Showing papers on "Reaction rate constant published in 2003"

Mechanism of Asymmetric hydrogenation of ketones catalyzed by BINAP/1,2-diamine-ruthenium(II) complexes

Abstract: Asymmetric hydrogenation of acetophenone with trans-RuH(η1-BH4)[(S)-tolbinap][(S,S)-dpen] (TolBINAP = 2,2‘-bis(di-4-tolylphosphino)-1,1‘-binaphthyl; DPEN = 1,2-diphenylethylenediamine) in 2-propanol gives (R)-phenylethanol in 82% ee. The reaction proceeds smoothly even at an atmospheric pressure of H2 at room temperature and is further accelerated by addition of an alkaline base or a strong organic base. Most importantly, the hydrogenation rate is initially increased to a great extent with an increase in base molarity but subsequently decreases. Without a base, the rate is independent of H2 pressure in the range of 1−16 atm, while in the presence of a base, the reaction is accelerated with increasing H2 pressure. The extent of enantioselection is unaffected by hydrogen pressure, the presence or absence of base, the kind of base and coexisting metallic or organic cations, the nature of the solvent, or the substrate concentrations. The reaction with H2/(CH3)2CHOH proceeds 50 times faster than that with D2/(...

Influence of chemical structure of dyes, of pH and of inorganic salts on their photocatalytic degradation by TiO2 comparison of the efficiency of powder and supported TiO2

Abstract: Anionic (Alizarin S (AS), azo-Methyl Red (MR), Congo Red (CR), Orange G (OG)) and cationic (Methylene Blue (MB)) dyes were degraded, either individually or in mixtures, by using UV-irradiated TiO 2 in suspension or supported on glass and on paper. The influence of the chemical structure of different dyes as well as that of pH and of the presence of inorganic salts on the photocatalytic properties of TiO 2 has been discussed. The role of adsorption is suggested, indicating that the reaction occurs at the TiO 2 surface and not in the solution. S and N hetero-atoms are respectively mineralized into SO 4 2− , NO 3 − and NH 4 + , except azo-groups which mainly formed N 2 which represents an ideal case for a decontamination reaction. The fate of nitrogen strongly depends on its initial oxidation degree. High photocatalytic activities have been found for TiO 2 coated on glass by the sol–gel method. Its efficiency was intermediate between those of PC-500 and P-25 powders. The efficiency of PC-500 TiO 2 sample, fixed on paper by using a binder, is slightly less important than that of the powder. The presence of a silica-binder with an acidic pzc is suggested to be at the origin of the decrease in efficiency.

Catalytic CO oxidation by free Au2-: experiment and theory.

Abstract: Temperature-dependent rf-ion trap mass spectrometry and first-principles simulations reveal the detailed reaction mechanism of the catalytic gas-phase oxidation of CO by free Au2- ions in the presence of O2. A metastable intermediate with a mass of Au2CO3- was observed at low temperatures. Two alternative structures corresponding to digold carbonate or peroxyformate are predicted for this intermediate. Both structures are characterized by low activation barriers for the formation of CO2. These combined experimental and theoretical investigations provide a comprehensive understanding of the kinetics, energetics, and atomic arrangements along the reaction path, thus allowing a formulation of the catalytic cycle for the oxidation reaction.

Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation

Abstract: The photochemical and photocatalytic degradation of aqueous solutions of Solophenyl Green (SG) BLE 155%, an azo dye preparation very persistent in heavy colored textile waters, has been investigated by means of ultraviolet (UV) irradiation. The pure photochemical process demonstrated to be very efficient for low initial concentrations of the dyestuff. For higher concentrations the photocatalyitic degradation was carried on using commercial titanium dioxide, and mixtures of this semiconductor with different activated carbons (AC) suspended in the solution. The kinetics of photocatalytic dyestuff degradation were found to follow a first-order rate law. It was observed that the presence of the activated carbon enhanced the photoefficiency of the titanium dioxide catalyst. Differently activated carbon materials induced different increases in the apparent first-order rate constant of the process. The effect was quantified in terms of a synergy factor (R) already described in the literature. The kinetic behavior could be described in terms of a modified Langmuir–Hinshelwood model. The values of the adsorption equilibrium constants for the organic molecules, KC, and for the reaction rate constants, kC, were 0.0923 l mg−1 and 1.58 mg l−1 min−1 for the TiO2/UV process and 0.0928 l mg−1 and 2.64 mg l−1 min−1 for the TiO2+AC/UV system with highest synergy factor, respectively. The mechanism of degradation was discussed in terms of the titanium dioxide photosensitization by the activated carbon.

Comparative kinetic studies of the copolymerization of cyclohexene oxide and propylene oxide with carbon dioxide in the presence of chromium salen derivatives. In situ FTIR measurements of copolymer vs cyclic carbonate production.

Abstract: The catalysis of the reaction of carbon dioxide with epoxides (cyclohexene oxide or propylene oxide) using the (salen)Cr(III)Cl complex as catalyst, where H(2)salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediimine (1), to provide copolymer and cyclic carbonate has been investigated by in situ infrared spectroscopy. As previously demonstrated for the cyclohexene oxide/CO(2) reaction in the presence of complex 1, coupling of propylene oxide and carbon dioxide was found to occur by way of a pathway first-order in catalyst concentration. Unlike the cyclohexene oxide/carbon dioxide reaction catalyzed by complex 1, which affords completely alternating copolymer and only small quantities of trans-cyclic cyclohexyl carbonate, under similar conditions propylene oxide/carbon dioxide produces mostly cyclic propylene carbonate. Comparative kinetic measurements were performed as a function of reaction temperature to assess the activation barrier for production of cyclic carbonates and polycarbonates for the two different classes of epoxides, i.e., alicyclic (cyclohexene oxide) and aliphatic (propylene oxide). As anticipated in both instances the unimolecular pathway for cyclic carbonate formation has a larger energy of activation than the bimolecular enchainment pathway. That is, the energies of activation determined for cyclic propylene carbonate and poly(propylene carbonate) formation were 100.5 and 67.6 kJ.mol(-1), respectively, compared to the corresponding values for cyclic cyclohexyl carbonate and poly(cyclohexylene carbonate) production of 133 and 46.9 kJ.mol(-1). The small energy difference in the two concurrent reactions for the propylene oxide/CO(2) process (33 kJ.mol(-1)) accounts for the large quantity of cyclic carbonate produced at elevated temperatures in this instance.

Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

Abstract: . The available database concerning rate constants for gas-phase reactions of the hydroxyl (OH) radical with alkanes through early 2003 is presented over the entire temperature range for which measurements have been made (~180-2000 K). Measurements made using relative rate methods are re-evaluated using recent rate data for the reference compound (generally recommendations from this review). In general, whenever more than one study has been carried out over an overlapping temperature range, recommended rate constants or temperature-dependent rate expressions are presented. The recommended 298 K rate constants, temperature-dependent parameters, and temperature ranges over which these recommendations are applicable are listed in Table 1.

Heterogeneous Electron-Transfer Kinetics for Ruthenium and Ferrocene Redox Moieties through Alkanethiol Monolayers on Gold

Abstract: The standard heterogeneous electron-transfer rate constants between substrate gold electrodes and either ferrocene or pentaaminepyridine ruthenium redox couples attached to the electrode surface by various lengths of an alkanethiol bridge as a constituent of a mixed self-assembled monolayer were measured as a function of temperature. The ferrocene was either directly attached to the alkanethiol bridge or attached through an ester (CO2) linkage. For long bridge lengths (containing more than 11 methylene groups) the rate constants were measured using either chronoamperometry or cyclic voltammetry; for the shorter bridges, the indirect laser induced temperature jump technique was employed to measure the rate constants. Analysis of the distance (bridge length) dependence of the preexponential factors obtained from an Arrhenius analysis of the rate constant versus temperature data demonstrates a clear limiting behavior at a surprisingly small value of this preexponential factor (much lower than would be expect...

