Kinetics of O2(a1Deltag) and I(2P1/2) in the photochemistry of N2O/I2 mixtures

The recent demonstration of a discharge-driven oxygen-iodine laser has generated renewed interest in the kinetics of iodine interacting with electronically excited O2 and atomic O. Kinetic measurements that are of relevance to the laser have been carried out using 193 nm pulsed laser photolysis of N2O/I2/CO2 mixtures. Singlet oxygen was generated in this system by the reaction O(1D)+N2O-->O2(a1Deltag, X3Sigma-g)+N2. The fraction of electronically excited O2 produced by this channel was shown to be >0.9. The secondary photochemistry of the N2O/I2/CO2 system was characterized by monitoring the time histories of I(2P1/2), I2, IO, and O2(a). Kinetic modeling of these data was used to determine the rate constant for the deactivation of I(2P1/2) by O(3P) (k=(1.2+/-0.1)x10(-11) cm3 s(-1)). Quenching of I(2P1/2) by O(3P) is suppressed in the discharge-driven laser by using NO2 to scavenge the O atoms. The reaction O(3P)+NO2-->O2+NO is sufficiently exothermic for the production of O2(a), and it has been speculated that this channel may be significant in the laser excitation kinetics. Photolysis of NO2 was used to probe this reaction. O2(a) was not detected, and an upper bound of <0.1 for its production in the reaction of O(3P) or O(1D) with NO2 was established.     

Kinetic studies of the deactivation of O2(1Σg+) and O(1D)

The kinetics of the deactivation of O₂(¹Σ_g⁺) is studied in real time. O₂(¹Σ_g⁺) is generated in this system by the O(¹D) + O₂ reaction following O₃laser flash photolysis in the presence of excess O₂, and it is monitored by its characteristic emission band at 762 nm. Quenching rate constants were obtained for O₂, O₃, N₂, CO₂, H₂O, CF₄and the rare gases. Since O(¹D) is the precursor for the formation of O₂(¹Σ_g⁺), the addition of an O(¹D) quencher effectively lowers the initial concentration of O₂(¹Σ_g⁺). By measuring the initial intensity of the 762 nm fluorescence signal, the relative quenching efficiencies were determined for O(¹D) quenching by N₂, CO₂, Xe, and Kr with respect to O₂; the results are in good agreement with literature values.     

Kinetic and mechanistic study of the reaction of O(1D) with CF2HBr

A laser flash photolysis–resonance fluorescence technique has been employed to investigate the kinetics and mechanism of the reaction of electronically excited oxygen atoms, O(¹D), with CF₂HBr. Absolute rate coefficients (k₁) for the deactivation of O(¹D) by CF₂HBr have been measured as a function of temperature over the range 211–425 K. The results are well described by the Arrhenius expression k₁(T) = 1.72 × 10^?10 exp(+72/T) cm³molecule^?1 s^?1; the accuracy of each reported rate coefficient is estimated to be ±15% (2σ). The branching ratio for nonreactive quenching of O(¹D) to the ground state, O(³P), is found to be 0.39 ± 0.06 independent of temperature, while the branching ratio for production of hydrogen atoms at 298 K is found to be 0.02_?0.02^+0.01. The above results are considered in conjunction with other published information to examine reactivity trends in O(¹D) + CF₂XY reactions (X,Y = H, F, Cl, Br). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 262–270, 2001     

Atmospheric chemistry of FCOx radicals: Kinetic and mechanistic study of the FC(O)O2 + NO2 reaction

A pulse radiolysis system was used to study the kinetics of the reaction of FC(O)O₂ radicals with NO₂. By monitoring the rate of the decay of NO₂ using its absorption at 400 nm the reaction rate constant was determined to be (5.5 ± 0.6) × 10^?12 cm³ molecule^?1 s^?1 at 296 K and 500–1000 mbar pressure of SF₆ diluent. A long path length Fourier transform infrared spectrometer was used to investigate the thermal stability of the product FC(O)O₂NO₂. The rate of thermal decomposition of FC(O)O₂NO₂ was independent of the total pressure of N₂ diluent over the range 100–700 torr and was fit by the expression k_?3 = 6.0 × 10¹⁶ exp(?14150/T) s^?1. The results are discussed in the context of the atmospheric chemistry of FCO_x radicals. © 1995 John Wiley & Sons, Inc.     

