The ratio of the rate constants for H atom abstraction and β-cleavage for bis-oxyisopropylidene biradical

The relative reactivity of bis-oxyisopropylidene biradical ‘OCMe₂O’ generated upon homolysis of the O−O bond of dimethyldioxirane was characterized by the ratio of the rate constants for H atom abstraction and β-cleavage:k ₃ /k ₂ =0.23±0.06 L mol⁻¹ (314 K). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1321–1323, July, 1998.     

Determination of dissociation energies of N-H bond in the 4-anilinodiphenylaminyl radical and O-H bond in the 2,5-dichloro-4-hydroxyphenoxyl radical from the equilibrium constants of chain reactions in quinoneimine-hydroquinone systems

The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)—2,5-dichlorohydroquinone systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine was estimated from these data (ΔH = − 14.4±1.6 kJ mol⁻¹) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (D _NH = 278.6±3.0 kJ mol⁻¹). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation energy in 2,5-dichlorohydroquinone: D _OH = 362.4±0.9 kJ mol⁻¹. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: D _OH = 253.6±1.9 kJ mol⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1661–1666, October, 2006.     

Kinetics and mechanism of the highly efficient generation of singlet oxygen in dimethyldioxirane decomposition induced by the chloride ion

The decomposition of dimethyldioxirane induced by the chloride anion has been investigated by methods of infrared chemiluminescence and quantum chemistry. The reaction leads to efficient generation of singlet excited molecular oxygen ¹O₂ (the excitation yield in acetone is 61%). A mechanism of peroxide decomposition is proposed in which the key reactions are the addition of the chloride ion to an oxygen atom of dioxirane, resulting in dioxirane ring opening and the formation of the 2-chlorooxy-2-hydroxy propane alcoholate (k ₁), and the interaction of the latter with another dimethyldioxirane molecule. This interaction results either in the formation of an adduct, which further decomposes to evolve ¹O₂, and catalyst regeneration (k ₂) or in the formation of the 2-chloroxyisopropyl radical, which leads to the irreversible consumption of the chloride ion catalyst (k ₃). The decay kinetics of the infrared chemiluminescence of ¹O₂ has been studied in a wide range of reactant concentrations. The temperature dependence of the rate constant of the reaction of dimethyldioxirane with the chloride ion has been determined by a kinetic analysis of the mechanism proposed: log(² k ₁) = (11.1 ± 0.7) − (46 ± 4)/Θ, where Θ = 2.3RT kJ/mol. Estimation of the ratio of the rates of the reaction of the 2-chlorooxy-2-hydroxy propane alcoholate with dimethyldioxirane via two pathways (k ₃/k ₂) has demonstrated that the fraction of the process involving electron transfer does not exceed 1.5% under the experimental conditions examined. Nevertheless, the latter reaction, which withdraws the chloride ion from the catalytic cycle of dimethyldioxirane decomposition yielding singlet oxygen, has a marked effect on the overall kinetics of the process.     

Electrochemical investigations of the reaction mechanism and kinetics between NADH and riboflavin immobilised on amorphous zirconium phosphate

Electrochemical investigations of the reaction mechanism and kinetics between riboflavin immobilised on zirconium phosphate (ZPRib) in carbon paste and NADH showed results yielding reliable information about aspects on the mechanism of the electron transfer reaction between the flavin and NADH. The formal potential (E°′) of the adsorbed riboflavin was −220 mV versus SCE at pH 7.0. A shift about 250 mV towards a more positive potential compared with its value in solution was assigned to the interaction between the basic nitrogen of riboflavin and the acid groups of ZP. The invariance of the E°′ with the pH of the contacting solution and the effect of different buffer constituents were attributed to the protection effect of ZP over the riboflavin. The electrocatalytic oxidation of NADH at the electrode was investigated using cyclic voltammetry and rotating disk electrode methodology using a potential of −50 mV versus SCE. The heterogeneous electron transfer rate constant, k _obs, was 816 M⁻¹ s⁻¹ and the Michaelis-Menten constant, K _M, was 1.8 mM (confirming a charge transfer complex intermediate in the reaction) for an electrode with a riboflavin coverage of 6.8 × 10⁻¹⁰ mol cm⁻². This drastic increase in the reaction rate between NADH and the immobilised riboflavin was assigned to the shift of the E°′. A surprising effect with addition of calcium or magnesium ion to the solution was also observed. The E°′ was shifted to −150 mV versus SCE and the reaction rate for NADH oxidation increased drastically. Received: 22 February 1999 / Accepted: 10 March 1999     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Kinetic study of reaction of [ReN(H2O)(CN)4]2− with quinoline-2-carboxylate and pyridine-2,3-dicarboxylate anions

