis, H-exchange reactivity, and structures of syn,anti-[Co(cyclen)(NH3)2]3+ and syn,anti-[Co(cyclen)(diamine)]3+ ions (diamine = H2N(CH2)2NH2, H2N(CH2)3NH2)

The synthesis of syn,anti-[Co(cyclen)en](ClO4)3 (1(ClO4)3) and syn,anti-[Co(cyclen)tn](ClO4)3 (2(ClO4)3) is reported, as are single-crystal X-ray structures for syn,anti-[Co(cyclen)(NH3)2](ClO4)3 (3(ClO4)3). 3(ClO4)3: orthorhombic, Pnma, a = 17.805(4) A, b = 12.123(3) A, c = 9.493(2) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.030. 1(ClO4)3: monoclinic, P2(1)/n, a = 8.892(2) A, b = 15.285(3) A, c = 15.466(3) A, alpha = 90 degrees, beta = 91.05(3) degrees, gamma = 90 degrees, Z = 4, R1 = 0.0657. 2Br3: orthorhombic, Pca2(1) a = 14.170(4) A, b = 10.623(3) A, c = 12.362(4) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.0289. Rate constants for H/D exchange (D2O, I = 1.0 M, NaClO4, 25 degrees C) of the syn and anti NH protons (rate law: kobs = ko + kH[OD-]) and the apical NH, and the NH3 and NH2 protons (rate law: kobs = kH[OD-]) in the 1, 2, and 3 cations are reported. Deprotonation constants (K = [Co(cyclen-H)(diamine)2+]/[Co(cyclen)(diamine)3+][OH-]) were determined for 1 (5.5 +/- 0.5 M-1) and 2 (28 +/- 3 M-1). In alkaline solution 1, 2, and 3 hydrolyze to [Co(cyclen)(OH)2]+ via [Co(cyclen)(amine)OH)]2+ monodentates. Hydrolysis of 3 is two step: kobs(1) = kOH(1)[OH-], kobs(2) = ko + kOH(2)[OH-] (kOH(1) = (2.2 +/- 0.4) x 10(4) M-1 s-1, ko = (5.1 +/- 1.2) x 10(-4) s-1, kOH(2) = 1.0 +/- 0.1 M-1 s-1). Hydrolysis of 2 is biphasic: kobs(1) = k1K[OH-]/(1 + K[OH-] (k1 = 5.0 +/- 0.2 s-1, K = 28 M-1), kobs(2) = k2K2[OH-]/(1 + K2[OH-]) (k2 = 3.5 +/- 1.2 s-1, K2 = 1.2 +/- 0.8 M-1). Hydrolysis of 1 is monophasic: kobs = k1k2KK2[OH-]2/(1 + K[OH-1])(k-1 + k2K2[OH-]) (k1 = 0.035 +/- 0.004 s-1, k-1 = 2.9 +/- 0.6 s-1, K = 5.5 M-1, k2K2 = 4.0 M-1 s-1). The much slower rate of chelate ring-opening in 1, compared to loss of NH3 from 3, is rationalized in terms of a reduced ability of the former system to allow the bond angle expansion required to produce the SN1CB trigonal bipyramidal intermediate.     

The reaction of S-nitroso-N-acetyl-D,L-penicillamine (SNAP) with the angiotensin converting enzyme inhibitor, captopril--mechanism of transnitrosation

