General pathway of sulfur-chain breakage of polythionates by iodine confirmed by the kinetics and mechanism of the pentathionate-iodine reaction

The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S(5)O(6)I(-) along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S(5)O(6)I(-), including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S(5)O(6)(2-) + 10I(2) + 14H(2)O → 5SO(4)(2-) + 20I(-) + 28H(+). A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.     

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.     

酸性介质中碘离子在铂电极上的电化学氧化行为

采用循环伏安法研究了酸性介质中碘离子在铂电极上不同电位区间, 不同酸度下的电化学反应行为. 结果表明, 当极化电位较低(小于0.6 V(vs Hg/Hg2SO4))时, 碘离子在铂电极上发生2I--2e→I2电氧化反应, 反应产物通过I2+I-=I-3被进一步溶解, 整个反应属于E-C(electrochemical-chemical)模式. 电氧化过程中可以形成碘膜, 其也可以被碘离子溶解. 当极化电位升高至0.6 V(vs Hg/Hg2SO4)或以上时, 碘离子会直接电氧化为高价态碘化合物, I-+3H2O→IO-3+6H++6e, 而析出的碘膜并不发生再氧化反应; 在电化学还原过程中, 出现了两个还原峰, 分别对应于I2、I-3的还原反应; 在无碘膜时, 碘离子电氧化过程受溶液中碘离子的液相扩散步骤控制; 碘膜形成后, 主要受碘膜中碘离子的固相扩散控制; 酸度对于碘离子的电化学氧化过程有很大的影响, 其线性极化曲线的起峰电位及电流峰值电位均随酸浓度升高而负移.     

Hypoiodous acid as guest molecule in protonated water clusters: a combined FT-ICR/DFT study of I(H2O)n+.

Cationic water clusters containing iodine, of the composition I(H2O)n+, n = 0-25, are generated in a laser vaporization source and investigated by FT-ICR mass spectrometry. An investigation of blackbody radiation-induced fragmentation of size-selected clusters I(H2O)n+, n = 3-15, under collision-free conditions revealed an overall linear increase of the unimolecular rate constant with cluster size, similar to what has been observed previously for other hydrated ions. Above a certain critical size, I(H2O)n+, n greater than or approx. 13, reacts with HCl by formation of the interhalide ICl and a protonated water cluster, which is the reverse of a known solution-phase reaction. Accompanying density functional calculations illustrate the conceptual differences between cationic and anionic iodine-water clusters I(H2O)n+/-. While I-(H2O)n is genuinely a hydrated iodide ion, the cationic closed-shell species I(H2O)n+ may be best viewed as a protonated water cluster, in which one water molecule is replaced by hypoiodous acid. In the strongly acidic environment, HOI is protonated because of its high proton affinity. However, similar to the well-known H3O+/H5O2+ controversy in protonated water clusters, a smooth transition between H2IO+ and H4IO2+ as core ions is observed for different cluster sizes.     

Kinetic-photometric determination of iodide based on its inhibitory effect on the bromate oxidation of Victoria Blue 4-R.

A new sensitive kinetic method has been developed for the determination of trace amounts of I- over the range of 50-300 ng cm(-3). The method was based on the inhibitory effect of iodide on the oxidation of Victoria Blue 4-R by KBrO3. The reaction is monitored spectrophotometrically at 596.3 nm. Method development includes optimization of reagent concentration and temperature. The kinetic parameters of the reaction were reported and a rate equation was suggested. The effects of certain foreign ions upon the reaction rate were determined for the assessment of the selectivity of the method. The proposed method was successfully applied to the determination of iodide in real samples. The new developed method was found to have fairly good selectivity, sensitivity, simplicity and rapidity.     