Formation of well-aligned ZnGa(2)O(4) nanowires from Ga(2)O(3)/ZnO core-shell nanowires via a Ga(2)O(3)/ZnGa(2)O(4) epitaxial relationship

Formation of well-aligned and single-crystalline ZnGa(2)O(4) nanowires on sapphire (0001) substrates has been achieved via annealing of the Ga(2)O(3)/ZnO core-shell nanowires. Ga(2)O(3)/ZnO core-shell nanowires were prepared using a two-step method. The thickness of the original ZnO shell and the thermal budget of the annealing process play crucial roles for preparing single-crystalline ZnGa(2)O(4) nanowires. Structural analyses of the annealed nanowires reveal the existence of an epitaxial relationship between ZnGa(2)O(4) and Ga(2)O(3) phases during the solid-state reaction. A strong CL emission band centered at 360 nm and a small tail at 680 nm are obtained at room temperature from the single-crystalline ZnGa(2)O(4) nanowires.     

Kinetic and mechanistic studies on the formation of bis(dicarboxylato)carbonatochromate(III) complexes

The reaction of the Cr(xx)₂(H₂O)₂ ⁻ (xx = oxalate, malonate and methylmalonate) complexes with dissolved CO₂ was studied by stopped-flow spectrophotometry in the 7 < pH < 9 range and between 20 to 30°C at an ionic strength of 0.5 mol dm⁻³ (NaCl). Under the experimental conditions the aqua complex ion consists of a pH-dependent mixture of Cr(xx)₂(H₂O)₂ ⁻, Cr(xx)₂(OH) (H₂O)²⁻ and Cr(xx)₂(OH)₂ ³⁻. The monohydroxo and dihydroxo species undergo CO₂ uptake and subsequent intramolecular carbonate ligand chelation independently, at rates which are readily distinguishable and are governed by the uptake rate constants k ₁ and k ₂ and chelation rate constants k ₃ and k ₄, respectively. Only the k ₁ values for oxalato, malonato and methylmalonato complexes could be calculated; k ₁ = 1084 and 1333 and 1650 mol⁻¹ dm³ s⁻¹, respectively. The results obtained were compared with those obtained from other systems that have either cobalt(III), iridium(III) or rhodium(III) as central atoms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetic studies of the reaction between dihalodicarbonylrhodate(I) anions and 2-aminopyridine

The kinetic behavior of the reaction between dihalodicarbonylrhodate(I) anions, [RhX₂(CO)₂]⁻¹, where X = Cl, Br, and the chelating agent 2-aminopyridine was investigated spectrophotometrically. The reaction for both halo analogues was found to obey third order kinetics, first order in the complex anion and second order in the 2-aminopyridine concentrations. The third order rate constants for the chloro and bromo complex anions had the values, at 25°C, of 779 and 156 l² mol⁻² min⁻¹, respectively, and the corresponding activation energies were 3.00 and 5.50 Kcal mol⁻¹. A mechanism is proposed to account for these observations.     

Kinetic and mechanistic studies of the electrocatalytic reduction of O2 TO H2O with mononuclear Cu complexes of substituted 1,10-phenanthrolines

Mononuclear Cu complexes with a 1,10-phenanthroline-based ligand adsorbed onto an edge-plane graphite electrode act as electrocatalysts for the 4-electron reduction of O2 to H2O. A mechanism is proposed for the electrocatalytic O2 reduction that accounts for the observed redox and kinetic dependences on coordinating anions and proton donors in the buffer. Systematic increases of ligand electron-withdrawing properties and/or the steric demands near the Cu center increase the E0 of the Cu catalysts but decrease the rate of O2 reduction. The kinetic rate of O2 reduction at E0, reported as kinetic current divided by catalyst redox charge, decreases as E0 increases: from 16 s(-1) measured at E0 in air-saturated solutions for adsorbed Cu(phen) to 0.4 s(-1) for Cu(2,9-Et2-phen). The maximum value of E for which catalytic activity can be attained is estimated to be +350 mV vs NHE. Near E0, the kinetic current deviates from that expected if O2 binding were the sole rate-limiting step. This indicates that one or more of the electrochemical reduction steps are rate limiting at potentials near E0.     

