Reactivity of the Hydrated Electron.

Chemical systems with standard reaction enthalpies (ΔH0r) and an activation energies (Ea) of a few tens of kcal/mol are generally well described by the unimolecular reaction rate theory. The goal of this work is to determine the limits of this model by studying the kinetics of various chemical reactions with a small ΔH0r and Ea respectively. The theoretical model for the evaluation of the experimental data has been developped by Troe to determine reaction rate constants in the different pressure regimes. Based on a relaxation method an experimental technique has been specially developped to investigate chemical reactions with small ΔH0r and for Ea. A fast temperature jump, induced by a pulsed CO2 laser, disturbes instantaneously the chemical system, and the subsequent relaxation of the system is observed spectrospically. First, this experimental technique has been proved with a chemical system of known chemical kinetics, i.e. the gas phase dimerization of nitrogen dioxide to dinitrogen tetroxid. The rate constants of this system have been determined by other methods, such as flash photolysis, laser photolysis or in shock wave tubes. T-jump experiments have been carried out in the presence of helium, from 0.3 to 200 bar total pressure, corresponding to the so called reaction fall-off. This wide range of pressures, extending over 4 decades, allows the extrapolation of the reaction rate constant in the low and high pressure limits with good confidence. Additionally, mesurements in the temperature range from 255 to 273 K lead to the following reaction rate constant : Experimental limits of the technique rise from non-thermalisation effects prior to the reaction, when direct V-V energy transfer occurs between the IR-sensitizer and dinitrogen tetroxid. This transfer is particularly observed with a small sensitizer dinitrogen tetroxid ratio. The next system studied is the N2O3 NO2 + NO reaction, which has a never smaller ΔH0r. In this case, non-thermalisation effects are not observed, leading to the conclusion of direct V-V energy transfer between the sensitizer and N2O4, and not NO2. Experiments between 225 and 260 K, and 0.5 to 200 bar of total argon pressure, allow the rate constant to be determined as: Like the N2O4/NO2 system, the N2O3 NO2 + NO reaction is also fairly well described by the kinetic theory of unimolecular reactions. Differences between the calculation according to Troe's model and experimental values are indeed below 30%. Finally, reactions with much smaller ΔH0r and Ea, like the cis/trans isomerisation of acrolein and 1,3-butadiene, have been studied. Experimental results are in good accordance with the theory for the low pressure limit reaction rate constant. However, there is a large discrepancy for the high pressure limit reaction rate constant, where the theory predicts rate constant about 100 times higher than the experimentals results. Further more, the reaction rate constant also depends on the concentration of the isomer present in the mixture. This surprising result can be explained by the formation of dimers or even polymers which decrease the reaction rates. These results are however in accordance with those obtained by Bauer and al. for the syn anti isomerisation of methylnitrite and aziridine, as well as those obtained by Quack in the HF dimer. These authors have also observed discrespancis between the theoretical prediction and experimental results in the high pressure regime.

High-level ab initio and density functional theory study on reaction path and rate constant of the hydrogen abstraction reaction SiH2Cl2+H→SiHCl2+H2

Reaction pathways and rate constants of gas-phase uranium and uranium oxide ions with O2 and H2O have been investigated using a quadrupole ion trap mass spectrometer (QIT-MS). A new reaction pathway is identified for the reaction between U2+ and H2O, which leads to the formation of UO+ via the intermediate UOH2+. Reaction rate constants are determined for several reactions by measuring the reaction rate at different partial pressures of the reagent gas and are found to be in reasonable agreement with the literature. These rate constants include the first known measurement for the reaction of U2+ with H2O (∼0.4 kADO). New limits on thermochemical values are also provided for certain species. These include ΔHf (UO2+) ≤ 1742 kJ mol-1 and 1614 ≤ ΔHf (UOH2+) ≤ 1818 kJ mol-1 and are based on the assumption that only exothermic or thermoneutral reactions are possible under the conditions used. This assumption is supported by simulations of the root-mean-square (RMS) ion kinetic energy of stored uranium ions in the QIT. Only a slight increase in the RMS ion kinetic energies, from 0.1 to 0.2 eV, is predicted over the range of trapping conditions studied (0.05 ≤ qz ≤ 0.75) corresponding to a theoretical reaction temperature of ∼384 K. The simulations also compare helium and neon as bath gases and show that the RMS kinetic energies are found to be very similar at long trapping times (>20 ms), although neon establishes steady state conditions in approximately half the time.

