Rate-determining Reaction Research Articles

Isolation, X-ray structure and properties of an unusual pentacarbonyl(2,2′-pyridyl-quinoxaline) tungsten complex

The first example of a monodentate complexation of 2-(2′-pyridyl)quinoxaline ( pq) to a metal centre through N 4 is reported. Photochemical exchange of the THF ligand in W(CO) 5THF by pq yields W(CO) 5(N 4-pq) ( 1), where the potentially bidentate pq ligand coordinates in an unusual monodentate fashion. Complex 1 is isolated as orange crystals and fully characterized on the basis of NMR, IR, UV–Vis and emission spectroscopy. The structure of 1 was determined by X-ray analysis. W(CO) 5(N 4-pq) ( 1) crystallizes in space group P2 1/ n , monoclinic crystal system with α = 7.0237(5) Å, b = 10.4618(8) Å, c = 23.7768(18) Å, Z = 4 and V = 1731.9(2) Å 3. Complex 1 exhibits intramolecular CH⋯N and intermolecular CH⋯O hydrogen bonds between the CH groups and nitrogen atoms of quinoxaline and CH groups and oxygen atoms of carbonyls, respectively, resulting in a supramolecular architecture in solid state. The preference to N 4 as coordination site is discussed in terms of electronic interactions. Solutions of 1 emits dually at 77 K while they are moderately instable at room temperature, as 1 undergoes chelation via a first-order kinetic process to form W(CO) 4pq ( 2). The determined reaction rate of 1 in toluene is 2.3 × 10 −5 s −1 (at 298 K) and is compared with literature values for other W(CO) 5L (L:diimine) complexes.

Modeling chemical growth processes in Titan’s atmosphere: 1. Theoretical rates for reactions between benzene and the ethynyl (C 2H) and cyano (CN) radicals at low temperature and pressure

Density functional theory calculations at the B3LYP/6-31+G ∗∗ level were employed to characterize the critical points for adducts, isomers, products, and intervening transition states for the reactions between benzene and the ethynyl (C 2H) or cyano (CN) radicals. Both addition reactions were found to have no barriers in their entrance channels, making them efficient at the low temperature and pressure conditions that prevail in the haze-forming region of Titan’s atmosphere as well as in the dense interstellar medium (ISM). The dominant products are ethynylbenzene (C 6H 5C 2H) and cyanobenzene (C 6H 5CN). Hydrogen abstraction reactions were also characterized but found to be non-competitive. Trajectory calculations based on potentials fit to about 600 points calculated at the ROMP2/6-31+G ∗∗ level for each interaction surface were used to determine reaction rates. The rates incorporated any necessary corrections for back reactions as ascertained from a multiwell treatment used to determine outcome distributions over the range of temperatures and pressures pertinent to Titan and the ISM and are in good agreement with the limited available experimental data.

Rhodium(I) hydrogenation in water: Kinetic studies and the detection of an intermediate using 13C{ 1H} PHIP NMR spectroscopy

The mechanism for hydrogenation of dimethylmaleate in water using cationic rhodium complexes with water-soluble bi-dentate phosphines has been investigated using kinetics and a novel method for the indirect detection of intermediates in catalytic hydrogenation reactions, whereby a late intermediate was detected. A mechanism is proposed involving fast, irreversible substrate binding followed by a rate-determining reaction with dihydrogen.

MATHEMATICAL MODELING OF LIVING CELL METABOLISM USING THE METHOD OF STEADY-STATE STOICHIOMETRIC FLUX BALANCE

This approach uses a set of algebraic linear equations for reaction rates (the method of steady-state stoichiometric flux balance) to model the purposeful metabolism of the living self-reproducing biochemical system (i.e. cell), which persists in steady-state growth. Linear programming (SIMPLEX method) is used to derive the solution for the model equations set (determining reaction rates which provide flux balance at given conditions). Here, we demonstrate the approach through the mathematical modeling of steady-state metabolism in Saccharomyces cerevisiae mitochondria.

Reaction enhancement of point sources due to vortex stirring

We investigate a class of reactive advection-diffusion problems motivated by an ecological mixing process. We use analytical and numerical methods to determine reaction rates between two initially distinct scalar point masses that are separated from one another by a third (nonreactive) scalar. The scalars are stirred by a single two-dimensional vortex in a variety of geometrical configurations. We show that the aggregate second-order reaction rate in the low-concentration limit is enhanced by the instantaneous stirring processes, relative to the rate predicted by an equivalent eddy diffusivity. The peak reaction rate grows as P(1/3), and the time to reach the peak decreases as P(-2/3), where P is the Péclet number. The results of this study have important implications not only for ecological modeling, but for the general understanding of turbulent reactive flows.