Crystal structure and heat capacity of the mixed-valence trinuclear iron monoiodoacetate complex, [Fe(III)2Fe(II)O(O2CCH2I)6(H2O)3][Fe(III)3O(O2CCH2I)6(H2O)3]I

The crystal structure of a trinuclear iron monoiodoacetate complex was determined. Although it has been incorrectly characterized as [Fe₃O(O₂CCH₂I)₆(H₂O)₃], the correct chemical formula turned out to be [Fe(III)₂Fe(II)O(O₂CCH₂I)₆(H₂O)₃]-[Fe(III)₃O(O₂CCH₂I)₆(H₂O)₃]I (1). The two kinds of Fe₃O molecules (Fe(III)₂Fe(II)O and Fe(III)₃O) are crystallographically indistinguishable. All the Fe atoms are crystallographically equivalent because of a crystallographic threefold symmetry. Heat capacities of 1 seem to exhibit no thermal anomaly in the temperature range 5.5–309 K, although the valence detrapping phenomenon has been observed in this temperature range. This fact indicates that the valence-detrapping phenomenon in 1 occurs without any phase transition, leading 1 to a glassy state, probably because the crystal of 1 is just like a solid solution of distorted mixed-valence Fe(III)₂Fe(II)O molecules and permanently undistorted Fe(III)₃O molecules which may act as an inhibitor for a cooperative valence-trapping.     

Solvent-free iodination of organic molecules using the I(2)/urea-H(2)O(2) reagent system

Introduction of iodine under solvent-free conditions into several aromatic compounds activated toward electrophilic functionalization was found to proceed efficiently using elemental iodine in the presence of a solid oxidizer, the urea-H(2)O(2) (UHP) adduct. Two types of iodo-functionalization through an electrophilic process were observed: iodination of an aromatic ring, and side-chain iodo-functionalization in the case of arylalkyl ketones. Two reaction routes were established based on the required substrate : iodine : oxidizer ratio for the most efficient iodo-transformation, and the role of UHP was elucidated in each route. The first, requiring a 1 : 0.5 : 0.6 stoichiometric ratio of substrate to iodine to UHP, followed the atom economy concept in regard to iodine and was valid in the case of aniline, 4-t-Bu-phenol, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,2,3-trimethoxy benzene, 1,2,4-trimethoxy benzene, 1,3,5-trimethoxy benzene, 1-indanone and 1-tetralone. The second reaction route, where a 1 : 1 : 1 stoichiometric ratio of substrate : I(2) : UHP was needed for efficient iodination, was suitable for side-chain iodo-functionalization of acetophenone and methoxy-substituted acetophenones. Moreover, addition of iodine to 1-octene and some phenylacetylenic derivatives was found to proceed efficiently without the presence of any oxidizer and solvent at room temperature.     

Kinetic and mechanistic studies on the interaction of adenosine with hydroxopentaaquarhodium(III) ion

The kinetics of the title reaction have been studied spectrophotometrically as a function of pH, [substrate], [adenosine] and temperature (50–65°C) by monitoring the appearance of a characteristic peak of the adenosine substituted product (λ_max = 289 nm). The reaction rate is pH dependent in the 3.0–4.3 range. With increase in [adenosine] the rate was found to increase and approached a limit at a higher adenosine concentration. The following rate law has been established at pH 4.3: d[Rh(H₂O)₃(OH)(adenosine)²⁺]/dt = k _a K _E[Rh(H₂O)₅(OH)²⁺]_total[adenosine]/ (1 + K _E[adenosine]) Rate and activation parameters are consistent with an associative interchange mechanism. Experimental results are discussed with reference to literature data for analogous systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quenching of I(2P1/2) by O3 and O(3P)

Oxygen-iodine lasers that utilize electrical or microwave discharges to produce singlet oxygen are currently being developed. The discharge generators differ from conventional chemical singlet oxygen generators in that they produce significant amounts of atomic oxygen. Post-discharge chemistry includes channels that lead to the formation of ozone. Consequently, removal of I(2P1/2) by O atoms and O3 may impact the efficiency of discharge driven iodine lasers. In the present study, we have measured the rate constants for quenching of I(2P1/2) by O(3P) atoms and O3 using pulsed laser photolysis techniques. The rate constant for quenching by O3, (1.8 +/- 0.4) x 10(-12) cm3 s-1, was found to be a factor of 5 smaller than the literature value. The rate constant for quenching by O(3P) was (1.2 +/- 0.2) x 10(-11) cm3 s-1.     

