Showing papers on "Reaction rate constant published in 1981"
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TL;DR: In this paper, simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated.
Abstract: Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and can...
2,062 citations
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TL;DR: The available rate data on the reactivity and physical deactivation of molecular oxygen in its first electronic excited state (1Δg) in liquid solution have been critically compiled as discussed by the authors.
Abstract: The available rate data on the reactivity and physical deactivation of molecular oxygen in its first electronic excited state (1Δg) in liquid solution have been critically compiled. Where possible, relative rates reported in the literature have been normalized to standard values selected by a statistical analysis of the experimental data. Second order rate constants for the deactivation and chemical reaction of singlet oxygen are reported for 670 compounds. Additionally, psuedo first order rate constants (kd) for solvent deactivation of singlet oxygen are reported for 50 different solvents.
555 citations
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TL;DR: In this article, the authors demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity for catalytic cracking of C6 to C9 hydrocarbons on ZSM-5.
Abstract: For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.
347 citations
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283 citations
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TL;DR: It is demonstrated that the interaction of polypeptide ligands with cells under physiological conditions can be described by a set of steady state equations that provide a foundation for computer simulations of ligand-cell interactions, which closely correspond to experimental data.
235 citations
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TL;DR: In this article, a gas-liquid reaction model has been studied for 1 x 10/sup -7/ less than or equal to P/sub NO2 less than and equal to 8 x 10 /sup -4/ atm.
Abstract: The reaction 2NO/sub 2/(g) + H/sub 2/O(l) ..-->.. 2H/sup +/ + NO/sub 3//sup -/ + NO/sub 2//sup -/ (l) has been studied for 1 x 10/sup -7/ less than or equal to P/sub NO2/ less than or equal to 8 x 10/sup -4/ atm. Since this reaction involves the transfer of a reactant from the gas into the aqueous phase, the rate depends upon the following: (a) physical mass transfer of the reactant, (b) the equilibrium solubility of NO/sub 2/, and (c) homogeneous aqueous-phase kinetics. In order for the observed rate to yield information most sensitive to (c), the rate of (a) must be comparable to that of (c). In the case of a second-order reaction, this can be achieved by working at low partial pressure of NO/sub 2/ as well as a high rate of physical mixing of the two phases. To facilitate the latter, the gas was brought into contact with the liquid as finely dispersed bubbles produced by flowing through a disk-frit; the mass transfer time constant, determined by uptake of CO/sub 2/, was tau/sub m/ = 1.7 to 5.3 s. The rate of reaction 1, monitored by observing the electrical conductivity of the aqueous solution, exhibitedmore » dependence on p/sub NO2/ and tau/sub m/ consistent with second-order kinetics and a steady-state aqueous-phase NO/sub 2/ concentration. Values of H/sub NO2/, the Henry's law coefficient, and k/sub 1/, the second-order aqueous-phase rate constant, were determined to be (7.0 +- 0.5) x 10/sup -3/M atm/sup -1/ and (1.0 +- 0.1) x 10/sup 8/M/sup -1/s/sup -1/, respectively, at 22/sup 0/C. Self-consistency of the gas-liquid reaction model has been demonstrated in terms of the identity of the diffusing species and the extent of mass-transport limitation.« less
199 citations
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TL;DR: In this paper, the authors studied the reaction of zerovalent palladium to aromatic iodides in tetrahydrofuran in the presence of the ligand triphenylphosphine.
189 citations
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TL;DR: In this paper, the absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 less than or equal to 298 K and 20 less than and equal to P less than 200 torr by using He, Ar, and SF/sub 6/ as diluent gases.
Abstract: Absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 less than or equal to T less than or equal to 1150 K and 20 less than or equal to P less than or equal to 200 torr by using He, Ar, and SF/sub 6/ as diluent gases. To help elucidate the variations in reaction mechanism with temperature, we also studied OH reactions with deuterated benzene (C/sub 6/D/sub 6/) and with selectively deuterated toluenes (C/sub 6/H/sub 5/CD/sub 3/, C/sub 6/D/sub 5/CD/sub 3/, and C/sub 6/D/sub 5/CH/sub 3/). Three major reaction channels were characterized kinetically. At T less than or equal to 298 K, electrophilic addition of the OH radical to the aromatic ring is the dominant reactive pathway in all systems studied. At temperatures above 500 K, rapid decomposition of the thermalized adduct back to reactants diminishes the importance of the addition channel and leads to bimolecular reaction rate-constant values significantly lower than those measured near room temperature. At elevated temperatures, the ring hydrogen abstraction (for benzene) and side-chain hydrogen abstraction (for toluene) pathways are shown to be predominant. The measured bimolecular rate constants increase monotonically withmore » increases in temperature above 500 K, and kinetic separation of the two hydrogen abstraction modes for toluene is achieved.« less
180 citations
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TL;DR: In this article, a transition state switching model is developed for use in systems where more than one transition state occurs along the reaction coordinate, cast in the perspective of both the unified statistical theory (UST) of Miller and of variational transition state theory.