The kinetics of the reaction between [ReN(H₂O)-(CN)₄]²⁻ with different κ² N,O-donor ligands (quin⁻ and 2,3-dipic⁻, respectively) have been studied in the pH 4–12 range in aqueous solution. Two consecutive reaction steps with the formation of the [ReN(η¹-quin)(CN)₄]³⁻ and [ReN(μ²-quin) (CN)₃]²⁻ complexes, respectively, were spectrophotometrically observed and kinetically investigated. The same reaction mechanism is proposed for these two ligands. The first fast reaction (for quin⁻) is attributed to the aqua substitution of [ReN(H₂O)(CN)₄]²⁻ with forward and reverse rate constants of 1.96(5) × 10⁻¹ M⁻¹ s⁻¹ and 5.6(3) × 10⁻² s⁻¹, while a rate of 2.64(3) M⁻¹ s⁻¹ was observed for the reaction between the conjugate base [ReN(OH)(CN)₄]³⁻ and quin⁻ at 40.2 °C. Due to small absorbance changes, it was difficult to obtain any good quality data for the fast reactions for 2,3-dipic⁻. The second, slower reaction is attributed to cyano substitution with rate constants (k ₃ K ₁) of 4.17(4) × 10⁻³ for quin⁻ and 4.68(7) × 10⁻³ M⁻¹ s⁻¹ for 2,3-dipic⁻, at 80.02 °C, respectively. The acid dissociation constant for the aqua complex was spectrophotometrically determined as 11.58(3) and 11.54(2) and kinetically as 11.51(8) and 11.41(1), at 80.4 °C, respectively. Negative values of −83.5(2) and −144.1(2) J K⁻¹ mol⁻¹ as well as the of 71.4(3) and 47.3(3) kJ mol⁻¹, for the slow quin⁻ and 2,3-dipic⁻ reactions, respectively, point to an ordered transition state where bond formation is responsible for the major driving force of the reaction. The and for the fast forward reaction of quin⁻ is indicative of expected associative activation in the transition state. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chemiluminescence and catalysis of decomposition of dimethyldioxirane adsorbed from the gas phase on silipore containing tris(bipyridyl)ruthenium complex Ru(bpy)3Cl2 and 9,10-diphenylanthracene

The kinetics of decomposition of dimethyldioxirane (DMD) adsorbed from the gas phase on the silipore surface was studied by the chemiluminescence (CL) technique. The lower boundary of the CL yield in the reaction of DMD decomposition (4·10⁻⁹ Einstein mol⁻¹) and chemiexcitation yield of methyl acetate (4·10⁻⁴) were estimated. Chemiluminescence upon decomposition of DMD on the silipore surface in the presence of activators of CL such as tris(bipyridyl)ruthenium complex Ru(bpy)₃Cl₂ and 9,10-diphenylanthracene was revealed. Ru(bpy)₃Cl₂ activates the luminescence according to chemically induced electron exchange mechanism. Devoted to Prof. R. W. Murray on his 70th birthday. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1516–1521, August, 1998.     

The influence of ionic medium on the kinetics of ligand replacement in the Ptdient H2O2+ complex in an aqueous solution

The influence of the ion background (NaClO₄, LiClO₄, and HClO₄) on the kinetics of the reaction PtdientH₂O²⁺+X⁻→PtdientX⁺+H₂O(X=Cl, Br, I, SCN, and N₃) was studied at 25°C by spectrophotometry. Changes in the rate constant with increase in the ionic strength are described by the Debye-Hückel and Gosh-Bjerrum equations. The reaction PtdienCl⁺+H₂O→PtdientH₂O²⁺+Cl⁻ was studied by potentiometry and its rate constant was established to depend weakly on variations of the medium. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1918–1921, October, 1998.     

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Hemoglobin-stabilized tetranitrosyl binuclear iron complex with pyridine-2-yl in aqueous solutions

The tetranitrosyl iron complex with pyridine-2-yl [Fe₂(SC₅H₄N)₂(NO)₄] (1) has higher NO-donating activity in 3% aqueous solutions of DMSO (pH 7.0, 25 °C) than the organic NO donor, viz., adduct of NO with diethylenetriamine (NO-adduct). The NO concentration was determined by the spectrophotometric method based on the formation of an NO complex with hemoglobin (Hb). The apparent first-order rate constants of the studied reactions are (6.15±0.6)·10⁻¹ s⁻¹ and (0.8±0.08)·10⁻¹ s⁻¹ for complex 1 and the NO-adduct, respectively, at an Hb concentration of 2·10⁻¹ mol L⁻¹ and the ratio [NO donor]/[Hb] = 10. The effect of Hb and [NO donor]/[Hb] ratio on the rate of NO generation from a solution of complex 1 was studied. For a fourfold decrease in the concentration of complex 1 the reaction rate constant decreases to 0.5·10⁻⁴ s⁻¹, whereas the fourfold increase in the Hb concentration results in the stabilization of complex 1. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 732–736, April, 2007.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

Photocatalytic water purification: Destruction of resorcinol by irradiated titania

Ethanedial, butanoic acid anhydride and trihydroxybenzenes have been determined as intermediates of resorcinol photocatalytic oxidation. The photoreaction rate shows a 1st order dependence on resorcinol concentration. The calculated reaction rate constant is 2.1×10⁻⁸ mol dm⁻³ s⁻¹, which is similar to the rate constant of phenol photocatalytic oxidation on TiO₂ reported beore.     