Kinetic and mechanistic studies on the interaction of thymidine with the hydroxopentaaquarhodium(III) ion

The interaction of thymidine, a nucleoside, with hydroxopentaaquarhodium(III), [Rh(H₂O)₅(OH)]²⁺ ion in aqueous medium is reported and the possible mode of binding is discussed. The kinetics of interaction between thymidine and [Rh(H₂O)₅OH]²⁺ has been studied spectrophotometrically as a function of [Rh(H₂O)₅OH²⁺], [thymidine], pH and temperature. The reaction has been monitored at 298 nm, the _max of the substituted complex, and where the spectral difference between the reactant and product is a maximum. The reaction rate increases with [thymidine] and reaches a limiting value at a higher ligand concentration. From the experimental findings an associative interchange mechanism for the substitution process is suggested. The activation parameters (H=47.8 ± 5.7 kJ mol^–1, S=–173 ± 17 J K^–1 mol^–1) supports our proposition. The negative G⁰ (–13.8 kJ mol^–1) for the first equilibrium step also supports the spontaneous formation of the outer sphere association complex.     

Synthetic scope and mechanistic studies of Ru(OH)x/Al2O3-catalyzed heterogeneous hydrogen-transfer reactions

Three kinds of hydrogen-transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH)x/Al2O3. A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent-free reaction is also demonstrated: the isomerization of 1-octen-3-ol with a substrate/catalyst ratio of 20,000/1 shows a very high turnover frequency (TOF) of 18,400 h(-1), with a turnover number (TON) that reaches 17,200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH)x/Al2O3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions were examined with monodeuterated substrates. After the racemization of (S)-1-deuterio-1-phenylethanol in the presence of acetophenone was complete, the deuterium content at the alpha-position of the corresponding racemic alcohol was 91%, whereas no deuterium was incorporated into the alpha-position during the racemization of (S)-1-phenylethanol-OD. These results show that direct carbon-to-carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the alpha-deuterium of 3-deuterio-1-octen-3-ol was selectively relocated at the beta-position of the corresponding ketones (99% D at the beta-position), suggesting the involvement of a 1,4-addition of ruthenium monohydride species to the alpha,beta-unsaturated ketone intermediate. The ruthenium monohydride species and the alpha,beta-unsaturated ketone would be formed through alcoholate formation/beta-elimination. Kinetic studies and kinetic isotope effects show that the Ru-H bond cleavage (hydride transfer) is included in the rate-determining step.     

Kinetic and mechanistic studies of NEt3-catalyzed intramolecular aminolysis of carbamate

The mechanism of the NEt 3-catalyzed intramolecular aminolysis of Z- 1 in acetonitrile or aqueous acetonitrile solutions is suggested to involve rate-determining collapse of T (+/-) through simultaneous H-abstraction and ethoxide expulsion of E2 process. The evidence for that includes the primary isotope effect of k H/ k D = 1.66 in acetonitrile, general-base catalysis in aqueous buffer solutions, salt effect, and the proposed water-stabilized rate-determining transition states (TS1 and TS2) in the water-titration experiments.     