Abstract: A transition state switching model is developed for use in systems where more than one transition state occurs along the reaction coordinate. The model is cast in the perspective of both the unified statistical theory (UST) of Miller and of variational transition state theory. The basic assumptions are those common to transition state theory and RRKM–QET. A reaction branching analysis leads to reaction probabilities for a number of potential surfaces and appropriate expressions are delineated for both unimolecular and bimolecular reactions. The theory is developed from a microcanonical viewpoint and rigorously conserves both energy E and angular momentum J. Comparison is made with experimental data for the C4H8 +⋅ system where absolute unimolecular rate constants and branching ratios have been measured as a function of energy, bimolecular rate constants, and branching ratios measured at room temperature (the ethylene ion–molecule reaction), the lifetime of C4H8 +⋅ measured when formed by the ethylene ion–...
174 citations
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TL;DR: The results permit the estimation that cysteine local environments may provide a means of achieving a 10(6)-fold range in rate constants in disulfide exchange reactions in random-coil proteins.
Abstract: The ionic strength dependence of the bimolecular rate constant for reaction of the negative disulfide 5,5'-dithiobis (2-nitrobenzoic acid) with cysteines in fragments of naturally occurring proteins was determined by stopped-flow spectroscopy. The Debye-Huckel relationship was applied to determine the effective charge at the cysteine and thereby determine the extent to which nearby neighbors in the primary sequence influence the kinetics. Corrections for the secondary salt effect on cysteine pKs were determined by direct spectrometric pH titration of sulfhydryl groups or by observation of the ionic strength dependence of kinetics of cysteine reaction with the neutral disulfide 2,2'-dithiodipyridine. Quantitative expressions was verified by model studies with N-acetyl-cystein. At ionic strengths equal to or greater than 20 mM, the net charge at the polypeptide cysteine site is the sum of the single negative charge of the thiolate anion and the charges of the amino acids immediately preceding and following the cysteine in the primary sequence. At lower ionic strengths, more distant residues influence kinetics. At pH 7.0, 23 degree C, and an ionic strength of 20 mM, rate constants for reaction of the negative disulfide with a cysteine having two positive neighbors, one positive and one neutral neighbor, or two neutral neighbors are 132000, 3350, and 367 s-1 M-1, respectively. This corresponds to a contribution to the activation energy of 0.65- 1.1 kcal/mol per ion pair involved in collision between the cysteine and disulfide regions. The results permit the estimation that cysteine local environments may provide a means of achieving a 10(6)-fold range in rate constants in disulfide exchange reactions in random-coil proteins. This range may prove useful in developing strategies for directing disulfide pairing in synthetic proteins.
169 citations
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TL;DR: In the photodecomposition of di-tert-butyl peroxide with phenols, the reaction rate of tert butoxy radicals was shown to be 3.3 x 10/sup 8/ and 1.6 x 10 /sup 9/M/sup -1/ s/sup 1/1/s/sup 0/C for phenol and p-methoxyphenol, respectively.
Abstract: Tert-butoxy radicals generated in the photodecomposition of di-tert-butyl peroxide react efficiently with phenols to yield the corresponding phenoxy radicals. Typical rate constants in benzene at 22/sup 0/C are 3.3 x 10/sup 8/ and 1.6 x 10/sup 9/ M/sup -1/ s/sup -1/ for phenol and p-methoxyphenol, respectively. The process is considerably slower in polar solvents; e.g., when pyridine is used as cosolvent, the rate constant for phenol drops to 4.1 x 10/sup 6/ M/sup -1/ s/sup -1/ as a result of strong hydrogen bonding which decreases the reactivity of the phenolic O-H group. Isotope effects (H/D) are typically in the 3 to 5 range. 5 figures, 4 tables.
