Experimental Rate Constant Data Research Articles

Temperature-Dependent Kinetic Study of the Reactions of Hydrogen Atoms with H2S and C2H4S

A discharge-flow reactor combined with modulated molecular beam mass spectrometry technique was employed to determine the rate constants of H-atom reactions with hydrogen sulfide and thiirane. The rate constants for both reactions were determined at a total pressure of 2 Torr from 220 to 950 K under pseudo-first-order conditions by monitoring either consumption of H atoms in excess of H2S (C4H4S) or the molecular species in excess of atomic hydrogen. For H + H2S reaction, a suggested previously strong curvature of the Arrhenius plot was confirmed: kl = 8.7 × 10−13 × (T/298)2.87 × exp(−125/T) cm3 molecule−1 s−1 with a conservative uncertainty of 15% at all temperatures. Non-Arrhenius behavior was also observed for the reaction of H-atom with C2H4S, with the experimental rate constant data being best fitted to a sum of two exponential functions: k2 = 1.85 × 10−10 exp(−1410/T) + 4.17 × 10−12 exp(−242/T) cm3 molecule−1 s−1 with an independent of temperature uncertainty of 15%.

Multi-target QSPR modeling for simultaneous prediction of multiple gas-phase kinetic rate constants of diverse chemicals

The reactions of molecular ozone (O3), hydroxyl (•OH) and nitrate (NO3) radicals are among the major pathways of removal of volatile organic compounds (VOCs) in the atmospheric environment. The gas-phase kinetic rate constants (kO3, kOH, kNO3) are thus, important in assessing the ultimate fate and exposure risk of atmospheric VOCs. Experimental data for rate constants are not available for many emerging VOCs and the computational methods reported so far address a single target modeling only. In this study, we have developed a multi-target (mt) QSPR model for simultaneous prediction of multiple kinetic rate constants (kO3, kOH, kNO3) of diverse organic chemicals considering an experimental data set of VOCs for which values of all the three rate constants are available. The mt-QSPR model identified and used five descriptors related to the molecular size, degree of saturation and electron density in a molecule, which were mechanistically interpretable. These descriptors successfully predicted three rate constants simultaneously. The model yielded high correlations (R2 = 0.874–0.924) between the experimental and simultaneously predicted endpoint rate constant (kO3, kOH, kNO3) values in test arrays for all the three systems. The model also passed all the stringent statistical validation tests for external predictivity. The proposed multi-target QSPR model can be successfully used for predicting reactivity of new VOCs simultaneously for their exposure risk assessment.

Quantum Mechanical/Molecular Mechanical Molecular Dynamics and Free Energy Simulations of the Thiopurine S-Methyltransferase Reaction with 6-Mercaptopurine

Quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations were performed to investigate the methylation of 6-mercaptopurine catalyzed by thiopurine S-methyltransferase. Several setups with different tautomeric forms and orientations of the substrate were considered. It is found that, with the orientation in chain A of the X-ray structure, the substrate can form an ideal near-attack configuration for the methylation reaction, which may take place after the deprotonation of the substrate by the conserved residue Asp23 through a water chain. The potential of mean force (PMF) of the methyl-transfer step for the most favorable pathway is 19.6 kcal/mol, which is in good agreement with the available experimental rate constant data.

Molecular Electrostatic Potential as a tool for Evaluating the Etherification Rate Constant

In this work the molecular electrostatic potential (MEP) is proposed as an effective approach in describing the influence of substituent on the rate constant of etherification reaction. A relationship on the basis of density functional theory has been established to show that the etherification rate constant should be proportional to the electrostatic potential on the atomic sites. Indeed, we employed the MEP at the atomic sites as a local quantum descriptor to estimate the reaction rate constant variation caused by substituent effect. Taking the experimental rate constants of 30 substituted phenols, the validity of the proposed method has been verified. Moreover, the atoms-in-molecules (AIM) charge scheme as another local descriptor was tested for its ability to represent variations in the kinetic data for etherification reaction of phenols. It was shown that the changes in these two descriptors were strongly correlated with the variation of experimental rate constant data. The outcome of good correlations in this study offers a relatively simple and effective method to compute the rate constant for etherification reaction of substituted phenols based on MEP and AIM charge.

Analysis of the reactivities of organic compounds in hydrogen atom abstraction from their C-H bonds by the sulfate radical anion SO 4 ·−

An experimental rate constant data array (>50 values) for the reactions of the sulfate radical anion with the C-H bonds of various organic compounds is analyzed by Denisov’s intersecting parabolas method. The data array is divided into the following four groups according to the type of the C-H bond attacked and the bre value: compounds with primary C-H bonds (bre = 13.14 ± 0.17), compounds with secondary C-H bonds remote from the polar groups (bre = 12.64 ± 0.34), compounds with secondary α-C-H bonds (bre = 13.28 ± 0.24), and compounds with tertiary C-H bonds. It is demonstrated by DFT calculations that the rate constant of the reaction SO4·− + RH is determined by nonspecific solvation and by the ion-dipole interaction in the transition state of the reaction.