Magnetite is precipitated by dissimilatory iron-reducing bacteria or forms through corrosion of zero-valent iron (ZVI) in permeable reactive barriers. Reduction of carbon tetrachloride (CCl4) by synthetic magnetite was examined in batch reactors to evaluate the pH dependence of the reaction rates and product distributions. This work presents the first data where magnetite promotes CCl4 dechlorination independent of added sorbed Fe(II) or coexisting minerals that maintained Fe2+ above the magnetite solubility limit. In this system, reaction rate constants increase with increasing pH values between 6 and 10. The pH dependence is explained by acid-base equilibrium between two surface sites, where the more deprotonated exhibits greater dechlorination reactivity. The distribution of reaction products was also found to depend on pH. The primary reaction product is carbon monoxide (CO) followed by chloroform (CHCl3). CHCl3 production is at a minimum at pH 6 but increases through pH 10. Formation rate constants for both products increase with increasing pH, but the values for CHCl3 increase at a much faster rate. A hypothesis is proposed that relates the CHCl3 rate enhancement to the reduced capacity of deprotonated surface sites to stabilize the trichlorocarbanion transition-state complex. These data form a basis to assess the natural attenuation capacity of magnetite formed under iron reducing conditions. Application of this information to permeable barrier technology suggests that, in the long term, oxidation of ZVI to magnetite may be accompanied by a shift toward more benign reaction products as well as a 2 order of magnitude decrease in reaction rate constants.

The influence of amine buffers on carbon tetrachloride (CCl4) reductive dechlorination by the iron oxide magnetite (FeIIFeIII2O4) was examined in batch reactors. A baseline was provided by monitoring the reaction in a magnetite suspension containing NaCl as a background electrolyte at pH 8.9. The baseline reaction rate constant was measured at 7.1 x 10(-5)+/-6.3 x 10(-6) L m(-2) h(-1). Carbon monoxide (CO) was the dominant reaction product at 82% followed by chloroform (CHCl3) at 5.2%. In the presence of 0.01 M tris-(deuteroxymethyl)aminomethane (TRISd), the reaction rate constant nearly tripled to 2.1 x 10(-4)+/-6.5 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 11% and did not cause any statistically significant changes to the CO yield. Reactions in the presence of triethylammonium (TEAd) (0.01 M) increased the rate constant by 17% to 8.6 x 10(-5)+/-8.1 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 8.8% while leaving the CO yield unchanged. The same concentration of N,N,N',N'-tetraethylethylenediamine (TEEN) increased the reaction rate constant by 18% to 8.7 x 10(-5)+/-4.8 x 10(-6) L m(-2) h(-1) but enhanced the CHCl3 yield to 34% at the expense of the CO yield that dropped to 35%. Previous work has shown that CHCl3 can be generated either through hydrogen abstraction by a trichloromethyl radical (radical CCl3), or through proton abstraction by the trichlorocarbanion (-:CCl3). These two possible hydrogenolysis pathways were examined in the presence of deuterated buffers. Deuterium tracking experiments revealed that proton abstraction by the trichlorocarbanion was the dominant hydrogenolysis mechanism in the magnetite-buffered TRISd and TEAd systems. The only buffer that had minimal influence on both the reaction rate and product distribution was TEAd. These results indicate that buffers should be prescreened and demonstrated to have minimal impact on reaction rates and product distributions prior to use. Alternatively, it may be preferable, to utilize the buffer capacity of the solids to avoid organic buffer interactions entirely.