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TL;DR: In this article, the rate of uptake of NO2 by liquid water accordingo (R1), 2NO2(g) + H20(I) - 2H + + NO3- + NO2-, is shown to be unaffected by O2 (0.2 atm).
Abstract: The rate of uptake of NO2 by liquid water accordingo (R1), 2NO2(g) + H20(I) - 2H + + NO3- + NO2-, is shown to be unaffected by O2 (0.2 atm). Hence the rate constant and Henry's law solubility constant of NO2 previously obtained may be employed to evaluate the rates of aqueous phase reactions of NO2 in the ambient atmosphere. Reactions (R1) and (R2), NO2(g) + NO(g) + H20(l) -- 2H + + 2NO2-, are quite slow at representative atmospheric partial pressures and cloud liquid water content; the characteristic times range upward from 103 - 104 hours at 10 -7 atm, increasing with decreasing partial pressures of the gases. Direct acidification of cloud liquid water by (R1) or (R2) is also unimportant. Catalytic enhancement of (R 1) is potentially important for catalyst concentrations of order 10 -7 M, assuming sufficiently fast rate constants (---108 M - s-). Iron-catalyzed reaction in particular, however, is found to be unimportant. Reaction of NO2 with dissolved S(IV) is potentially important, based upon an assumed upper limit rate constant of 2.5 x 10 7 M- s- I. Deposition of NO2 to surface (ocean or lake) water is shown to be controlled by aqueous phase mass transport and/or reaction and is much slower than heretofore assumed.
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01 Jan 1981
TL;DR: In this paper, the dissolution of a single microscopic oil droplet adhering to a thin fiber suspended in an aqueous surfactant solution is recorded photomicrographically and the solubilization rate (rate of volume change per unit interfacial area) is calculated from the shape of the droplet using previously published theory.
Abstract: A quantitative study of the rate of solubilization of nonpolar oils in aqueous surfactant solutions has been made using a new technique. The dissolution of a single microscopic oil droplet adhering to a thin fiber suspended in an aqueous surfactant solution is recorded photomicrographically and the solubilization rate (rate of volume change per unit interfacial area) is calculated from the shape of the droplet using previously published theory (B. J. Carroll, J. Colloid Interface Sci.57, 488 (1976)), which in this case reduces to a simple approximate form. The new technique has been used to study solubilization kinetics in some well-characterized systems containing a high-purity nonionic surfactant. The effects of surfactant concentration, temperature, and oil type were studied. The rate of solubilization is proportional to the surfactant concentration above the CMC and depends on the oil polarity and molecular weight. The rate is strongly temperature dependent in the region of the nonionic cloud point: 15°K below the cloud point, the rate is extremely small relative to that at the cloud point. It is shown that in the case of highly insoluble oils the solubilization mechanism must involve the adsorption-desorption of micelles at the oil/water interface, rather than the diffusion of molecules of oil into the micelle via the aqueous phase. The experimental data are shown to be consistent with the slow stage of the process being the adsorption, not desorption, of the micelle at the interface. The magnitude of the measured rate constant is consistent with theory if dissociation of the micelle occurs prior to the adsorption step and an estimate of the relaxation time for demicellization is obtained. The temperature dependence of the rate in the cloud point region is discussed in terms of changes in the properties of the nonionic solution in this region.
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TL;DR: In this paper, the laser flash photolysis method has been used to determine the bimolecular rate constants for the reaction between O2(1Δ9) and several lipid-soluble and water soluble substrates.
Abstract: — The laser flash photolysis method has been used to determine the bimolecular rate constants for the reaction between O2(1Δ9) and several lipid-soluble and water-soluble substrates. Values for lipid-soluble substrates have been obtained using aqueous dispersions of surfactants above the critical micelle concentration with 1,3 diphenylisobenzofuran as monitor of singlet oxygen. Under these conditions the hydrophobic substances are solubilized by the micellar phase. For substrates which are water-soluble, 9,10-anthracene dipropionic acid disodium salt was used as singlet oxygen monitor. For several substances, the values obtained are comparable to the values found in homogeneous nonaqueous solutions. In cases where significant differences have been found these have been rationalized according to the individual case. The only major unexpected result concerned β-carotene which, in micellar dispersion, failed to react at all with O2(1Δ9) This may be due to multi-molecular aggregations occurring in the polar medium. The work described herein shows clearly that, under appropriate conditions, singlet oxygen kinetics can be effectively followed in aqueous solutions by time resolved methods. The indiscriminate use of β-carotene as a quencher of O2(1Δ9)in mainly aqueous media is questioned.