Photochemical fate of pharmaceuticals in the environment: cimetidine and ranitidine.

Abstract: The photochemical fates of the histamine H2-receptor antagonists cimetidine and ranitidine were studied. Each of the two environmentally relevant pharmaceuticals displayed high rates of reaction with both singlet oxygen (1O2, O2(1delta(g))) and hydroxyl radical (*OH), two transient oxidants formed in sunlit natural waters. For cimetidine, the bimolecular rate constant for reaction with *OH in water is 6.5 +/- 0.5 x 10(9) M(-1) s(-1). Over the pH range 4-10, cimetidine reacts with 1O2 with bimolecular rate constants ranging from 3.3 +/- 0.3 x 10(6) M(-1) s(-1) at low pH to 2.5 +/- 0.2 x 10(8) M(-1) s(-1) in alkaline solutions. The bimolecular rate constants for ranitidine reacting with 1O2 in water ranges from 1.6 +/- 0.2 x 10(7) M(-1) s(-1) at pH 6-6.4 +/- 0.2 x 10(7) M(-1) s(-1) at pH 10. Reaction of ranitidine hydrochloride with *OH proceeds with a rate constant of 1.5 +/- 0.2 x 10(10) M(-1) s(-1). Ranitidine was also degraded in direct photolysis experiments with a half-life of 35 min under noon summertime sunlight at 45 degrees latitude, while cimetidine was shown to be resistant to direct photolysis. The results of these experiments, combined with the expected steady-state near surface concentrations of 1O2 and *OH, indicate that photooxidation mediated by 1O2 is the likely degradation pathway for cimetidine in most natural waters, and photodegradation by direct photolysis is expected to be the major pathway for ranitidine, with some degradation caused by 1O2. These predictions were verified in studies using Mississippi River water. Model compounds were analyzed by laser flash photolysis experiments to assess which functionalities within ranitidine and cimetidine are most susceptible to singlet-oxygenation and direct photolysis. The heterocyclic moieties of the pharmaceuticals were clearly implicated as the sites of reaction with 1O2, as evidenced by the high relative rate constants of the furan and imidazole models. The nitroacetamidine portion of ranitidine has been shown to be the moiety active in direct photolysis.

Kinetics of the reaction of CO2 with aqueous potassium salt of taurine and glycine

Abstract: The kinetics of the reaction between CO2 and aqueous potassium salts of taurine and glycine was measured at 295 K in a stirred-cell reactor with a flat gas-liquid interface. For aqueous potassium taurate solutions, the temperature effect on the reaction kinetics was measured at 285 and 305 K. Unlike aqueous primary alkanolamines, the partial reaction order in amino acid salt changes from one at low salt concentration to approximately 1.5 at salt concentrations as high as 3,000 mol·m-3. At low salt concentrations, the measured apparent rate constant (kapp) for potassium glycinate is comparable to the values in literature. In the absence of reliable information in the literature on the kinetics and mechanism of the reaction, the applicability of the zwitterion and termolecular mechanism (proposed originally for alkanolamines) was explored. For the zwitterion mechanism, the forward second-order reaction rate constant (k2) of the CO2 reaction with amino acid salt seems to be much higher than for alkanolamines of similar basicity, indicating that the Bronsted plot for amino acid salts might differ from that of alkanolamines. The contribution of water to the deprotonation of zwitterion seems to be more significant than reported values for aqueous secondary alkanolamines.

Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

Abstract: The molar amounts of defective sites (Md) in several titanium(IV) oxide (TiO2) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO2 in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of Md with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that Md could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between Md and the specific surface area suggests that the surface area was not directly reflected on Md. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and Md revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO2 powders in this reaction system.