Thermodynamics of oxygen activation by macrocyclic complexes of rhodium

The oxidation of ABTS2- [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)] with a superoxorhodium(III) complex, L2(H2O)RhOO2+ (L2 = meso-hexamethylcyclam) is characterized by an acid-dependent equilibrium constant, log(Ke/[H+]) = 4.91 +/- 0.10 in the pH range of 4.89-6.49. This equilibrium constant was used to calculate the reduction potential for the L2(H2O)RhOO2+/L2(H2O)RhOOH2+ couple, E0 = 0.97 V vs NHE. The pH dependence of the kinetics of the L2(H2O)RhOOH2+/I- reaction yielded the acid dissociation constant for the coordinated water in L2(H2O)RhOOH2+, pKa = 6.9. Spectrophotometric pH titrations provided pKa = 6.6 for the superoxo complex, L2(H2O)RhOO2+. The combination of the two pKa values with the reduction potential measured in acidic solutions yielded the reduction potential E0 = 0.95 V for the L2(HO)RhOO+/L2(HO)RhOOH+ couple. Thermochemical calculations yielded the bond-dissociation free energy of the L2(H2O)RhOO-H2+ bond as 315 kJ/mol at 298 K.     

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O(2). The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: [see text] The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H(2)SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O+SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T-->S process in H2SiOO. This process has a small spin-orbit coupling matrix element, consistent with an estimate of its rate constant of 1x10(9) s-1 obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2+O2 and SiCl2+O2.     

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.     

Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H(2)O)MOO(2+) (L = (H(2)O)(4), (NH(3))(4), or N(4)-macrocycle; M = Cr(III), Rh(III)) react with iodide ions according to the stoichiometry L(H(2)O)MOO(2+) + 3I(-) + 3H(+) --> L(H(2)O)MOH(2+) + 1.5I(2) + H(2)O. The rate law is -d[L(H(2)O)MOO(2+)]/dt = k [L(H(2)O)MOO(2+)][I(-)][H(+)], where k = 93.7 M(-2) s(-1) for Cr(aq)OO(2+), 402 for ([14]aneN(4))(H(2)O)CrOO(2+), and 888 for (NH(3))(4)(H(2)O)RhOO(2+) in acidic aqueous solutions at 25 degrees C and 0.50 M ionic strength. The Cr(aq)OO(2+)/I(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H)()2(O)/k(D)2(O) = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related Cr(aq)OOH(2+)/I(-) reaction has k(H)2(O)/k(D)2(O) = 0.6. The oxidation of (NH(3))(5)Rupy(2+) by Cr(aq)OO(2+) exhibits an [H(+)]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H(+)])[Ru(NH(3))(5)py(2+)][Cr(aq)OO(2+)]. Diiodine radical anions, I(2)(*)(-), reduce Cr(aq)OO(2+) with a rate constant k = 1.7 x 10(9) M(-1) s(-1).     

Oxyhalogen-sulfur chemistry: kinetics and mechanism of oxidation of N-acetylthiourea by chlorite and chlorine dioxide

The oxidation reactions of N-acetylthiourea (ACTU) by chlorite and chlorine dioxide were studied in slightly acidic media. The ACTU-ClO(2)(-) reaction has a complex dependence on acid with acid catalysis in pH > 2 followed by acid retardation in higher acid conditions. In excess chlorite conditions the reaction is characterized by a very short induction period followed by a sudden and rapid formation of chlorine dioxide and sulfate. In some ratios of oxidant to reductant mixtures, oligo-oscillatory formation of chlorine dioxide is observed. The stoichiometry of the reaction is 2:1, with a complete desulfurization of the ACTU thiocarbamide to produce the corresponding urea product: 2ClO(2)(-) + CH(3)CONH(NH(2))C=S + H(2)O --> CH(3)CONH(NH(2))C=O + SO(4)(2-) + 2Cl(-) + 2H(+) (A). The reaction of chlorine dioxide and ACTU is extremely rapid and autocatalytic. The stoichiometry of this reaction is 8ClO(2)(aq) + 5CH(3)CONH(NH(2))C=S + 9H(2)O --> 5CH(3)CONH(NH(2))C=O + 5SO(4)(2-) + 8Cl(-) + 18H(+) (B). The ACTU-ClO(2)(-) reaction shows a much stronger HOCl autocatalysis than that which has been observed with other oxychlorine-thiocarbamide reactions. The reaction of chlorine dioxide with ACTU involves the initial formation of an adduct which hydrolyses to eliminate an unstable oxychlorine intermediate HClO(2)(-) which then combines with another ClO(2) molecule to produce and accumulate ClO(2)(-). The oxidation of ACTU involves the successive oxidation of the sulfur center through the sulfenic and sulfinic acids. Oxidation of the sulfinic acid by chlorine dioxide proceeds directly to sulfate bypassing the sulfonic acid. Sulfonic acids are inert to further oxidation and are only oxidized to sulfate via an initial hydrolysis reaction to yield bisulfite, which is then rapidly oxidized. Chlorine dioxide production after the induction period is due to the reaction of the intermediate HOCl species with ClO(2)(-). Oligo-oscillatory behavior arises from the fact that reactions that form ClO(2) are comparable in magnitude to those that consume ClO(2), and hence the assertion of each set of reactions is based on availability of reagents that fuel them. A computer simulation study involving 30 elementary and composite reactions gave a good fit to the induction period observed in the formation of chlorine dioxide and in the autocatalytic consumption of ACTU in its oxidation by ClO(2).     

