Frank Striebel

The thermal unimolecular decomposition rate constants of ethoxy radicals

We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C2H5O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH3CH2O+M→CH2O+CH3+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k1a,0=[He] 3.3×10-8 exp(-58.5 kJ mol-1/RT) cm3 s-1 and k1a,∞=1.1×1013 exp(-70.3 kJ mol-1/RT) s-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH3CH2O+M→CH3CHO+H+M of k1b,∞=1.3×1013 exp(-84 kJ mol-1/RT) s-1. Atmospheric implications are discussed.

A detailed experimental and theoretical study on the decomposition of methoxy radicals

We present a detailed experimental and theoretical study on the pressure and temperature dependence of the rate constant for the thermal unimolecular decomposition of methoxy radicals, according to CH3O + M → CH2O + H + M. Experimentally, we studied the decomposition of the methoxy radical at temperatures between 680 and 810 K and pressures ranging from 1 to 90 bar helium. The methoxy radicals have been generated by laser flash photolysis of methylbenzoate [C6H5C(O)OCH3] at 193 nm and detected by laser-induced fluorescence. Additionally, we characterized the important features of the potential energy surface by ab initio calculations. The results of these calculations were used to analyze the thermal rate constant applying both the Troe formalism as well as a master equation approach. The following falloff parameters have been extracted: k1,∞ = 6.8 × 1013 exp(−109.5 kJ mol−1/RT) s−1, k1,0 = [He] 1.9 × 10−8(T/1000 K)−2.4 exp(−101.7 kJ mol−1/RT) cm3 s−1 and FC(He) = 0.715–T/4340 K. Additionally, we reanalyzed the literature data for N2 as bath gas and we recommend the following falloff parameters for this: k1,0 = [N2] 3.1 × 10−8(T/1000 K)−3.0 exp(−101.7 kJ mol−1/RT) cm3 s−1 and FC(N2) = 0.97–T/1950 K. In contradiction to earlier studies we did not find any indications that tunneling markedly contributes to the thermal rate constant under our experimental conditions. We calculated the specific and the high-pressure limiting rate constants using RRKM theory and obtained satisfactory agreement with experimental results. We attribute the strong fluctuations of the specific rate constants to be essentially caused by the properties of the density of states. For the β C–H scission reactions in alkoxy radicals we suggest for the high-pressure limiting rate constants a common A factor and activation energy of A = 1013.8 ± 0.3 s−1 and Ea = 94 ± 6 kJ mol−1. Consequently, the reverse reactions, i.e. the H-atom additions to the carbon site of the CO π bond in aldehydes and ketones, always compete with the direct H-atom abstraction.

Shock-Tube Study of the Thermal Decomposition of CH3CHO and CH3CHO + H Reaction

The thermal decomposition of acetaldehyde, CH3CHO + M --> CH3 + HCO + M (eq 1), and the reaction CH3CHO + H --> products (eq 6) have been studied behind reflected shock waves with argon as the bath gas and using H-atom resonance absorption spectrometry as the detection technique. To suppress consecutive bimolecular reactions, the initial concentrations were kept low (approximately 10(13) cm(-3)). Reaction was investigated at temperatures ranging from 1250 to 1650 K at pressures between 1 and 5 bar. The rate coefficients were determined from the initial slope of the hydrogen profile via k1 = [CH3CHO]0(-1) x d[H]/dt, and the temperature dependences observed can be expressed by the following Arrhenius equations: k1(T, 1.4 bar) = 2.9 x 10(14) exp(-38 120 K/T) s(-1), k1(T, 2.9 bar) = 2.8 x 10(14) exp(-37 170 K/T) s(-1), and k1(T, 4.5 bar) = 1.1 x 10(14) exp(-35 150 K/T) s(-1). Reaction was studied with C2H5I as the H-atom precursor under pseudo-first-order conditions with respect to CH3CHO in the temperature range 1040-1240 K at a pressure of 1.4 bar. For the temperature dependence of the rate coefficient the following Arrhenius equation was obtained: k6(T) = 2.6 x 10(-10) exp(-3470 K/T) cm(3) s(-1). Combining our results with low-temperature data published by other authors, we recommend the following expression for the temperature range 300-2000 K: k6(T) = 6.6 x 10(-18) (T/K) (2.15) exp(-800 K/T) cm(3) s(-1). The uncertainties of the rate coefficients k1 and k6 were estimated to be +/-30%.