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH3CH2(SINGLE BOND)O(SINGLE BOND)CH2CH2C(O)O(SINGLE BOND)CH2CH3). EEP reacts with OH with a bimolecular rate constant of (22.9±7.4)×10−12 cm3 molecule−1s−1 at 297±3 K and 1 atmosphere total pressure. In order to more clearly define EEP's atmospheric reaction mechanism, an investigation into the OH+EEP reaction products was also conducted. The OH+EEP reaction products and yields observed were: ethyl glyoxate (EG, 25±1% HC((DOUBLE BOND)O)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl (2-formyl) acetate (EFA, 4.86±0.2%, HC((DOUBLE BOND)O)(SINGLE BOND)CH2(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl (3-formyloxy) propionate (EFP, 30±1%, HC((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH2(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl formate (EF, 37±1%, HC((DOUBLE BOND)O)O(SINGLE BOND)CH2CH3), and acetaldehyde (4.9±0.2%, HC((DOUBLE BOND)O)CH3). Neither the EEP's OH rate constant nor the OH/EEP reaction products have been previously reported. The products' formation pathways are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry. © 1997 John Wiley & Sons, Inc.

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH3CH2(SINGLE BOND)O(SINGLE BOND)CH2CH2C(O)O(SINGLE BOND)CH2CH3). EEP reacts with OH with a bimolecular rate constant of (22.9±7.4)×10−12 cm3 molecule−1s−1 at 297±3 K and 1 atmosphere total pressure. In order to more clearly define EEP's atmospheric reaction mechanism, an investigation into the OH+EEP reaction products was also conducted. The OH+EEP reaction products and yields observed were: ethyl glyoxate (EG, 25±1% HC((DOUBLE BOND)O)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl (2-formyl) acetate (EFA, 4.86±0.2%, HC((DOUBLE BOND)O)(SINGLE BOND)CH2(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl (3-formyloxy) propionate (EFP, 30±1%, HC((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH2(SINGLE BOND)C((DOUBLE BOND)O)(SINGLE BOND)O(SINGLE BOND)CH2CH3), ethyl formate (EF, 37±1%, HC((DOUBLE BOND)O)O(SINGLE BOND)CH2CH3), and acetaldehyde (4.9±0.2%, HC((DOUBLE BOND)O)CH3). Neither the EEP's OH rate constant nor the OH/EEP reaction products have been previously reported. The products' formation pathways are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry. © 1997 John Wiley & Sons, Inc.

Effect of β-cyclodextrin, surfactants and solvent on the reactions of the recently synthesized Schiff base and its Cu(II) complex with cyanide ion

Limona ketone, as a significant intermediate in the oxidation of limonene, plays a crucial role in environmental and human health impacts. In this article, we present a study on the reaction kinetics and mechanism of ozonolysis of limona ketone. The rate constant for the reaction between limona ketone and ozone was measured under standard conditions of 298 K and atmospheric pressure by the absolute rate method and was determined to be (1.2 ± 0.1) × 10-16 cm3 molecule-1 s-1. The reaction products were probed online with a vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometer and utilized to elucidate the reaction mechanism with the aid of kinetics. The atmospheric lifetime of limona ketone was discussed with the present reaction rate constant of limona ketone with O3, combined with the literature-reported rate constants for OH and NO3 reaction. The present results show that in urban areas with elevated ozone levels, ozonolysis is the primary pathway for the degradation of limona ketone, and its low-volatility organic products could significantly contribute to the formation of secondary organic aerosols.