Kinetics and mechanism of the complexation and anation of protonated dioxotetracyanomolybdate(IV) with 1,10-phenanthroline

The anation kinetics of the protonated dioxotetracyanomolybdate(IV) ion with 1,10-phenanthroline have been studied spectrophotometrically. The effect of the H+ ion, ionic strength and temperature on the reaction rate has been determined; the rate increases with increasing H+ ion concentration, is independent of ionic strength, and increases with temperature. The reaction follows first order kinetics with respect to [Mo(OH)2- (H2O)2(CN)4]2– and is considered to proceed through the formation of a 1,10-phenanthroline–Mo(OH)2- (H2O)2(CN)4M2– complex (outer sphere) which converts into an inner sphere complex. The formation constant (Kn) for the outer sphere complex has been calculated from the kinetic data by two different methods. Anation of Mo(OH)2-(H2O)2(CN)42– is discussed in terms of an associative interchange (Ia) mechanism. The activation parameters have been calculated using the Arrhenius equation. A substitution mechanism is proposed and the rate equation derived:kobs = kKnKa[H+] [phen] /{1 + KnKa[H+] [phen]}.     

Unusual oxidation of phosphines employing water as the oxygen atom source and tris(benzene-1,2-dithiolate)molybdenum(VI) as the oxidant. A functional molybdenum hydroxylase analogue system

The kinetics of the reaction of Mo(VI)(S2C6H4)3 with organic phosphines to produce the anionic Mo(V) complex, Mo(V)(S2C6H4)3-, and phosphine oxide have been investigated. Reaction rates, monitored by UV-vis stopped-flow spectrophotometry, were studied in THF/H2O media as a function of the concentration of phosphine, molybdenum complex, pH, and water concentration. The reaction exhibits pH-dependent phosphine saturation kinetics and is first-order in complex concentration. The water concentration strongly enhances the reaction rate, which is consistent with the formation of Mo(VI)(S2C6H4)3(H2O) adduct as a crucial intermediate. The observed pH dependence of the reaction rate would arise from the distribution between acid and basic forms of this adduct. Apparently, the electrophilic attack by the phosphine at the oxygen requires the coordinated water to be in the unprotonated hydroxide form, Mo(VI)(S2C6H4)3(HO)-. This is followed by the concerted abstraction of 2e-, H+ by the Mo(VI) center to give Mo(IV)(S2C6H4)3(2-), H+, and the corresponding phosphine oxide. However, this Mo(IV) complex product is oxidized rapidly to Mo(V)(S2C6H4)3- via comproportionation with unreacted Mo(VI)(S2C6H4)3. The Mo(V) complex thus formed can be oxidized to the starting Mo(VI) complex upon admission of O2. Consequently, Mo(VI)(S2C6H4)3 is a catalyst for the autoxidation of phosphines in the presence of water. Additionally, there was a detectable variation in the reactivity for a series of tertiary phosphines. The rate of Mo(VI) complex reduction increases as does the phosphine basicity: (p-CH3C6H4)3P > (C6H5)3P > (p-ClC6H4)3P. Oxygen isotope tracing confirms that water rather than dioxygen is the source of the oxygen atom which is transferred to the phosphine. Such reactivity parallels oxidase activity of xanthine enzyme with phosphine as oxygen atom acceptor and Mo(VI)(S2C6H4)3 as electron acceptor.     