Studies of the reaction of the hydroxyl radical with the oxalate ion in an acidic aqueous solution by pulse radiolysis

The reaction of the · OH radical with the oxalate ion in an acidic aqueous solution was studied by pulse radiolysis. The rate constant for the reaction of formation of the radical HOOC-COO·(λ_max = 250 nm, ɛ = 1800 L mol⁻¹ cm⁻¹) is (5.0±0.5)·107 L mol⁻¹ s⁻¹. In the reaction with the hydrogen ion (k = 1.1·10⁷ L mol⁻¹ s⁻¹), the radical HOOC-COO· is transformed into a nonidentified radical designated arbitrarily as H⁺(HOOC-COO)· (λ_max = 260 nm, ɛ = 4000 L mol⁻¹ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1165–1167, June, 2008.     

Studies on electron transfer reactions: oxidation of phenol and ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V) in aqueous acidic medium

The kinetics of oxidation of phenol and a few ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V), [PV^VW₁₁O₄₀]⁴⁻ (HPA) have been studied spectrophotometrically in aqueous acidic medium containing perchloric acid and also in acetate buffers of several pH values at 25 °C. EPR and optical studies show that HPA is reduced to the one-electron reduced heteropoly blue (HPB) [PV^IVW₁₁O₄₀]⁵⁻. In acetate buffers, the build up and decay of the intermediate biphenoquinone show the generation of phenoxyl radical (ArO^·) in the rate-determining step. At constant pH, the reaction shows simple second-order kinetics with first-order dependence of rate on both [ArOH] and [HPA]. At constant [ArOH], the rate of the reaction increases with increase in pH. The plot of apparent second-order rate constant, k ₂, versus 1/[H⁺] is linear with finite intercept. This shows that both the undissociated phenol (ArOH) and the phenoxide ion (ArO⁻) are the reactive species. The ArO⁻–HPA reaction is the dominant pathway in acetate buffer and it proceeds through the OH⁻ ion triggered sequential proton transfer followed by electron transfer (PT-ET) mechanism. The rate constant for ArO⁻–HPA reaction, calculated using Marcus theory, agrees fairly well with the experimental value. The reactivity of substituted phenoxide ions correlates with the Hammett σ⁺ constants, and ρ value was found to be −4.8. In acidic medium, ArOH is the reactive species. Retardation of rate for the oxidation of C₆H₅OD in D₂O indicates breaking of the O–H bond in the rate-limiting step. The results of kinetic studies show that the HPA-ArOH reaction proceeds through a concerted proton-coupled electron transfer mechanism in which water acts as proton acceptor (separated-CPET).     

Measurement of the rate constants for the reactions of the IO• radical with sulfur-containing compounds H2S, (CH3)2S, and SO2

The reactions of iodine monoxide (IO^•) with sulfur-containing compounds, which are important for the atmospheric chemistry, are studied. An attempt is made to distinguish between the heterogeneous and homogeneous reaction pathways. It is shown that, under the experimental conditions, the reactions proceed on the wall and generate iodine atoms into the gas phase. It is found that, at room temperature, the rate constants for the gas-phase reactions of IO^• with (CH₃)₂S and H₂S are lower than 2.5 × 10⁻¹⁴ and 8.0 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹, respectively; the rate constant for the gas-phase reaction of iodine monoxide with SO₂ ≤ 5.6 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹.     

A kinetic investigation of the oxidation of <Emphasis Type="SmallCaps">dl</Emphasis>-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion

Kinetics of oxidation of dl-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO₆)₂]⁵⁻, has been studied in aqueous alkaline medium in the temperature range of 25–40 °C. The oxidation kinetics is first order in the silver(III) and pipecolinate concentrations. The observed second-order rate constant, decreasing with increasing [periodate] is virtually independent of [OH⁻]. α-Aminoadipate as the major oxidation product of pipecolinate has been identified by chromatographic analysis. A reaction mechanism is proposed that involves a pre-equilibrium between [Ag(HIO₆)₂]⁵⁻ and [Ag(HIO₆)(H₂O)(OH)]²⁻, a mono-periodate coordinated silver(III) complex. Both Ag(III) complexes are reduced in parallel by pipecolinate in rate-determining steps (described by k ₁ for the former Ag(III) species and k ₂ for the latter). The determined rate constants and their associated activation parameters are k ₁ (25 °C) = 0.40 ± 0.02 M⁻¹ s⁻¹, ∆H ₁^≠ = 53 ± 2 kJ mol⁻¹, ∆S ₁^≠ = −74 ± 5 J K⁻¹ mol⁻¹ and k ₂ (25 °C) = 0.64 ± 0.02 M⁻¹ s⁻¹, ∆H ₂^≠ = 41 ± 2 kJ mol⁻¹, ∆S ₂^≠ = −110 ± 5 J K⁻¹ mol⁻¹. The time-resolved spectra, a positive dependence of the rate constants on ionic strength of the reaction medium, and the consistency of pre-equilibrium constants derived from different reaction systems support the proposed reaction mechanism.     

Electrocatalytic oxidation of nitrite at a terpyridine manganese(II) complex modified carbon past electrode

A carbon past electrode modified with [Mn(H₂O)(N₃)(NO₃)(pyterpy)], ( \textpyterpy = 4￠- ( 4 - \textpyridyl ) - 2,2￠:\text6￠,\text2￠￠- \textterpyridine ) \left( {{\text{pyterpy}} = 4\prime - \left( {4 - {\text{pyridyl}}} \right) - 2,2\prime:{\text{6}}\prime,{\text{2}}\prime\prime - {\text{terpyridine}}} \right) complex have been applied to the electrocatalytic oxidation of nitrite which reduced the overpotential by about 120 mV with obviously increasing the current response. Relative standard deviations for nitrite determination was less than 2.0%, and nitrite can be determined in the ranges of 5.00 × 10⁻⁶ to 1.55 × 10⁻² mol L⁻¹, with a detection limit of 8 × 10⁻⁷ mol L⁻¹. The treatment of the voltammetric data showed that it is a pure diffusion-controlled reaction, which involves one electron in the rate-determining step. The rate constant k′, transfer coefficient α for the catalytic reaction, and diffusion coefficient of nitrite in the solution, D, were found to be 1.4 × 10⁻², 0.56× 10⁻⁶, and 7.99 × 10⁻⁶ cm² s⁻¹, respectively. The mechanism for the interaction of nitrite with the Mn(II) complex modified carbon past electrode is proposed. This work provides a simple and easy approach to detection of nitrite ion. The modified electrode indicated reproducible behavior, anti-fouling properties, and stability during electrochemical experiments, making it particularly suitable for the analytical purposes.     

Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO₄, and H₂SO₄) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k ₂ {ie17-1} k ₁. The temperature dependences of k ₁ (s⁻¹) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H₂SO₄ between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E _a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k ₂ for the reaction in 64.16% H₂SO₄ between 80 and 130°C are IogA= 9.1 ± 2.5 and E _a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H₂SO₄ shows that I is a weak base $ (pK_{BH^ + } \approx - 6) $ (pK_{BH^ + } \approx - 6) . The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N₂, NO, and N₂O. The pseudo-first-order rate constant k ₁ of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br⁻, Cl⁻, or H₂O). The relationship between k ₁ and the rate constant k of the bimolecular nucleophilic substitution reaction (S_N2) is given by the linear equation log$ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) $ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) , where $ C_{H^ + } $ C_{H^ + } and C _Nu are the concentrations of H+ and nucleophile, respectively; X ₀ is the excess acidity; and m ^≠ and m* are coefficients. The Swain-Scott equation log$ (k_{Nu} /k_{H_2 O} ) = ns $ (k_{Nu} /k_{H_2 O} ) = ns , where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the S_N2 reactions of the protonated molecule of I with Br⁻, Cl⁻, and H₂O.     

Ferrocytochrome c and deoxyhemoglobin in the reaction with the iron cysteamine nitrosyl complex {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O

By an example of the iron cysteamine nitrosyl complex {Fe₂[S(CH₂)₂NH₃]₂(NO)₄}SO₄··2.5H₂O (CAC), it was shown for the first time that the hydrolysis of this NO donor in the presence of ferrocytochrome c (cyt c ²⁺) affords the iron nitrosyl complex NO-cyt c ²⁺, which serves as the NO depot. The rate constant of NO release from CAC was determined from the kinetics of the formation of NO-cyt c ²⁺. At pH 3.0 the rate constant is (2.7±0.1)·10⁻³ s⁻¹. Ferrocytochrome c produces a less stabilizing effect on CAC than deoxyhemoglobin (Hb). Thus in the presence of cyt c ²⁺, the reaction is completed in 1 h, whereas NO is released from a solution of CAC (2·10⁻⁴ mol L⁻¹) in the presence of Hb during 40 h. The previously unknown stabilization of iron nitrosyl complexes by hemoglobin was found.