We report real wave packet (WP) calculations of reaction probabilities, cross sections, rate constants, and product distributions of the reaction N(4S)+O2(X 3∑g−)→NO(X 2∏)+O(3P). We propagate initial WPs corresponding to several O2 levels, and employ reactant coordinates and a flux method for calculating initial-state-resolved observables, or product coordinates and an asymptotic analysis for calculating state-to-state quantities. Exact or J-shifting calculations are carried out at total angular momentum J=0 or J>0, respectively. We employ the recent X 2A′ S3 potential energy surface (PES) by Sayós et al. and the earlier a 4A′ PES by Duff et al. In comparing S3 results with the WP ones of a previous X 2A′ S2 PES, we find lower S3 energy thresholds and larger S3 probabilities, despite the higher S3 barrier. This finding is due to the different features of the doublet PESs in the reactant and product channels, at the transition state, and in the NO2 equilibrium region. We analyze the effects of the O2 initial level and show that tunneling through the S3 barrier enhance the room-temperature rate constant by ∼3.7 times with respect to the previous S2 WP rate. The agreement with the room-temperature experimental result is thus notably improved. The NO vibrational distribution is inverted and the rotational ones are strongly oscillating. We explain these nonstatistical results showing that the reaction partners approach each other with a large impact parameter. The WP vibrational distribution is however different from that observed, which is oscillating. WP calculations show that the new S3 PES describes accurately several features of the X 2A′ state, although a lowering of its barrier height by ∼0.56 kcal/mol should bring calculated and observed rate constants in full agreement.

Objectives : Determination of reaction order (n) and rate constants (k) of the CaCO<sub>3</sub> scale formation reaction that was accelerated by the HVI (high voltage impulse) induction.Methods : HVI was inducted to the synthetic solution containing 2.5 mM of Ca<sup>2+</sup> ion at different temperatures of 25, 40, 60℃. The concentration of Ca<sup>2+</sup> ion has been monitored as voltages of the HVI increased from 0 to 5, 10, 15 kV. Reaction order and the rate constants of the CaCO<sub>3</sub> formation reaction were determined with the experimental dataset of Ca<sup>2+</sup> concentration vs. time plots.Results and Discussion : The CaCO<sub>3</sub> formation was determined to follow two-molecules 2<sup>nd</sup> order reaction. The reaction rate constant, k increased as temperature and the applied voltages of HVI increased. The rate constant, k at 25℃ and 15 kV of HVI was 8.2×10<sup>-3</sup> L/(mmol･hr), which was 2.7 times greater than the k of the control at 25℃, 3.0×10<sup>-3</sup> L/(mmol･hr).Conclusions : The reaction of CaCO<sub>3</sub> formation was accelerated by HVI as the applied voltages of HVI increased, indicating that the HVI could be used as an alternative desalting technology for scale control.

Problem statement: For chemical reactions, the determination of the rate constants is both very difficult and a time consuming process. The aim of this research was to develop computer programs for determining the rate constants for the general form of any complex reaction at a certain temperature. The development of such program can be very helpful in the control of industrial processes as well as in the study of the reaction mechanisms. Determination of the accurate values of the rate constants would help in establishing the optimum conditions of reactor design including pressure, temperature and other parameters of the chemical reaction. Approach: From the experimental concentration-time data, initial values of rate constants were calculated. Experimental data encountered several types of errors, including temperature variation, impurities in the reactants and human errors. Simulations of a second order consecutive irreversible chemical reaction of the saponification of diethyl ester were presented as an example of the complex reactions. The rate equations (system of simultaneous differential equations) of the reaction were solved to get the analytical concentration versus time profiles. The simulation results were compared with experimental results at each measured point. All deviations between experimental and calculated values were squared and summed up to form a new function. This function was fed into a minimizer routine that gave the optimal rate constants. Two optimization techniques were developed using FORTRAN and MATLAB for accurately determining the rate constants of the reaction at certain temperature from the experimental data. Results: Results showed that the two proposed programs were very efficient, fast and accurate tools to determine the true rate constants of the reaction with less 1% error. The use of the MATLAB embedded subroutines for simultaneously solving the differential equations and minimization of the error function was very fast in solving such problems, as compared to the FORTRAN program, which, although resulting in fast and accurate results, yet, requiring the use of a library of external subroutines. Conclusion: Any of the two proposed methodologies could be used to determine the rate constants of any complex reaction at a certain temperature. The proposed programs were independent of the nature of the reaction, only the rate equations and the initial conditions had to be modified for any new reaction.