High-temperature shock tube measurements of methyl radical decomposition

We have studied the two-channel thermal decomposition of methyl radicals in argon, involving the reactions CH3 + Ar --> CH + H2 + Ar (1a) and CH3 + Ar --> CH2 + H + Ar (1b), in shock tube experiments over the 2253-3527 K temperature range, at pressures between 0.7 and 4.2 atm. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1311 nm. The collision-broadening coefficient for CH in argon, 2gamma(CH-Ar), was measured via repeated single-frequency experiments in the ethane pyrolysis system behind reflected shock waves. The measured 2gamma(CH-Ar) value and updated spectroscopic and molecular parameters were used to calculate the CH absorption coefficient at 431.1311 nm (23194.80 cm(-1)), which was then used to convert raw traces of fractional transmission to quantitative CH concentration time histories in the methyl decomposition experiments. The rate coefficient of reaction 1a was measured by monitoring CH radicals generated upon shock-heating highly dilute mixtures of ethane, C2H6, or methyl iodide, CH3I, in an argon bath. A detailed chemical kinetic mechanism was used to model the measured CH time histories. Within experimental uncertainty and scatter, no pressure dependence could be discerned in the rate coefficient of reaction 1a in the 0.7-4.2 atm pressure range. A least-squares, two-parameter fit of the current measurements, applicable between 2706 and 3527 K, gives k(1a) (cm(3) mol(-1) s(-1)) = 3.09 x 1015 exp[-40700/T (K)]. The rate coefficient of reaction 1b was determined by shock-heating dilute mixtures of C2H6 or CH3I and excess O2 in argon. During the course of reaction, OH radicals were monitored using the well-characterized R(1)(5) line of the OH A-X (0,0) band at 306.6871 nm (32606.52 cm(-1)). H atoms generated via reaction 1b rapidly react with O2, which is present in excess, forming OH. The OH traces are primarily sensitive to reaction 1b, reaction 9 (H + O2 --> OH + O) and reaction 10 (CH3 + O2 --> products), where the rate coefficients of reactions 9 and 10 are relatively well-established. No pressure dependence could be discerned for reaction 1b between 1.1 and 3.9 atm. A two-parameter, least-squares fit of the current data, valid over the 2253-2975 K temperature range, yields the rate expression k(1b) (cm(3) mol(-1) s(-1)) = 2.24 x 10(15) exp[-41600/T (K)]. Theoretical calculations carried out using a master equation/RRKM analysis fit the measurements reasonably well.     

pH oscillations in the BrO3--SO3(2-)/HSO3- reaction in a CSTR

Large-amplitude pH oscillations have been measured during the oxidation of sulfur (IV) species by the bromate ion in aqueous solution in a continuous-flow stirred tank reactor in the absence of any additional oxidizing or reducing reagent. The source of the oscillation in this simple chemical reaction is a two-way oxidation of sulfur (IV) by the bromate ion: (1) the hydrogen-ion-producing self-accelerating oxidation to sulfur (VI) (SO4(2-)), and (2) a hydrogen-ion-consuming oxidation to sulfur (V) (S2O6(2-)). In such a way, both the H+-producing and H+-consuming composite processes required for a pH oscillator take place in parallel in a reaction between two reagents in this system. A simple reaction scheme, consisting of the protonation equilibria of SO3(2-) and HSO3-, the oxidation of HSO3- and H2SO3 by BrO3- to SO4(2-), and the oxidation of H2SO3 to S2O6(2-) has successfully been used to simulate the observed dynamical behavior. Simulation with this simple scheme shows that oscillations can be calculated even if only about 1% of sulfur (IV) is oxidized to S2O6(2-) along with the main product SO4(2-). Agreement between calculated and measured dynamical behavior is found to be quite good. Increasing temperature decreases both the period length of oscillations in a CSTR and the Landolt time measured in a closed reactor. No temperature compensation of the oscillatory frequency is found in this reaction.     

Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(III) to oxo-chromium(V) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).     