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TL;DR: In this paper, the authors studied the kinetics and mechanism of oxidation of carbon atoms exposed at monolayer steps on graphite surface and two pieces of direct evidence are shown for the importance of surface diffusion in the overall kinetics.
Abstract: The kinetics and mechanism of oxidation of carbon atoms exposed at monolayer steps on graphite surface have been studied with the etch‐decoration and transmission electron microscopy technique and two pieces of direct evidence are shown for the importance of surface diffusion in the overall kinetics. It is found that the rate of carbon removal, or the turnover frequency, depends on the population density of these edge carbon atoms, being substantially higher on low‐density surfaces. In the argon flush experiment, it is found that surfaces with low edge carbon densities continue to ’’burn’’ for prolonged periods of time after O2 is cut off from the gas phase. Two independent mechanisms are revealed by the experimental results: (1) reaction resulted from direct collision of O2 on the edge carbon, and (2) reaction of the edge carbon with the migrated oxygen which is first chemisorbed on the basal carbon. Furthermore, from the results of the argon flush experiments, the following results can be calculated: surface diffusion coefficient = 5×10−12 cm2/s (for O/C = 0.34) and 2×10−12 cm2/s (for O/C = 0.06); rate constant for the edge carbon reacting with the migrated O atom = 4.5 s−1, all at 650 °C. The amount of chemisorption on the basal plane can also be obtained from the data of the argon flush technique as demonstrated in this work.
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TL;DR: Arrhenius parameters for the decomposition reactions of various alkoxy radicals are evaluated in this paper, and the recommended values for tert-butoxy radical decomposition reaction are log A (s−1) = 14.1, Ea = 15.3 kcal/mol.
Abstract: Arrhenius parameters for the decomposition reactions of various alkoxy radicals are evaluated. The recommended values for tert-butoxy radical decomposition reaction are log A (s−1) = 14.1, Ea = 15.3 kcal/mol.
The parameters give reasonably self-consistent results with related rate constants, product distributions of hydrocarbon oxidation reactions, and the values reported in different laboratories in different environments.
A reworking of data by Batt et al. on RO + NO and RO + NO2 leads to a lowering of his A factors for these reactions by a factor of 5. It also leads to a similar lowering of the A factors for RO decompositions. The intrinsic activation energies E for the addition of alkyl radicals R to the carbonyl group are not only found to be a function of the exothermicity of reaction, but they are also dependent on the nucleophilicity of the alkyl radicals. For each radical R (Me, Et, i-Pr) the data can be fit to an Evans–Polanyi plot E = a + b (δH), where a uniform slope b = 0.58 can be found for all R. The intercepts a are then found to decrease nearly linearly with the decreasing ionization potential of R, namely, a = 2.1 [IP] – 6.2 (kcal/mol), with the ionization potential given in electron volts.
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TL;DR: The chemical stability of 5-aza-2'-deoxycytidine (I) in acidic, neutral, and alkaline solutions was analyzed by high-performance liquid chromatography and was most stable when stored in neutral solution at low temperature.
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TL;DR: In this paper, the reaction kinetics of CO/sub 2/ with triethanolamine (TEA) and methyldiethanolamine (MDEA) in aqueous solution have been studied by using a stopped-flow technique with pH detection.
Abstract: Reaction kinetics of CO/sub 2/ with triethanolamine (TEA) and methyldiethanolamine (MDEA) in aqueous solution have been studied by using a stopped-flow technique with pH detection. Rate constants are obtained from the comparison of experimental and theoretical curves giving the optical density as a function of time. At concentrations of CO/sub 2/ well below the saturation limit, the results are consistent with the hydration reactions of the CO/sub 2/ molecules either with neutral water molecules or with hydroxide ions, depending upon the pH, itself governed by the ionization equilibrium of the dissolved amine. Moreover, a specific (catalytic) reaction, first order with respect to both carbon dioxide and amine (rate constant, 2.85 M/sup -1/ s/sup -1/ at 25/sup 0/C), has been shown to contribute significantly to the reaction rate in the case of the first amine (TEA) only.
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TL;DR: In this article, the Vegard-Kaplan system was studied in a room temperature discharge-flow apparatus by monitoring the temporal decay of the 0,6 and 1,10 bands of the system.