Using relative rate methods, rate constants for the gas-phase reactions of divinyl sulfoxide [CH 2CHS(O)CHCH 2; DVSO] with NO 3 radicals and O 3 have been measured at 296 +/- 2 K, and rate constants for the reaction with OH radicals have been measured over the temperature range of 277-349 K. Rate constants obtained for the NO 3 radical and O 3 reactions at 296 +/- 2 K were (6.1 +/- 1.4) x 10 (-16) and (4.3 +/- 1.0) x 10 (-19) cm (3) molecule (-1) s (-1), respectively. For the OH radical reaction, the temperature-dependent rate expression obtained was k = 4.17 x 10 (-12)e ((858 +/- 141)/ T ) cm (3) molecule (-1) s (-1) with a 298 K rate constant of (7.43 +/- 0.71) x 10 (-11) cm (3) molecule (-1) s (-1), where, in all cases, the errors are two standard deviations and do not include the uncertainties in the rate constants for the reference compounds. Divinyl sulfone was observed as a minor product of both the OH radical and NO 3 radical reactions at 296 +/- 2 K. Using in situ Fourier transform infrared spectroscopy, CO, CO 2, SO 2, HCHO, and divinyl sulfone were observed as products of the OH radical reaction, with molar formation yields of 35 +/- 11, 2.2 +/- 0.8, 33 +/- 4, 54 +/- 6, and 5.4 +/- 0.8%, respectively, in air. For the experimental conditions employed, aerosol formation from the OH radical-initiated reaction of DVSO in the presence of NO was minor, being approximately 1.5%. The data obtained here for DVSO are compared with literature data for the corresponding reactions of dimethyl sulfoxide.

Absolute rate constants have been determined by the pulse radiolysis technique for several electrophilic reactions of the benzyl, the benzhydryl, and the trityl cation in 1,2-dichloroethane solution. The rate constants for the reactions of these carbonium ions with chloride ion, with bromide ion, and with iodide ion are all very nearly the same, namely 6 x 10/sup 10/ M/sup -1/ s/sup -1/ at 24/sup 0/C. The values very likely represent the diffusion controlled limit for the ion combination reactions. The rate constants for the reactions with triethylamine, tri-n-propylamine, and tri-n-butylamine range from 2.0 x 10/sup 9/ to 7 x 10/sup 6/ M/sup -1/ s/sup -1/ at 24/sup 0/C. With increasing phenyl substitution, the decreasing trend in the magnitude of the rate constant is consistent with the combined electronic and steric effects. With increasing size of the amine, the decrease in the value of the rate constant seems to indicate that the steric effect predominates. The values of the rate constants for reactions of benzyl and benzhydryl cation with methanol, ethanol, and 2-propanol indicate the following. The rate constant is higher for reaction with the alcohol dimer in solution than with alcohol monomer. The rate constants for reaction with alcohol monomermore » have values of 1 x 10/sup 8/ M/sup -1/ s/sup -1/ or lower.« less

ABSTRACTThe chemistry of Cu-CVD from the precursor (hfac)Cu(tmvs) on sputtered TiN films was investigated using a simple tubular reactor analysis. In the present study, the validity of a proposed reaction mechanism and its rate constants were investigated using a well characterized test structure (Macrocavity). The macrocavity was prepared by stacking silicon wafers with spacing ranging from 0.1–0.5mm, and placing the stacked structure in the tubular reactor. The growth rate profile within a macrocavity is simulated from a derived equation using the reaction mechanism and rate constants obtained from previous work. The variation of the absolute growth rate around the center of a macrocavity with varying wafer spacing indicates that the Cu deposition is proceeded via gas phase reaction to produce a reaction intermediate. As a whole, the simulated growth rate profiles agree very well the experimental data. Therefore, we conclude that the mechanism and reaction rate constants from our research are valid and can be applied to the prediction of growth rate of Cu from the (hfac)Cu(tmvs) system.