Kinetics and mechanism of peroxymonocarbonate formation

The kinetics and mechanism of peroxymonocarbonate (HCO(4)(-)) formation in the reaction of hydrogen peroxide with bicarbonate have been investigated for the pH 6-9 range. A double pH jump method was used in which (13)C-labeled bicarbonate solutions are first acidified to produce (13)CO(2) and then brought to higher pH values by addition of base in the presence of hydrogen peroxide. The time evolution of the (13)C NMR spectrum was used to establish the competitive formation and subsequent equilibration of bicarbonate and peroxymonocarbonate following the second pH jump. Kinetic simulations are consistent with a mechanism for the bicarbonate reaction with peroxide in which the initial formation of CO(2) via dehydration of bicarbonate is followed by reaction of CO(2) with H(2)O(2) (perhydration) and its conjugate base HOO(-) (base-catalyzed perhydration). The rate of peroxymonocarbonate formation from bicarbonate increases with decreasing pH because of the increased availability of CO(2) as an intermediate. The selectivity for formation of HCO(4)(-) relative to the hydration product HCO(3)(-) increases with increasing pH as a consequence of the HOO(-) pathway and the slower overall equilibration rate, and this pH dependence allows estimation of rate constants for the reaction of CO(2) with H(2)O(2) and HOO(-) at 25 °C (2 × 10(-2) M(-1) s(-1) and 280 M(-1) s(-1), respectively). The contributions of the HOO(-) and H(2)O(2) pathways are comparable at pH 8. In contrast to the perhydration of many other common inorganic and organic acids, the facile nature of the CO(2)/HCO(3)(-) equilibrium and relatively high equilibrium availability of the acid anhydride (CO(2)) at neutral pH allows for rapid formation of the peroxymonocarbonate ion without strong acid catalysis. Formation of peroxymonocarbonate by the reaction of HCO(3)(-) with H(2)O(2) is significantly accelerated by carbonic anhydrase and the model complex [Zn(II)L(H(2)O)](2+) (L = 1,4,7,10-tetraazacyclododecane).     

Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A)

The [H(+)]-catalyzed dissociation rate constants of several trivalent lanthanide (Ln) complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (LnDO2A(+), Ln = La, Pr, Eu, Er and Lu) have been determined in two pH ranges: 3.73-5.11 and 1.75-2.65 at four different temperatures (19-41.0 °C) in aqueous media at a constant ionic strength of 0.1 mol dm(-3) (LiClO(4)). For the study in the higher pH range, i.e. pH 3.73-5.11, copper(II) ion was used as the scavenger for the free ligand DO2A in acetate/acetic acid buffer medium. The rates of Ln(III) complex dissociation have been found to be independent of [Cu(2+)] and all the Ln(III) complexes studied show [H(+)]-dependence at low acid concentrations but become [H(+)]-independent at high acid concentrations. Influence of the acetate ion content in the buffer on the dissociation rate has also been investigated and all the complexes exhibit a first-order dependence on [Acetate]. The dissociation reactions follow the rate law: k(obs) = k(Ac)[Acetate] + K'k(lim)[H(+)]/(1 + K'[H(+)]) where k(AC) is the dissociation rate constant for the [Acetate]-dependent pathway, k(lim) is the limiting rate constant, and K' is the equilibrium constant for the reaction LnDO2A(+) + H(+) ? LnDO2AH(2+). In the lower pH range, i.e. pH 1.75-2.65, the dye indicator, cresol red, was used to monitor the dissociation rate, and all the Ln(III) complexes also show [H(+)]-dependence dissociation pathways but without the rate saturation observed at higher pH range. The dissociation reactions follow the simple rate law: k(obs) = k(H)[H(+)], where k(H) is the dissociation rate constant for the pathway involving monoprotonated species. The absence of an [H(+)]-independent pathway in both pH ranges indicates that LnDO2A(+) complexes are kinetically rather inert. The obtained k(AC) values follow the order: LaDO2A(+) > PrDO2A(+) > EuDO2A(+) > ErDO2A(+) > LuDO2A(+), whereas the k(lim) and k(H) values follow the order: LaDO2A(+) > PrDO2A(+) > ErDO2A(+) > EuDO2A(+) > LuDO2A(+), mostly consistent with their thermodynamic stability order, i.e. the more thermodynamically stable the more kinetically inert. In both pH ranges, activation parameters, ΔH*, ΔS* and ΔG*, for both acetate-dependent and proton-catalyzed dissociation pathways have been obtained for most of the La(III), Pr(III), Eu(III), Er(III) and Lu(III) complexes, from the temperature dependence measurements of the rate constants in the 19-41 °C range. An isokinetic (linear) relationship is found between ΔH* and ΔS* values, which supports a common reaction mechanism.     