Carbon Tunneling from a Single Quantum State

Abstract: We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 × 10 −6 per second in nitrogen and 4 × 10 −5 per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.

A New Rate Law Describing Microbial Respiration

Abstract: The rate of microbial respiration can be described by a rate law that gives the respiration rate as the product of a rate constant, biomass concentration, and three terms: one describing the kinetics of the electron-donating reaction, one for the kinetics of the electron-accepting reaction, and a thermodynamic term accounting for the energy available in the microbe's environment. The rate law, derived on the basis of chemiosmotic theory and nonlinear thermodynamics, is unique in that it accounts for both forward and reverse fluxes through the electron transport chain. Our analysis demonstrates how a microbe's respiration rate depends on the thermodynamic driving force, i.e., the net difference between the energy available from the environment and energy conserved as ATP. The rate laws commonly applied in microbiology, such as the Monod equation, are specific simplifications of the general law presented. The new rate law is significant because it affords the possibility of extrapolating in a rigorous manner from laboratory experiment to a broad range of natural conditions, including microbial growth where only limited energy is available. The rate law also provides a new explanation of threshold phenomena, which may reflect a thermodynamic equilibrium where the energy released by electron transfer balances that conserved by ADP phosphorylation.

Catalysis of electron transfer during activation of O2 by the flavoprotein glucose oxidase

Abstract: Two prototropic forms of glucose oxidase undergo aerobic oxidation reactions that convert FADH− to FAD and form H2O2 as a product. Limiting rate constants of kcat/KM(O2) = (5.7 ± 1.8) × 102 M−1⋅s−1 and kcat/KM(O2) = (1.5 ± 0.3) × 106 M−1⋅s−1 are observed at high and low pH, respectively. Reactions exhibit oxygen-18 kinetic isotope effects but no solvent kinetic isotope effects, consistent with mechanisms of rate-limiting electron transfer from flavin to O2. Site-directed mutagenesis studies reveal that the pH dependence of the rates is caused by protonation of a highly conserved histidine in the active site. Temperature studies (283–323 K) indicate that protonation of His-516 results in a reduction of the activation energy barrier by 6.0 kcal⋅mol−1 (0.26 eV). Within the context of Marcus theory, catalysis of electron transfer is attributed to a 19-kcal⋅mol−1 (0.82 eV) decrease in the reorganization energy and a much smaller 2.2-kcal⋅mol−1 (0.095 eV) enhancement of the reaction driving force. An explanation is advanced that is based on changes in outer-sphere reorganization as a function of pH. The active site is optimized at low pH, but not at high pH or in the H516A mutant where rates resemble the uncatalyzed reaction in solution.

Calculations of the F?+?HD reaction on three potential energy surfaces

Abstract: In this paper, the three-dimensional time-dependent quantum wave packet calculation has been employed to study the non-adiabatic reaction dynamics of F + HD on three, electronically non-adiabatic potential energy surfaces fitted by Alexander, Stark and Werner (J. Chem. Phys., 2000, 113, 11 084; named ASW). The integral cross-sections and branching ratio of the reaction as a function of collision energy are calculated. The integral cross-sections are compared with the experimental measurements and other theoretical results. For the reaction of F in its excited state with HD, the calculated function of integral cross sections as collision energy shows that a steplike feature near 0.9 kcal mol−1 probably arises from the collinear FHD geometry.

Measurements, theory, and modeling of OH formation in ethyl + O2 and propyl + O2 reactions

Abstract: The time-resolved formation of OH from ethyl + O2 and propyl + O2 reactions has been measured by OH laser-induced fluorescence in pulsed-photolytic Cl-initiated oxidation of ethane and propane between 296 and 700 K. The propane oxidation produces more OH at each temperature than does ethane oxidation. Above 600 K, the peak amplitudes of the OH signals from both reactions increase sharply with increasing temperature. Solutions to the time-dependent master equation for the C2H5 + O2, i-C3H7 + O2, and n-C3H7 + O2 reactions, employing previously published ab initio characterizations of the stationary points of the systems, have been used to produce temperature-dependent parameterizations that predict the rate constants for formation of all of the products (R + O2, RO2, QOOH, OH + aldehydes, OH + O-heterocycles, HO2 + alkene). These parameterizations are utilized in rate equation models to compare to experimental results for HO2 and OH formation in Cl-initiated ethane and propane oxidation. The models accurate...