High Rate Reversibility Anode Materials of Lithium Batteries from Vapor-Grown Carbon Nanofibers

We demonstrate the high-rate capability of lithium ion insertion-deinsertion reactions in carbon nanofibers (CNFs). The morphology of CNFs with structural and surface defects, due to the mixed features of disordered and graphitic carbon, plays an important role in both the enhancement of the lithium ion storage and the rate-determining reactions during the topotactic process. The reversible specific capacity of the CNFs at a 0.1 C rate was 461 mA x h/g. The most promising property, which is expected to overcome the hurdles of lithium batteries for high-power applications, is that they deliver considerably high specific capacity even at a very high charge-discharge current, i.e., at a cycling rate of 10 C the reversible capacity is around 170 mA x h/g with a 95% Coulombic efficiency.

Determination of Adjusted Neutron Spectra in Different MUSE Configurations by Unfolding Techniques

Abstract Investigations of spectrum adjustment by unfolding techniques for different locations of critical and driven subscritical MUSE4 configurations are presented. The current analysis is based on the recent accomplishment of specific foil activation measurements and on the use of the unfolding code STAY'SL in conjunction with IRDF-90 and IRDF-2002 dosimetry libraries. Specific developments have been conducted to consider the uncertainties associated with the determination of the experimental absolute reaction rates and the initial calculated spectrum in the most realistic manner. In this context, the iterative adjustment process for the neutron spectrum is established on the basis of variance/covariance matrices defined for the three types of input parameters, i.e., the experimental reaction rates, the initial spectrum, and the dosimetry nuclear data files. The results highlight the spectral variations between different locations of the MUSE4 cores, in particular the characteristic behavior of a subcritical configuration driven by an external T(d,n)He4 (14 MeV neutrons) source.

Comment on: “The dissolution rates of gibbsite in the presence of chloride, nitrate, silica, sulfate and citrate in open and closed systems at 20 °C” by M. Dietzel and G. Böhme

The recent paper by Dietzel and B hme (2005) reports the rate of dissolution of gibbsite under far-from equilibrium conditions and low pH at 20 C, indicating that the rates are about the same in chloride and nitrate solutions, faster in silica-bearing solutions, and fastest in sulfate- and citrate-bearing solutions. This information is highly relevant to environmental geochemistry and soil science. However, the authors failed to mention our earlier study of the dissolution of gibbsite in NaClH2SO4 and NaClHCl solutions (0.1 molal ionic strength, 0.01 molal H+) at 5 C (Ridley et al., 1997), in which we demonstrated that the release rate of Al3+ from gibbsite under the specified conditions, via the reaction (1) was approximately 10 times faster in the sulfate medium than in the chloride medium, and appeared to be controlled by a zero-order rate-determining reaction in batch experiments at far-from-equilibrium conditions.

Aminolysis of 2,4‐Dinitrophenyl X‐Substituted Benzoates and Y‐Substituted Phenyl Benzoates in MeCN: Effect of the Reaction Medium on Rate and Mechanism

Second-order rate constants (k(N)) have been determined spectrophotometrically for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1 a-f) and Y-substituted phenyl benzoates (2 a-h) with a series of alicyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C. The k(N) values are only slightly larger in MeCN than in H2O, although the amines studied are approximately 8 pK(a) units more basic in the aprotic solvent than in H2O. The Yukawa-Tsuno plot for the aminolysis of 1 a-f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate-determining step (RDS) or reaction mechanism. The Hammett correlation with sigma- constants also exhibits good linearity with a large slope (rho(Y) = 3.54) for the reactions of 2 a-h with piperidine, implying that the leaving-group departure occurs at the rate-determining step. Aminolysis of 2,4-dinitrophenyl benzoate (1 c) results in a linear Brønsted-type plot with a beta(nuc) value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a-f in MeCN. The medium change from H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T+/-) in aprotic solvent.

Computational design of a human butyrylcholinesterase mutant for accelerating cocaine hydrolysis based on the transition-state simulation.

Breaking down cocaine: The transition-state simulation predicts a significantly more stable transition-state structure (see model; C: green, O: red, H: gray, N: blue, cocaine atoms: yellow) for the rate-determining first reaction step (TS1) of the (−)-cocaine hydrolysis catalyzed by a mutant of human butyrylcholinesterase (BChE).

