Approximate Rate Coefficients Research Articles

Kinetics of the reaction of OH with methyl nitrate (223-343 K).

Rate coefficients (k4) for the reaction of hydroxyl radicals (OH) with methyl nitrate (CH3ONO2) were measured over the temperature range 232-343 K using pulsed laser photolysis to generate OH and pulsed laser-induced fluorescence to detect it in real-time and under pseudo-first-order conditions. In order to optimize the accuracy of the rate coefficients obtained, the concentration of CH3ONO2 (the reactant in excess) was measured on-line by absorption spectroscopy at 213.86 nm for which the absorption cross-section was also measured (σ213.86 = 1.65 ± 0.09 × 10-18 cm2 molecule-1). The temperature-dependent rate coefficient is described by k4(T) = 7.5 × 10-13 exp[(-1034 ± 40)/T] cm3 molecule-1 s-1 with a room temperature rate coefficient of k4(296 ± 2 K) = (2.32 ± 0.12) × 10-14 cm3 molecule-1 s-1 where the uncertainty includes the statistical error of 2σ and an estimation of the potential systematic bias of 5%. This new dataset helps to consolidate the database for this rate coefficient and to reduce uncertainty in the atmospheric lifetime of CH3ONO2. As part of this study, an approximate rate coefficient for the reaction of H-atoms with CH3ONO2 (k9) was also derived at room temperature: k9(298 K) = (1.68 ± 0.45) × 10-13 cm3 molecule-1 s-1.

A workflow for automatic generation and efficient refinement of individual pressure-dependent networks

Manual chemical kinetic model construction often requires modelists to at least implicitly guess all possible decay pathways and their relative fluxes for all important species. We present a workflow and associated tool for automatic generation and refinement of pressure-dependent networks (multiwell potential energy surfaces) that should enable modelists to efficiently and comprehensively identify decay pathways and estimate respective parameters for chemical species. This tool within the Arkane software combines the capabilities of the Reaction Mechanism Generator (RMG) software to generate possible reaction paths, determine thermochemistry, approximate rate coefficients and estimate frequencies with the capabilities of Arkane to use quantum chemical parameters to compute thermochemistry and pressure-dependent rate coefficients. A flux-based algorithm is used to decide which isomers to add to the network. Isomers added to the network are reacted to form other channels. When enough of the flux is accounted for by the isomers and bimolecular product channels, the generation process terminates to yield a comprehensive network. Network sensitivity analysis is applied to identify the most important wells and barriers that can be refined using quantum chemistry calculations. Iterative refinement can then be used to achieve accuracy. The comprehensive network can be easily reduced using energy and flux-based algorithms to the most important channels and isomers.

First quantum study of the rotational excitation of HCN by para-H2O: Convergence of quantum results, influence of the potential energy surface, and approximate rate coefficients of interest for cometary atmospheres.

The rotational excitation of HCN by H2O, the main perturber in cometary atmospheres, is investigated using quantum methodologies. We provide approximate rotational de-excitation rate coefficients among the first levels of HCN perturbed by thermalized para-water in the temperature range T=5K to T=150K. Because of the novelty of the system for quantum rotational excitation, the current study includes a detailed appreciation of the parameters involved in the convergence of the cross-sections and of rate coefficients calculations. A compromise on the convergence of the rate coefficients with respect to the rotational basis set is taken because of the computing time cost. Moreover, because of the cost also involved in calculating the 5D potential energy surfaces necessary for the current dynamical calculations, several potential energy surfaces of increased quality are tested and it is shown that, within the current approximations on the collisional calculations, average quality potential energy surfaces are sufficient for cometary applications.

Chemistry of fullerenes C60 and C70 formation in flames

A kinetic mechanism is constructed for the formation of fullerenes C[sub 60] and C[sub 70] in flames, based on types of reactions used in describing growth of polycyclic aromatic hydrocarbons (PAH) and including additional chemical processes needed to describe evolution of the unique structural features of fullerenes. The mechanism consists of types of reactions, each characterized by an approximate rate coefficient, including processes for ring formation (via H atom abstraction, C[sub 2]H[sub 2] addition, and cyclization leading to ring closing), reactive coagulation of aromatic molecules, and cage closing via H[sub 2] elimination and ring closing but also allowing for additional processes such as intramolecular rearrangements. Curved PAH, including benzo[ghi]fluoranthene (C[sub 18]H[sub 10]) and dibenzo-[ghi,mno]fluoranthene (corannulene, C[sub 20]H[sub 10]), are likely fullerene precursors. Corannulene is considered a key immediate in the fullerene formation mechanism. Although alternatives to corannulene as an intermediate are mentioned, the proposed mechanism is based on corannulene and other related PAH of C[sub 50] symmetry. Trivial nomenclature for the C[sub 50] intermediates is also introduced. Preliminary kinetic testing of the mechanism, using approximate rate coefficients based on analogous flat PAH reactions, shows the mechanism to be plausible. 84 refs., 12 figs., 4 tabs.