Kinetic studies involving the use of both stopped-flow and diode array spectrophotometers, show that the reaction between SNAP and captopril in the presence of the metal ion sequestering agent, EDTA, occurs in two well-defined stages. The first stage is a fast reaction while the second stage is slow. The first stage has been postulated to be transnitrosation, and the second stage involves the decay of the newly formed RSNO to effect nitric oxide (NO) release. Both stages are found to be dependent on captopril and H+ concentration. The rates of the transnitrosation increased drastically with increasing pH in the first stage, signifying that the deprotonated form of captopril is the more reactive species. In the case of the second stage the variation in pH showed an increase in rate up to pH 8 after which the rate remained unchanged. Both stages were clearly distinguishable and easily monitored separately. Transnitrosation is a reversible reaction with the tendency for the equilibrium to break down at high thiol concentration. Second-order rate constants were calculated based on the following derived expressions: -d[SNAP]/dt=k(f)((K(SHCapSH)[CapSH](t))/(K(SHCapSH)+[H+]))[SNAP]. k(f) is the second-order rate constant for the forward reaction of the reversible transnitrosation process. At 37 degrees C, k(f)= 785 +/- 14 M(-1) s(-1), activation parameters [Delta]H(f)++= 49 +/- 2 kJ mol(-1), (Delta)S(f)++=-32 +/- 2 J K(-1) mol(-1). The activation parameters demonstrate the associative nature of the transnitrosation mechanism. The second stage has been found to be very complex, as a variety of nitrogen products form as predicted before. However, the following expression was derived from the initial kinetic data: rate =k1K[SNOCap][CapS-]/(K[CapS-]+ 1) to give k1= 13.3 +/- 0.4 x 10(-4) s(-1) and K= 5.59 +/- 0.53 x 10(4) M(-1), at 37 degrees C, where k1 is the first-order rate constant for the decay of the intermediate formed during the reaction between SNOCap and the remaining excess CapSH present at the end of the first stage reaction. Activation parameters are (Delta)H1++= 37 +/- 1 kJ mol(-1), (Delta)S1++=-181 +/- 44 J K(-1) mol(-1).     

General-acid-catalyzed reactions of hypochlorous acid and acetyl hypochlorite with chlorite ion

The rate of oxidation of ClO2- by HOCl is first order in each reactant and is general-acid catalyzed. In the initial steps of the proposed mechanism, a steady-state intermediate, HOClOClO-, forms (k1 = 1.6 M-1 s-1) and undergoes general-acid (HA)-catalyzed reactions (k2HA) to generate a metastable intermediate, ClOClO. Values of k2HA/k-1 are 1.6 x 10(4) M-1 (H3O+), 20 M-1 (HOAc), and 8.5 M-1 (H2PO4-). Subsequent competitive reactions of ClOClO with ClO2- (k3) to give 2ClO2 and with OH- (k4OH) and other bases (k5B) to give ClO3- are very rapid. The relative yields of these products give k4OH/k3 = 1.3 x 10(5), k5HPO4/k3 = 0.20, and k5OAc/k3 = 0.06. At low pH and low buffer concentrations, the apparent yield of ClO2, based on 2ClO2 per initial HOCl, reaches 140%. This anomaly is attributed to the induced disproportionation of ClO2- by ClOClO to give ClO3- and additional HOCl. A highly reactive intermediate, ClOCl(O)OClO-, is proposed that can undergo Cl-O bond cleavage to give 2ClO2 + Cl- via one path and ClO3- + 2HOCl via another path. The additional HOCl recycles in the presence of excess ClO2- to give more ClO2. Ab initio calculations show feasible structures for the proposed reaction intermediates. Acetic acid has a second catalytic role through the formation of acetyl hypochlorite, which is much more reactive than HOCl in the transfer of Cl+ to ClO2- to form ClOClO.     