Improvement of the spatial resolution of prototypes using infrared laser stereolithography on thermosensitive resins

In this paper, the use of infrared laser radiation to achieve localized curing in thermosensitive materials, is presented. In stereolithography, the objective is to cure a localized region in a material by precisely confining the laser energy to the area that is to be cured. In the experiment, a CO 2 laser beam at 10.6 μm (infrared radiation) was focused onto a sample with a composition of epoxy resin, diethylene triamine (curing agent) and silica powder (filler). Using a differential scanning calorimeter (DSC) we were able to determine reaction rates as a function of temperature as well as the enthalpy involved in the phase transition and activation energy of the curing process. This paper presents a numerical model for the two-dimensional curing problem with infrared radiation laser by using a Finite Element Technique in Ansys program. The solution of the heat equation simulating this process was in general agreement with our previous observations of the stereolithographic results. Results and discussion about prototypes of different geometries and sizes were performed and presented as well as discussion about width and depth of the layer cured in the prototype construction.

Sensitivity ofr‐Process Nucleosynthesis to Light‐Element Nuclear Reactions

We study the efficiency and sensitivity of r-process nucleosynthesis to 18 light-element nuclear reaction rates. We adopt empirical power-law relations to parameterize the reaction sensitivities. We utilize two different hydrodynamic models for the neutrino-driven winds in order to study the dependence of our result on supernova wind models. We also utilize an exponential model to approximate a wide variety of other plausible conditions for the r-process. We identify several specific nuclear reactions among light neutron-rich nuclei that play a critical role in determining the final r-process nucleosynthesis yields. As an illustration, we examine ``semi-waiting'' points among the carbon isotopes. We show that not only neutron capture and $\beta$-decay, but also $(\alpha, \mathrm{n})$ reactions are important in determining waiting points along the r-process path. Our numerical results from this sensitivity analysis serve foremost to clarify which light nuclear reactions are most influential in determining the final r-process abundances. We also quantify the effects of present nuclear uncertainties on the final r-process abundances. This study thus emphasizes and motivates which future determinations of nuclear reaction rates will most strongly impact our understanding of r-process nucleosynthesis.

Mineral CO2Sequestration by Steel Slag Carbonation

Mineral CO2 sequestration, i.e., carbonation of alkaline silicate Ca/Mg minerals, analogous to natural weathering processes, is a possible technology for the reduction of carbon dioxide emissions to the atmosphere. In this paper, alkaline Ca-rich industrial residues are presented as a possible feedstock for mineral CO2 sequestration. These materials are cheap, available near large point sources of CO2, and tend to react relatively rapidly with CO2 due to their chemical instability. Ground steel slag was carbonated in aqueous suspensions to study its reaction mechanisms. Process variables, such as particle size, temperature, carbon dioxide pressure, and reaction time, were systematically varied, and their influence on the carbonation rate was investigated. The maximum carbonation degree reached was 74% of the Ca content in 30 min at 19 bar CO2 pressure, 100 degrees C, and a particle size of <38 microm. The two most important factors determining the reaction rate are particle size (<2 mm to <38 microm) and reaction temperature (25-225 degrees C). The carbonation reaction was found to occur in two steps: (1) leaching of calcium from the steel slag particles into the solution; (2) precipitation of calcite on the surface of these particles. The first step and, more in particular, the diffusion of calcium through the solid matrix toward the surface appeared to be the rate-determining reaction step. The Ca diffusion was found to be hindered by the formation of a CaCO3-coating and a Ca-depleted silicate zone during the carbonation process. Research on further enhancement of the reaction rate, which would contribute to the development of a cost-effective CO2-sequestration process, should focus particularly on this mechanism.

Mechanism of Cis/Trans Equilibration of Alkenes via Iodine Catalysis

Iodine is commonly used to speed the equilibration of Wittig cis/trans alkene products. This study uses computational chemistry to study the catalyzed isomerization mechanism in detail for seven different examples of 1,2-disubstituted alkenes. We find that the iodo intermediates of the conventional three-step reaction path are weakly stable, bound by less than 7 kJ mol(-1) in five cases and nonexistent in the other two. These variations in relative stability appear to be closely related to the degree of conjugation interruption in the alkene upon attachment of iodine. The rate-determining reaction barrier always occurs in the middle step, the internal rotation of the iodo intermediate, and the variations in the barrier heights are dictated by varying levels of steric hindrance in the seven cases. Regiospecificity of I-atom addition and noticeable hyperconjugative effects are discussed. Comparisons between various theoretical approximations are performed to demonstrate the great difficulty in obtaining accurate results for iodine-atom bond-forming and bond-breaking energies.