Zinc Dialkyldithiophosphate Oxidation by Cumene Hydroperoxide: Kinetic Studies by Raman and31P NMR Spectroscopy

The oxidation of zinc dialkyldithiophosphates (ZDDPs) by cumene hydroperoxide in cyclohexane is studied by Raman and 31P NMR spectroscopy. The reaction proceeds in two main stages: oxidation of ZDDP to basic ZDDP, and oxidation of basic ZDDP to other products. Observation of phosphorus-containing reaction products by the two techniques yields approximate rate coefficients for the two stages. 31P NMR spectra of the basic ZDDP peak at high resolution gives evidence of the progressive break up of this cluster molecule, by loss of (RO)2PS2 coordinating groups, as the oxidation proceeds.

ChemInform Abstract: The Gas Phase Pyrolysis of Phenol

Phenol pyrolysis has been studied in a turbulent flow reactor by analyzing concentration-time profiles of three major decomposition products: carbon monoxide, cyclopentadiene, and benzene. Experimental conditions were P = 1 atm, T = 1064 − 1162 K, and initial phenol concentrations of 500−2016 ppm. The major experimental observations were that the decomposition product profiles were nearly linear as a function of time and that the overall rate of carbon monoxide production was greater than that of cyclopentadiene. The rate difference is explained by a mechanism which includes a radical combination reaction of cyclopentadienyl and phenoxy. With literature and approximate rate coefficient data, the mechanism reproduced the experimental observations very well. The mechanism and data provide estimates of rate coefficients for the phenol decomposition initiation step, abstraction of hydrogen from phenol by cyclopentadienyl, and the phenoxy-cyclopentadienyl combination, all of which have not been available in the literature.

The gas phase pyrolysis of phenol

AbstractPhenol pyrolysis has been studied in a turbulent flow reactor by analyzing concentration‐time profiles of three major decomposition products: carbon monoxide, cyclopentadiene, and benzene. Experimental conditions were P = 1 atm, T = 1064 − 1162 K, and initial phenol concentrations of 500−2016 ppm. The major experimental observations were that the decomposition product profiles were nearly linear as a function of time and that the overall rate of carbon monoxide production was greater than that of cyclopentadiene. The rate difference is explained by a mechanism which includes a radical combination reaction of cyclopentadienyl and phenoxy. With literature and approximate rate coefficient data, the mechanism reproduced the experimental observations very well. The mechanism and data provide estimates of rate coefficients for the phenol decomposition initiation step, abstraction of hydrogen from phenol by cyclopentadienyl, and the phenoxy‐cyclopentadienyl combination, all of which have not been available in the literature.

Homolytic reactions of ligated boranes. Part 6. Electron spin resonance studies of dialkyl sulphide–boryl radicals

Hydrogen-atom abstraction by ButO˙ or (Me3Si)2N˙ from dialkyl sulphide–boranes (R2S→BH3; R = Me or Et) gives the corresponding dialkyl sulphide–boryl radicals (R2S→BH2), the structures and reactions of which have been studied in solution using e.s.r. spectroscopy. The equilibrium geometry at the radical centre in R2S→BH2 appears to be planar or very nearly so, as evidenced by the 11B and Hα hyperfine coupling constants and by the effect of deuterium substitution at boron. Sulphide–boryl radicals undergo ready β-scission [equation (A)], in common with the amine–boryls R3N→BH2 but in marked contrast to, R2S→BH2→ R˙+ RS[graphic omitted]BH2(A) the phosphine–boryls R3P→BH2. Sulphide–boryl radicals abstract halogen rapidly from alkyl bromides and, at 197 K, t-butyl bromide reacts with Me2S→BH2 2.6-times more rapidly than n-propyl bromide. Approximate rate coefficients for the formation and transformation of sulphide–boryl radicals have been obtained from competition experiments. Ab initio molecular orbital calculations at the 6-31G** level for sulphide–, phosphine–, and amine–boryl radicals and for related species provide a basis for interpretation of the experimental results.