Peroxydisulfate activation by [RuII(tpy)(pic)(H2O)]+. Kinetic, mechanistic and anti-microbial activity studies

The oxidation of [Ru(II)(tpy)(pic)H(2)O](+) (tpy = 2,2',6',2'-terpyridine; pic(-) = picolinate) by peroxidisulfate (S(2)O(8)(2-)) as precursor oxidant has been investigated kinetically by UV-VIS, IR and EPR spectroscopy. The overall oxidation of Ru(II)- to Ru(IV)-species takes place in a consecutive manner involving oxidation of [Ru(II)(tpy)(pic)H(2)O](+) to [Ru(III)(tpy)(pic)(OH)](+), and its further oxidation of to the ultimate product [Ru(IV)(tpy)(pic)(O)](+) complex. The time course of the reaction was followed as a function of [S(2)O(8)(2-)], ionic strength (I) and temperature. Kinetic data and activation parameters are interpreted in terms of an outer-sphere electron transfer mechanism. Anti-microbial activity of Ru(II)(tpy)(pic)H(2)O](+) complex by inhibiting the growth of Escherichia coli DH5α in presence of peroxydisulfate has been explored, and the results of the biological studies have been discussed in terms of the [Ru(IV)(tpy)(pic)(O)](+) mediated cleavage of chromosomal DNA of the bacteria.     

Kinetic and mechanistic aspects of the NO oxidation by O2 in aqueous phase

The oxidation kinetics of NO by O₂ in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO]² · [O₂] in analogy to gas-phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 10⁶ M^?2 s^?1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 10⁸ M^?2 s^?1. The rate constant displays a very slight pH dependence corresponding to less than a factor of three over the range 0 to 12. The system NO/O₂ in aqueous solution is an efficient nitrosating agent which has been tested using phenol as a substrate over the pH range 0 to 12. The rate limiting step leading to formation of 4-nitrosophenol is the formation of the reactive intermediate whose competitive hydrolysis yields HONO or NO₂^?. The absence of NO₃^? in the autoxidation of NO, the exclusive presence of NO₂^? as a product of the nitrosation reaction of phenol, and the kinetic results of the N₃^? trapping experiments point towards N₂O₃ as the reactive intermediate. © 1994 John Wiley & Sons, Inc.     

A homogeneous catalyst for selective O(2) oxidation at ambient temperature. diversity-based discovery and mechanistic investigation of thioether oxidation by the Au(III)Cl(2)NO(3)(thioether)/O(2) system.

A library of inorganic complexes with reversible redox chemistry and/or the ability to catalyze homogeneous oxidations by peroxides, including but not limited to combinations of polyoxometalate anions and redox-active cations, was constructed. Evaluation of library members for the ability to catalyze aerobic sulfoxidation (O(2) oxidation of the thioether, 2-chloroethyl ethyl sulfide, CEES) led to the discovery that a combination of HAuCl(4) and AgNO(3) forms a catalyst that is orders of magnitude faster than the previously most reactive such catalysts (Ru(II) and Ce(IV) complexes) and one effective at ambient temperature and 1 atm air or O(2). If no O(2) but high concentrations of thioether are present, the catalyst is inactivated by an irreversible formation of colloidal Au(0). However, this inactivation is minimal in the presence of O(2). The stoichiometry is R(2)S + (1)/(2)O(2) --> R(2)S(O), a 100% atom efficient oxygenation, and not oxidative dehydrogenation. However, isotope labeling studies with H(2)(18)O indicate that H(2)O and not O(2) or H(2)O(2) is the source of oxygen in the sulfoxide product; H(2)O is consumed and subsequently regenerated in the mechanism. The rate law evaluated for every species present in solution, including the products, and other kinetics data, indicate that the dominant active catalyst is Au(III)Cl(2)NO(3)(thioether) (1); the rate-limiting step involves oxidation of the substrate thioether (CEES) by Au(III); reoxidation of the resulting Au(I) to Au(III) by O(2) is a fast subsequent step. The rate of sulfoxidation as Cl is replaced by Br, the solvent kinetic isotope effect (k(H)(2)(O)/k(D)(2)(O) = 1.0), and multiparameter fitting of the kinetic data establish that the mechanism of the rate-limiting step involves a bimolecular attack of CEES on a Au(III)-bound halide and it does not involve H(2)O. The reaction is mildly inhibited by H(2)O and the CEESO product because these molecules compete with those needed for turnover (Cl(-), NO(3)(-)) as ligands for the active Au(III). Kinetic studies using DMSO as a model for CEESO enabled inhibition by CEESO to be assessed.