Kinetics and mechanisms of the ozone/bromite and ozone/chlorite reactions

Ozone reactions with XO(2)(-) (X = Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO(2)(-). The O(3)/XO(2)(-) reactions are first-order in [O(3)] and [XO(2)(-)], with rate constants k(1)(Cl) = 8.2(4) x 10(6) M(-1) s(-1) and k(1)(Br) = 8.9(3) x 10(4) M(-1) s(-1) at 25.0 degrees C and mu = 1.0 M. The proposed rate-determining step is an electron transfer from XO(2)(-) to O(3) to form XO(2) and O(3)(-). Subsequent rapid reactions of O(3)(-) with general acids produce O(2) and OH. The OH radical reacts rapidly with XO(2)(-) to form a second XO(2) and OH(-). In the O(3)/ClO(2)(-) reaction, ClO(2) and ClO(3)(-) are the final products due to competition between the OH/ClO(2)(-) reaction to form ClO(2) and the OH/ClO(2) reaction to form ClO(3)(-). Unlike ClO(2), BrO(2) is not a stable product due to its rapid disproportionation to form BrO(2)(-) and BrO(3)(-). However, kinetic spectra show that small but observable concentrations of BrO(2) form within the dead time of the stopped-flow instrument. Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O(3)/BrO(2)(-) reaction. Ion chromatographic analysis shows that O(3) and BrO(2)(-) react in a 1/1 ratio to form BrO(3)(-) as the final product. Variation of k(1)(X) values with temperature gives Delta H(++)(Cl) = 29(2) kJ mol(-1), DeltaS(++)(Cl) = -14.6(7) J mol(-1) K(-1), Delta H(++)(Br) = 54.9(8) kJ mol(-1), and Delta S(++)(Br) = 34(3) J mol(-1) K(-1). The positive Delta S(++)(Br) value is attributed to the loss of coordinated H(2)O from BrO(2)(-) upon formation of an [O(3)BrO(2)(-)](++) activated complex.     

Thermal decomposition of ClOOCl

ClOOCl was prepared in situ in a temperature controlled photoreactor (v = 420 L) by photolyzing OClO/N2 mixtures in the wavelength range 300-500 nm at temperatures between 242 and 261 K and total pressures between 2 and 480 mbar. After switching off the lights, excess NO2 was added, and IR and UV spectra were monitored simultaneously as a function of time. By spectral stripping of all other known UV absorbers (in particular, other chlorine oxides and chlorine nitrate), we determined rate constants k-1 of the reaction ClOOCl (+M) --> ClO + ClO (+M) from the first-order decay of the residual UV absorption of ClOOCl at 246 and 255 nm. k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1) (2sigma) was derived for the low-pressure limiting rate constant. Application of Troe's expression for the limiting low-pressure rate constants of unimolecular decomposition reactions leads to E0 = Delta(r)H0(0)(ClOOCl-->ClO+ClO) = 66.4 +/- 3.0 kJ mol(-1). k-1,0 started to fall off from the pressure proportional low pressure behavior at p approximately 30 mbar; however, reliable extrapolation to the high pressure limit was not possible. The decomposition rate constants of ClOOCl were directly measured for the first time, and they are higher, depending on temperature and pressure, by factors between 1.5 and 4.2 as compared to experimental data on k-1 by Nickolaisen et al. [J. Phys. Chem. 1994, 98, 155] which were derived from the approach of ClO to thermal equilibrium with its dimer ClOOCl. Combination of the present dissociation rate constants with recommended temperature and pressure dependent data on the reverse reaction (k1) demonstrate inconsistencies between the dissociation and recombination rate constants. Summarizing laboratory data on k1 and k-1 above 250 K and field measurements on the ClO + ClO <= => ClOOCl equilibrium in the nighttime polar stratosphere close to 200 K, the expression Kc = k1/k-1 = 3.0 x 10(-27) exp(8433 K/T) cm3 molecule(-1) is derived for the temperature range 200-300 K.     

Three autocatalysts and self-inhibition in a single reaction: a detailed mechanism of the chlorite-tetrathionate reaction

The chlorite-tetrathionate reaction has been studied spectrophotometrically in the pH range of 4.65-5.35 at T = 25.0 +/- 0.2 degrees C with an ionic strength of 0.5 M, adjusted with sodium acetate as a buffer component. The reaction is unique in that it demonstrates autocatalysis with respect to the hydrogen and chloride ion products and the key intermediate, HOCl. The thermodynamically most-favorable stoichiometry, 2S(4)O(6)2- + 7ClO2- + 6H2O --> 8SO(4)2- + 7Cl- + 12H+, is not found. Under our experimental conditions, chlorine dioxide, the chlorate ion, or both are detected in appreciable amounts among the products. Initial rate studies reveal that the formation of chlorine dioxide varies in an unusual way, with the chlorite ion acting as a self-inhibitor. The reaction is supercatalytic (i.e., second order with respect to autocatalyst H+). The autocatalytic behavior with respect to Cl- comes from chloride catalysis of the chlorite-hypochlorous acid and hypochlorous acid-tetrathionate subsystems. A detailed kinetic study and a model that explains this unusual kinetic behavior are presented.     