Kinetics and mechanism of oxygen atom abstraction from a dioxo-rhenium(VII) complex

The kinetics of reaction between triarylphosphanes and two newly prepared dioxorhenium(VII) compounds has been evaluated. The compounds are MeRe(VII)(O)(2)("O,S") in which "O,S" represents an alkoxo, thiolato chelating ligand. With MeReO(3), ligands derived from 1-mercaptoethanol and 1-mercapto-2-propanol form MeRe(O)(2)(met), 2, and MeRe(O)(2)(m2p), 3. These compounds persist in chloroform solution for several hours at room temperature and for 2-3 weeks at -22 degrees C, particularly when water is carefully excluded. They were obtained as red oils with clean (1)H NMR spectra, but attempts to obtain pure, crystalline products were not successful because one decomposition pathway shows a kinetic order >1. The fastest reaction occurs between P(p-MeOC(6)H(4))(3) and 2; k(298) = 215(7) L mol(-1) s(-1) in chloroform at 25(1) degrees C. The other rate constants follow a Hammett correlation against 3sigma, with rho = -0.69(7). This study relates to oxygen atom transfer reactions catalyzed by MeReO(mtp)PPh(3), 1, in which MeRe(O)(2)(mtp), 4, is a postulated intermediate that does not build up to a measurable concentration during the catalytic cycle. Compound 2 does not react with MeSTol, but MeS(O)Tol was formed when tert-butyl hydroperoxide was added. This suggests that equilibrium lies to the left in this reaction, 2 + MeSTol + L = MeReO(met)L + MeS(O)Tol, and is drawn to the right by a reaction between MeReO(met)L and the hydroperoxide. Triphenyl arsane does not react with 2, but thermodynamic versus kinetic barriers were not resolved.     

Kinetics of oxygen exchange between the two isomers of bisulfite ion,disulfite Ion (S(2)O(5)(2-)), and water as studied by oxygen-17 nuclear magnetic resonance spectroscopy

The nuclear magnetic transverse relaxation time of oxygen-17 in aqueous sodium bisulfite solutions in the pH range from 2.5 to 5 was measured over a range of temperatures, pH, and S(IV) concentrations at an ionic strength of 1.0 m. From these data the rate law for oxygen exchange between bisulfite ion and water was determined and found to be consistent with oxygen exchange occurring via the reactions SHO(3)(-) + H(+) SO(2) + H(2)O, SO(3)H(-) + SHO(3-) SO(3)(2-) + SO(2) + H(2)O, and SO(3)H(-) + SHO(3-) S(2)O(5)(2-) + H(2)O, where the symbol SHO(3-) refers to both isomeric forms of bisulfite ion, one in which the hydrogen is bonded to the sulfur (denoted HSO(3-)) and another in which the hydrogen is bonded to an oxygen atom (denoted SO(3)H(-)). The SO(3)H(-) isomer exchanges oxygen atoms with water much more rapidly than does the HSO(3-) isomer. The value of k(-1) was determined and is in essential agreement with the results of a previous determination by relaxation measurements. The value of k(16a) + k(16b) was also found, and k(16b) is at least as large as k(16a). The rate and mechanism of oxygen exchange between the two bisulfite ion environments were studied by analyzing the broadening of the HSO(3-) resonance. Oxygen exchange occurs through isomerization caused by proton transfers.     

The reaction of nitrogen atoms with methyl radicals: Are spin-forbidden channels important?

A computational study of the N(4S) + CH3 reaction has been carried out. The reactants approach through an attractive potential surface leading to an intermediate, H3CN, whose formation does not involve any barrier. In agreement with the experimental results, the dominant channel for this reaction is H2CN+H. The theoretically estimated rate coefficient for the overall process at 298 K is 9.1 x 10(-12) cm3 s(-1) molecule(-1), which is nearly 1 order of magnitude lower than the experimental result, but also much larger than those computed for the reactions of ground-state nitrogen atoms with halomethyl radicals. The analysis of the singlet potential energy surface, and the corresponding computational kinetic study, shows that for the reaction of excited nitrogen atoms with methyl radicals, the preferred product from the kinetic point of view is also H2CN+H, but in this case production of HCN is significant (with branching ratios around 0.185). According to our calculations, spin-forbidden processes are highly unlikely for the N(4S) + CH3 reaction. However, further evolution of the preferred products, H2CN+H, might explain the experimental observation of hydrogen cyanide as a minor product in this reaction.