Abstract: The removal of N2(A 3Σ+u, v′ = 0,1) by O has been studied in a room temperature discharge–flow apparatus by monitoring the temporal decay of the 0,6 and 1,10 bands of the Vegard–Kaplan system. The measured rate constants are (2.8±0.4) and (3.4±0.6)×10−11 cm3 molecule−1 s−1 for v′ = 0 and 1, respectively.
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TL;DR: In this paper, the photochemical oxygenation of 2,5-dimethylfuran (DMF) in water was studied under a variety of reaction conditions employing various humic substances as photosensitizers.
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TL;DR: In this article, a recently developed kinetic model has been applied to describe the polyester-styrene addition copolymerization, and temperature profiles inside plate sections of SMC parts during molding are predicted by a mathematical model in which addition polymerization is coupled with heat transfer.
Abstract: A recently developed kinetic model has been applied here to describe the polyester-styrene addition copolymerization. By assuming that the termination step is negligible and the reaction rate between inhibitor and initiator free radical is much, faster than any other reactions, the kinetic mechanism can be simplified to be expressed as a single equation. The parameters, rate constant of initiator decomposition and rate constant of propagation, are estimated from the induction time and the time to the peak exotherm of isothermal differential scanning calorimetry (DSC) curves. Temperature profiles inside plate sections of SMC parts during molding are predicted by a mathematical model in which addition polymerization is coupled with heat transfer. The predicted temperature profiles compare well with the experimental results. The model is also used to predict the cure time of different part thicknesses, mold temperature and initiator concentration. Glass fibers playa role as a heat sink as well as heat conductor during curing. Adding glass fibers to SMC not only lowered the maximum exotherm but also reduced the cure time.
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TL;DR: In this article, the authors used convolution analysis to evaluate the thermodynamic and kinetic parameters of two charge transfer systems at the water/nitrobenzene interface: Cs+ ion transfer and the electron transfer between ferrocene in nitrobenzinene and hexacyanoferrate(III) in water.
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TL;DR: Although isolated mitochondria were inhibited by the 2-formylpyridine thiosemicarbazonato copper (II) complex, any cellular reaction was obscured by the rapid oxygen reduction that occurred in the thiol oxidation process.
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TL;DR: In this article, the inhibition of Cu 2+, Cd 2+ and Zn 2+ discharge reactions was studied in the presence of C 4 -C 6 alcohols, phenol, benzyl alcohol and cyclohexanol (SAS).
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TL;DR: Findings lead to a view that the proton-catalyzed nucleophilic displacement of O2- from MbO2 by an entering water molecule, or SN2 mechanism with proton assistance, is the basis for most of the autoxidation reaction under normal conditions.
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TL;DR: In this paper, a theoretical analysis of the 1e, 1H+ square scheme is presented for a heterogeneous reaction with protonations occurring in solution near the electrode and assumed to be at equilibrium.
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TL;DR: In this paper, the effect of the lateral interactions of adsorbed molecules of two kinds on the rate constants of monomolecular desorption and adsorption was analyzed.
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TL;DR: In this paper, the authors used second order classical perturbation theory to solve the vibrational Hamilton-Jacobi equation and assigned final H2O vibrational states using the histogram method.
Abstract: Quasiclassical trajectories have been used to determine reaction rate constant enhancements and product state energy partitioning in the OH(v)+H2(v′)→H2O+H reaction for (v,v′)= (0,0), (1,0), (0,1), and (1,1). An analytical fit to the accurate ab initio potential surface of Walch and Dunning was used in the Monte Carlo calculations. Final H2O vibrational states were assigned using the histogram method to bin the good action variables governing H2O vibrational motions. These actions were calculated by using second order classical perturbation theory to solve the vibrational Hamilton–Jacobi equation. The resulting integral reaction cross sections and thermal rate constants, indicate that OH vibrational excitation leads to a very small enhancement (only a factor of 1.28) in the thermal rate constant at 300 K. H2 excitation, on the other hand, causes a large reduction in the reaction activation energy (from 0.18 eV to 0.03 eV) and a large enhancement in the rate constant (a factor of 393 at 300 K). These resul...
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TL;DR: In this article, the first observable species in the nanosecond region is Fe(OH) aq 2+ with λ max = 300 nm and ϵ 300 = 2400 M −1 cm −1.