Strong collision rate coefficients in unimolecular reaction rate theory

The traditional and a recently proposed renormalized approximation to the rate coefficient obtained from a strong collision master equation are derived and compared. It is shown that they deviate significantly when applied to the unimolecular reactions of large molecules with low activation energies at elevated temperatures. The derivation is extended to yield a method whereby the exact rate coefficient can be calculated without increasing the level of numerical effort. An analysis of the exact rate coefficient in the high and low collision frequency limits is found to verify the validity of the traditional approximate rate coefficient in these limits. The renormalized rate coefficient fails in the low frequency limit. Both approximations can be significantly in error at intermediate collision frequencies and the use of the exact expression is recommended.

Some reactions of N 2 · H 2O + ions

A drift-chamber mass spectrometer has been used to explore reactions of N 2 · H 2O + ions with CO 2, N 2O, SO 2, C 2H 2, NH 3, and NO. The first two reactions yield displacement products, the others proceed predominantly by charge transfer. Approximate rate coefficients are derived and are found to be close to the upper limit values calculated from average dipole orientation theory.

Production of hydrogen atoms during the thermal dissociation of ethylene between 1700 and 2200°K

The technique of atomic resonance absorption spectrophotometry (ARAS) was used to study the generation of H atoms during the thermal decomposition of C 2 H 4 . Highly diluted mixtures with relative concentrations between 20 and 800 ppm of C 2 H 4 in Ar were used. The experiments were performed behind reflected shock waves at temperatures between 1700 and 2200°K and densities between 6·10 18 to 1.6·10 19 particles/cm 3 . For reaction (1) C 2 H 4 + M →C 2 H 3 +H+ M we obtained the rate coefficient expression: k 1 =(3.8±1.3)·10 17 exp(− E 1 / R·T ) cm 3 /mol·sec E I / R =49 400±900°K Together with previous measurements for reaction (1a) C 2 H 4 + M →C 2 H 2 +H 2 + M which gave k 1a =(2.6±0.5)·10 17 ·exp(− E 1a / R·T ) cm 3 /mol·sec E 1a / R =39 900±500°K it became apparent that C 2 H 4 decays simultaneously via two reaction channels under the chosen conditions. In addition it was possible to derive approximate rate coefficients for reaction (3) H+C 2 H 4 →C 2 H 3 +H 2 k 3 ≅1.5·10 13 cm 3 /mol·sec; T =2000°K k 3 ≅5·10 12 cm 3 /mol·sec, T =1700°K

Excitation of iodine by singlet molecular oxygen. Part 1.—Mechanism of the I2chemiluminescence

The strong I2 chemiluminescence produced when iodine is added to oxygen gas containing singlet molecular oxygen is shown to arise by efficient stepwise excitation I2(X1Σ+g)+O2(1Σ+g)→I2(A3π1u)+O2(3Σ–g)(2), I2(A3π1u)+O2(1Δg)→I2(B3π0+u)+ O2(3Σ–g)(3)Process (2) also yields atomic iodine which recombines to I2 on the flow tube walls. Approximate rate coefficients are derived for these processes.

The separation of polar and steric effects. Part VI. The kinetics of the acid-catalysed esterification of substituted benzoic acids by methanol

Rate coefficients have been determined for the acid-catalysed esterification of benzoic acid and of o-XC6H4·CO2H (X = F, Cl, Br, I, Me, Et, or Pri) by methanol, with toluene-p-sulphonic acid as catalyst, at several temperatures. The acids m-XC6H4·CO2H (X = Cl, Me, or NO2) have been similarly studied at 60°, and approximate rate coefficients have been determined for the esterification of o-t-butylbenzoic acid at 100 and 120°.Arrhenius parameters and the related quantities of transition-state theory have been evaluated for the reactions of the ortho-substituted acids. The effect of ortho-substitution on the entropy of activation (ΔS‡) in esterification is discussed in terms of a ‘bulk effect’ which tends to decrease ΔS‡, and steric inhibition of solvation of the transition state which tends to increase ΔS‡. All the ortho-substituents studied (except t-butyl) depress the enthalpy of activation (ΔH‡) relative to that for the reaction of the parent acid and two effects are tentatively suggested to account for this, namely, the secondary steric effect of ortho-substituents, and a steric inhibition of ‘ring solvation’ of the initial state, which may be brought about by substituents in any position in the ring.Taft steric substituent constants (Es) have been evaluated for the ortho-substituted acids, and these are compared with values obtained from other systems. The differences found are discussed in terms of the Hammett reaction constant (ρ) and of the nature of the solvent.It is also concluded that results from the kinetics of esterification cannot replace those from acid-catalysed ester hydrolysis for the purpose of accurately satisfying the conditions of the Taft relation.