Isodesmic Reactions for Transition States: Reactions of Cl Atoms with Methane and Halogenated Methanes

The performance of the technique of isodesmic reactions for transition states (IRTS) has been analyzed via application to 17 reactions of abstraction of hydrogen atoms from methane and halogenated methanes by Cl atom. A variety of quantum chemical methods and basis sets was used. The calculated energy barriers demonstrate linear correlations with those derived from modeling of the experimental rate constant data, in agreement with the prediction based on the IRTS formalism. The results of the study confirm the validity of the technique of isodesmic reactions for transition states for calculation of reaction rates and demonstrate the existence of method-specific systematic errors in calculations of reaction barriers. The technique of isodesmic reactions is directed at factoring out and eliminating these systematic errors. The predictive ability of the technique is directly related to the quality of the observed correlations. Average and maximum deviations from the best fit lines on the correlation plots de...

Kinetics of the Unimolecular Decomposition of the C2Cl3 Radical

The C2Cl3 → C2Cl2 + Cl (1) reaction was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (569−683 K) and bath gas density ([He] = (3−24) × 1016 atoms cm-3, [N2] = 12 × 1016 molecules cm-3). C2Cl2 was observed as a primary product of reaction 1. Rate constants of reaction 1 are in the falloff, closer to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the PES study indicate that the minimum energy path of the C2Cl3 dissociation proceeds through a shallow-well symmetric structure with the departing Cl atom equidistant from both carbon atoms; a PES saddle point exists between the nonsymmetric equilibrium C2Cl3 structure and the symmetric one. The results of quantum chemical calculations and the experimental rate constant data were used to create a model of reaction 1. The experimental dependence of the rate constant of C2Cl3 decomposition on temperature and pressure was reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors: k1∞ = (1.36 × 1013)T0.29 exp(−13625 K/T) s-1, k1°(He) = (4.89 × 1012)T-6.14 exp(−14497 K/T) cm3 molecule-1 s-1, k1°(N2) = (1.525 × 1012)T-5.95 exp(−14393 K/T) cm3 molecule-1 s-1, Fcent(He) = 0.33 exp(−T/1836 K) + 0.67 exp(−T/98 K) + exp(−3650 K/T), and Fcent(N2) = 0.35 exp(−T/1864 K) + 0.65 exp(−T/103 K) + exp(−3614 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.

A New Model for Laser-induced Thermal Damage in the Retina

We describe a new model for laser-induced retinal damage. Our treatment is prompted by the failure of the traditional approach to accurately describe the image size dependence of laser-induced retinal injuries and by a recently reported study which demonstrated that laser injuries to the retina might not appear for up to 48 h post exposure. We propose that at threshold a short-duration, laser-induced, temperature rise melts the membrane of the melanosomes found in the pigmented retinal epithelial cells. This results in the generation of free radicals which initiate a slow chain reaction. If more than a critical number of radicals are generated then cell death may occur at a time much later than the return of the retina to body temperature. We show that the equations consequent upon this mechanism result in a good fit to the recent image size data although more detailed experimental data for rate constants of elementary reactions is still required. This paper contributes to the current understanding of damage mechanisms in the retina and may facilitate the development of new treatments to mitigate laser injuries to the eye. The work will also help minimize the need for further animal experimentation to set laser eye safety standards.

Electronic factor in electron self‐exchange reaction of hydrated redox ion pairs from thermodynamic data

AbstractAn accurate scheme for determining the electronic factor of the electron self‐exchange reaction in solution is presented in this paper. The wed various activation parameters and slopes of potential energy surfaces are obtained in terms of an improved activation model and the accurate potential function determined from the vibrational spectroscopic and thermodynamic data. The coupling matrix elements are determined using numerical integral method over the perturbed double‐zeta Slater‐type state functions. Theoretical results of electronic factor in this work an found in close agreement with those extracted from experimental rate constant data and to be less than unity. Results indicate that outer‐sphere electron transfer reactions in solution involving hydrated transition metal ions are nonadiabatic in nature.

Destruction of benene in high-temperature flames: Chemistry of benene and phenol

A flat, laminar, premixed H 2 /O 2 /Ar flame was seeded with C 6 H 6 to study the destruction chemistry of single-ring aromatics. Molecular-beam mass spectrometry was used to measure mode fractions of 44 species at =1.79, 2.93 kPa (22 torr), T max =1940 K and a cold-gas velocity of 101 cm/s. The results were used to investigate the C 6 H 6 and C 6 H 5 OH reaction networks proposed in three current literature models. Large errors in the predictions of important reaction partners limited the usefulness of simple mole fraction comparisons or reaction path analysis. A technique called net rate analysis was developed from existing mechanism screening techniques and modified reaction path analyses. Using this method, the reaction networks involving benzene are found to be adequate except for the need of falloff corrections for several reactions, in particular for H addition to C 6 H 6 under the present conditions. The phenol submechanisms strongly overpredict net destruction at high temperatures. Correction of mechanistic problems and inclusion of pressure effects for H elimination from C 6 H 5 OH result in a much-improved net rate prediction. Analysis reveals a need for experimental rate constant data for key phenol reactions above 1200 K. The results were found to hold for the =1.8, C 6 H 6 /O 2 /Ar flame data of Bittner. Comparisons of model-predicted mole fractions with the data lead to incorrect conclusions about phenol chemistry, demonstrating that net rate analysis gives better results even when most important species mole fractions are well predicted.