Producing chromium carbide using reduction of chromium oxide with methane

AbstractIn the present work, reduction of chromium oxide using methane is investigated. The thermogravimetric method was used to obtain kinetic parameters of the reaction in the temperature range of 870‐975°C under atmospheric pressure. The experimental data was analyzed using the grain model. The reaction product, chromium carbide, was observed to be produced at a temperature of about 140°C lower than by other conventional methods. Carbon deposition, which is highly undesirable in the synthesis of chromium carbide, was negligible in the present experiments. The experiments were conducted in two modes, either under the conditions of reaction control or under the conditions of diffusion control for determining reaction rate constants and diffusion rates to study pore diffusion, respectively. The pore diffusion studies were carried out with and without considering the bulk flow effect. The gas phase diffusion coefficient through the product layer around the grains was calculated with an approximate method. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Indirect techniques in nuclear astrophysics: The ANC method

Asymptotic normalization coefficients (ANCs) have proven to be useful for determining reaction rates of interest in nuclear astrophysics. ANCs, which provide the normalization of the tail of the overlap function, determine S factors for direct capture reactions at astrophysical energies. During the past eight years, many ANCs have been measured by peripheral transfer reactions. Following an introduction to ANCs, recent experiments are briefly described. The astrophysical implications of some of these measurements are discussed and a future outlook for ANC measurements with radioactive beams is given.

Effect of Elevated Temperature and Reduced Relative Humidity on ORR Kinetics for PEM Fuel Cells

As a measure of catalytic activity, , the current density at 0.3 V overpotential, was chosen to evaluate the oxygen reduction reaction (ORR) at elevated temperatures and various relative humidities (RH) for polymer exchange membrane (PEM) fuel cells. The purely kinetic reaction order of the ORR with respect to oxygen partial pressure is less than 1.0 and changes with the RH. The activation energy is at 100% RH and at 50% RH. The active electrochemical surface area of platinum changes little with RH. RH has a strong effect on the catalytic activity under dry conditions (0-60% RH), but under wet conditions its influence is unclear. The Tafel slope obtained in the current density range changes significantly with RH: wet conditions produce low Tafel slopes , dry conditions produce high Tafel slopes . Dependence of the RH on the oxygen reduction reaction (ORR) may be explained by the changes of the rate-determining reaction, proton activity, and adsorbed on the platinum surface. The ORR kinetic parameters obtained here are instructive for high-temperature fuel cell data analysis and performance improvement.

Hydrolytic and Reductive Transformations of Nifuroxazide

The elimination of nitro groups in the form of nitrite anions in the course of reduction and alkaline hydrolysis of the antitumor drug nifuroxazide has been studied using polarographic and photometric techniques. The experimentally determined reaction rate constants are used to calculate the thermodynamic parameters (ΔG≠, Δ H≠, ΔS≠). It is suggested that the therapeutic action of nifuroxazide is related not only to the formation of nitro radical anions in vivo, but also to the production of nitric oxide as a result of the biotransformation of this drug.

Progress and Challenges in Determining Reaction Rates for the s and p Processes

Advances in abundance measurements in primitive meteorites and stars as well as progress in astrophysical models are driving the need for more as well as more accurate (n,γ) reaction-rate measurements. I will describe some recent successes as well as some remaining challenges in measuring the needed rates. In addition, I will briefly discuss how (n,α) measurements are helping to constrain (γ,α) rates for the p process, but at the same time have turned up some surprises.

Hydrolysis Process of the Second Generation Platinum-Based Anticancer Drug cis-Amminedichlorocyclohexylamineplatinum(II)

The hydrolysis process of the anticancer drug cis-amminedichlorocyclohexylamineplatinum(II) (JM118 or cis-[PtCl2(NH3)cyclohexylamine]) and the influence of solvent models therein have been studied using hybrid density functional theory (B3LYP). The aquation reactions leading to the activated drug forms a key step for the reaction with the target DNA. In this study, the stepwise hydrolysis, cis-[PtCl2(NH3)cyclohexylamine] + 2 H2O --> cis-[Pt(NH3)cyclohexylamine(OH2)2]2+ + 2 Cl- was explored, using three different models. Implicit solvent effects were incorporated through polarized continuum models. The stationary points on the potential energy surfaces for the first and second hydrolysis steps, proceeding via a general S(N)2 pathway, were fully optimized and characterized. It was found that the explicit solvent effects originating from the inclusion of extra water molecules into the system are significantly stronger than those arising from the bulk aqueous medium, especially for the second aquation step, emphasizing the use of appropriate models for these types of problems. In comparison with previous work on the parent compound cisplatin, a slower rate of hydrolysis is determined for the first (rate determining) reaction. The results furthermore imply that the doubly aquated form of JM118 will be the main DNA binding form of the drug. The results provide detailed energy profiles for the mechanism of hydrolysis of JM118, which may assist in understanding the reaction mechanism of the drug with the DNA target and in the design of novel Pt-containing anticancer drugs.