Kinetics and mechanisms of chlorine dioxide and chlorite oxidations of cysteine and glutathione

Chlorine dioxide oxidation of cysteine (CSH) is investigated under pseudo-first-order conditions (with excess CSH) in buffered aqueous solutions, p[H+] 2.7-9.5 at 25.0 degrees C. The rates of chlorine dioxide decay are first order in both ClO2 and CSH concentrations and increase rapidly as the pH increases. The proposed mechanism is an electron transfer from CS- to ClO2 (1.03 x 10(8) M(-1) s(-1)) with a subsequent rapid reaction of the CS* radical and a second ClO2 to form a cysteinyl-ClO2 adduct (CSOClO). This highly reactive adduct decays via two pathways. In acidic solutions, it hydrolyzes to give CSO(2)H (sulfinic acid) and HOCl, which in turn rapidly react to form CSO3H (cysteic acid) and Cl-. As the pH increases, the (CSOClO) adduct reacts with CS- by a second pathway to form cystine (CSSC) and chlorite ion (ClO2-). The reaction stoichiometry changes from 6 ClO2:5 CSH at low pH to 2 ClO2:10 CSH at high pH. The ClO2 oxidation of glutathione anion (GS-) is also rapid with a second-order rate constant of 1.40 x 10(8) M(-1) s(-1). The reaction of ClO2 with CSSC is 7 orders of magnitude slower than the corresponding reaction with cysteinyl anion (CS-) at pH 6.7. Chlorite ion reacts with CSH; however, at p[H+] 6.7, the observed rate of this reaction is slower than the ClO2/CSH reaction by 6 orders of magnitude. Chlorite ion oxidizes CSH while being reduced to HOCl, which in turn reacts rapidly with CSH to form Cl-. The reaction products are CSSC and CSO3H with a pH-dependent distribution similar to the ClO2/CSH system.     

Kinetic studies of the ClO + ClO association reaction as a function of temperature and pressure

The kinetics of the association reaction of ClO radicals: ClO + ClO + M --> Cl2O2+ M (1), have been investigated as a function of temperature T between 206.0-298.0 K and pressure p between 25-760 Torr using flash photolysis with time-resolved UV absorption spectroscopy. ClO radicals were generated following the photolysis of Br2/Cl2O mixtures in nitrogen diluent gas. Charge coupled device (CCD) detection of time resolved absorptions was used to monitor ClO radicals over a broad wavelength window covering the ClO (A 2Pi<-- X 2Pi) vibronic absorption bands. The high pass filtered ClO absorption cross sections were calibrated as a function of temperature between T = 206.0-320 K, and exhibit a negative temperature dependence. The ClO association kinetics were found to be more rapid than those reported in previous studies, with limiting low and high pressure rate coefficients, in nitrogen bath gas, k0 = (2.78 +/- 0.82) x 10(-32) x (T/300)(-3.99 +/- 0.94) molecule(-2) cm6 s(-1) and k(infinity) = (3.37 +/- 1.67) x 10(-12) x (T/300)(-1.49 +/- 1.81) molecule(-1) cm3 s(-1), respectively, (obtained with the broadening factor F(c) fixed at 0.6). Errors are 2sigma. The pressure dependent ClO association rate coefficients (falloff curves) exhibited some discrepancies at low pressures, with higher than expected rate coefficients on the basis of extrapolation from high pressures (p > 100 Torr). Reanalysis of data excluding kinetic data recorded below p = 100 Torr gave k0 = (2.79 +/- 0.85) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1) and k(infinity) = (3.44 +/- 1.83)x 10(-12) x (T/300)(-1.73 +/- 1.91) molecule(-1) cm3 s(-1). Potential sources of the low pressure discrepancies are discussed. The expression for k(0) in air bath gas is k0 = (2.62 +/- 0.80) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1). These results support upward revision of the ClO association rate coefficient recommended for use in stratospheric models, and the stratospheric implications of the results reported here are briefly discussed.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Influence of terpyridine as pi-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.