Oxidation of C-H bonds by [(bpy)2(py)RuIVO]2+ occurs by hydrogen atom abstraction.

Abstract: Anaerobic oxidations of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)2(py)RuIVO]2+ in acetonitrile solution give mixtures of products including oxygenated and non-oxygenated compounds. The products include those formed by organic radical dimerization, such as 9,9‘-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidation of DHA vs DHA-d4 gives kH/kD ≥ 35 ± 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidations of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate constants for H-atom abstraction from these substrates correlate well with the strength of the C−H bond that is cleaved. Rate constants for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate constant for H-atom tr...

Hydrogen Peroxide Photolysis in Acidic Aqueous Solutions Containing Chloride Ions. I. Chemical Mechanism

Abstract: Equilibrium constants and rate constants involving Cl•(aq), Cl2•-(aq), H2O, and H2O2(aq) are measured at 297 ± 2 K by analyzing the kinetics of formation and decay of Cl2•-. The new results are in good agreement with previous studies, when available. The reaction between solvated chlorine atoms and hydrogen peroxide is reported for the first time: Cl•(aq) + H2O2(aq) → H+(aq) + Cl-(aq) + HO2•(aq), k10 = (2.0 ± 0.3) × 109 M-1 s-1 (±σ). The new results are used along with evaluations of literature data to arrive at recommendations for several key rate constants and equilibrium constants. Of particular interest, the recommended equilibrium constant for the reaction Cl•(aq) + Cl-(aq) ↔ Cl2•-(aq) is K5 = (1.4 ± 0.2) × 105 M-1(±σ).

Diffusion-Controlled Electron-Transfer Reactions in Ionic Liquids

Abstract: The reactions of pyridinyl and alkylpyridinyl radicals with duroquinone involve an electron transfer to produce the durosemiquinone radical in a diffusion-controlled process. The rate constants for such reactions in several ionic liquids and molecular organic solvents are determined by pulse radiolysis. The ionic liquids used are N-butylpyridinium tetrafluoroborate (BuPyBF4) and the following tetraalkylammonium (R4N+) salts of bis(trifluoromethylsulfonyl)imide (-NTf2): methyltributylammonium (MeBu3NNTf2), hexyltributylammonium (HxBu3NNTf2), methyltrioctylammonium (MeOc3NNTf2), and methylbutylpyrrolidinium (MeBuPyrNTf2). The molecular solvents used are triethanolamine (TEOA) and cyclohexanol (c-HxOH). The experimental rate constants in the molecular solvents are only slightly higher than the diffusion-controlled rate constants estimated from the viscosity. On the other hand, the experimental values in the ionic liquids are about an order of magnitude higher than the values estimated from the measured visc...

The Influence of Anion on the Coarsening Kinetics of ZnO Nanoparticles

Abstract: In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We report on the synthesis of ZnO nanoparticles from Zn(CH3CO2)2, ZnBr2, and Zn(ClO4)2 in 2-propanol. For synthesis from Zn(CH3CO2)2 and ZnBr2, nucleation and growth are fast and are followed by diffusion-limited coarsening. For synthesis from Zn(ClO4)2, diffusion-limited coarsening is observed at shorter times whereas at longer times the particle size increases more rapidly. The rate constant for coarsening at constant temperature increases in the order Br- < CH3CO2- < ClO4- indicating that the rate is dependent on anion adsorption. The temperature dependence of the rate constant for coarsening is due to the temperature dependence of the solvent viscosity and the temperature dependence of the bulk solubility of ZnO.