Electron transfer reactions of iron (III) - polypyridyl complexes with organic sulphides

The redox reactions of FeL 3 3+ (where L = 2,2 ′-bipyridine, 4,4 ′-dimethyl-2,2 ′-bipyridine and 1,10-phenanthroline) with alkyl aryl sulphides in aqueous methanol have been studied by the spectrophotometric technique. The comparison of the experimental rate constant data with the values estimated from Marcus cross-reaction relationship led to the conclusion that the single electron transfer from sulphide to FeL 3 3+ is the rate controlling step. The kinetic data yield 1.0 × 10 7 M −1s −1 for the ArSR/ArS + . R self-exchange rate constant at 25°C. The reactions of iron(III)-polypyridyl complexes with alkyl aryl sulphides in aqueous methanol proceed through electron transfer mechanism and form sulphoxides as products.

Isotope effects in acid‐catalyzed aromatic hydrogen exchange

AbstractDeuterium‐ or tritium‐substituted aromatics undergo isotope exchange in fairly concentrated aqueous sulfuric acid media. The rate constant for this process is complex, being composed of that for the slow electrophilic attack, and the isotope effect on the breakup of the Wheland intermediate. Using experimental rate constant data for both deuterium and tritium exchange on the same substrate, the excess acidity method and the Swain–Schaad relationship, true protonation rate constants and isotope effects have been separated out for several positions in several simple aromatic substrates. Most of the electrophilic attack rate constants are described by a Hammett‐type linear free energy relationship with a ρ+ of − 6.5. The average observed deuterium isotope effect on Wheland intermediate breakup was 5.3. Using rate constants obtained at different temperatures, enthalpies and entropies of activation were also obtained for benzene, naphthalene and thiophene; the activation entropy averaged − 8 cal mol−1 K−1. Some evidence was found for general acid catalysis at the higher acidities.

Rotational stark effect and asymptotic value for the rate constant for charge transfer from a linear dipolar molecule to an ion

Using the quantum description of the rotational Stark effect the basis for the good agreement of experimental data for rate constants for charge transfer from an ion to a polar molecule with those following from the theory of isothermal capture has been justified. The calculations also show that in the capture process isotopic effects different from those expected on the grounds of classical capture theories may take place.

A survey of reactions involving H',H'', and chlorine atoms

Experimental and theoretical rate constants for the reactions H/sub 2/ + Cl in equilibrium H + HCl, H' + ClH'' ..-->.. H'Cl + H'', and H' + H''Cl(upsilon = 1) ..-->.. H' + H''Cl(upsilon = 0) are discussed in detail (H' and H'' are H, D, or T). For the reaction H/sub 2/ + Cl in equilibrium H + HCl, experimental rate constant data for the forward and reverse reactions are in good shape. Semiempirical potential energy surfaces have been found which, combined with trajectory calculations or activated complex theory, give rate constants in reasonable agreement with experiment. However, these potential energy surfaces cannot be extended to include the exchange reaction, H' + H''Cl ..-->.. H'Cl + H'', and neither the experimental determinations or theoretical predictions of the abstraction/exchange branching ratios are well established. Relaxation of HCl(upsilon = 1) by H or D atoms may result from either reactive or nonreactive collisions; theory and experiment disagree as to the relative importance of these two channels. 8 figures, 8 tables.

A study of the rates of catalyzed Boudouard reaction

The rate of Boudouard (C-CO 2) reaction catalyzed by Li 2O and Li 2CO 3 was investigated in the temperature range 839°–1050°C. The rate data is reported in the form of Fractional burn-off ( F) vs time ( t) plots. The experimental rate constant ( K) data deduced from the initial linear region of these plots is represented by log K = 2.901(±0.186) − 22,240(±1,020) 2.303RT ; K is in gm/gm min. The experimental rate constant data was corrected for pore-diffusion and film mass transfer effects. The intrinsic rate constant ( K v ) was found to exhibit the following temperature-dependence: log K v = 5.471(±0.353) − 48,040(±1,920) 2.303RT ; K v is in moles/cm 3atm sec. An electrochemical mechanism was proposed to explain the catalytic activities of Li 2O and Li 2CO 3.

Self-Diffusion of Iron in Iron Oxides and The Wagner Theory of Oxidation

The rates of self-diffusion of iron in artifically prepared wustites of various compositions have been determined using the decrease in surface activity technique. Similar measurements are reported for artificial magnetites of nearly stoichiometric composition and for natural hematite single crystals. These data, together with appropriate thermodynamic data derived from the present study or from the literature, are used to calculate the rates of oxidation of iron and its oxides, taking as a basis the theoretical rate equations developed by Wagner. The calculations are compared with experimental rate constant data for these same reactions and are shown to provide essential confirmation for the Wagner theory. For the most part, the results are also consistent with the previously proposed transport mechanisms in the oxides of iron.