Complete falloff curves for the unimolecular decomposition of i-propoxy radicals between 330 and 408 K

The temperature and pressure dependence of the rate constant for the unimolecular decomposition of i-propoxy radicals has been determined using the laser photolysis/laser induced fluorescence technique. Important features of the potential energy surface have been calculated by abinitio methods. Experiments have been performed at total pressures between 0.01 and 60 bar of helium and in the temperature range 330–408 K. The low and the high pressure limiting rate constants have been extracted from a complete falloff analysis: k0=[He]×1.0×10-8 exp(-43.8 kJ mol-1/RT) cm3 s-1 and k∞=1.2×1014 exp(-63.7 kJ mol-1/RT) s-1. We estimate an uncertainty for these rate constants of ±30%. Both rate constants have been discussed in terms of statistical unimolecular rate theory. Very good agreement between the calculated and the experimental rate constants has been found.

The thermal unimolecular decomposition of HCO: effects of state specific rate constants on the thermal rate constant

An experimental study on the thermal decomposition of formyl radicals (HCO) under high pressure conditions is presented for the first time. The experimental conditions covered in this study range from 1 to 140 bar of helium and temperatures between 590 and 800 K. Furthermore, the reaction was studied at 700 K and pressures from 1 to 14 bar of nitrogen to obtain information on the kinetics of the reaction under combustion conditions. An analysis of the pressure dependence of the rate constants shows that standard treatments like a Lindemann–Hinshelwood model or a master equation formalism are not able to describe the experimentally observed pressure dependence adequately. An isolated resonance model, however, which is based on calculated resonance lifetimes from H.-M. Keller, H. Floetmann, A. J. Dobbyn, R. Schinke, H.-J. Werner, C. Bauer and P. Rosmus (J. Chem. Phys., 1996, 102, 4983) allows for a description of the observed T- and p-dependence. Based on this model and our experimental data obtained in nitrogen as well as experimental results obtained by R. S. Timonen, E. Ratajczak, D. Gutman and A. F. Wagner (J. Phys. Chem., 1987, 91, 5325) in nitrogen we are able to give a simple expression for the rate constant at temperatures between 500 and 1000 K and pressures between 0.01 and 100 bar: k(T,[M]) = 3.3 × 109 × {[N2]/(1019 cm−3)}0.865 × exp{−(70.15 kJ mol−1)/RT} s−1. The uncertainty of the rate constant calculated by this expression is estimated to be 30%.

Reaction of OH + NO2 + M: kinetic evidence of isomer formation

The objective of this paper is twofold: First, we present new experimental data on the reaction of OH + NO2 at room temperature, which show that the results obtained earlier in our laboratory (see []) are about 30% too high. Secondly, we provide direct experimental evidence, that peroxynitrous acid (HOONO) is formed in the title reaction: At temperatures between 430 and 480 K we observe bi-exponential OH loss over the complete pressure range (1–130 bar) studied, which certainly supports the formation of the HOONO isomer. We determined the equilibrium constant for the OH + NO2 ⇔ HOONO reaction as a function of temperature and extracted from a 3rd-law analysis an O–O bond energy in the HOONO of ΔH°(0 K) = 83 ± 1 kJ mol−1.

A Saturated LIF Study on the High Pressure Limiting Rate Constant of the Reaction CN + NO + M → NCNO + M between 200 and 600 K

The reaction CN + NO + M ↔ NCNO + M was studied in the bath gas helium at temperatures between 200 and 600 K and in the pressure range between 1 and 100 bar. CN radicals were generated by laser flash photolysis of BrCN at 193 nm in the presence of NO and high helium pressures. The concentration of CN radicals was detected by recording their non-resonant fluorescence yield at 420 nm after delayed excitation in the (0, 1)-band of the X2∑+ → B2∑+ transition near 387.8 nm. Thermal second order rate constants were extracted from the fluorescence-yield time profiles under pseudo-first order conditions. We obtained access to wide parts of the falloff range and constructed complete falloff curves using the Troe formalism. We expressed the low and high pressure limiting rate constants as k1,0 = [He] (2.1±0.7)x 10-30 (T/300 K)-(2.1±0.3) cm6 s-1 and as k1,∞ = (5.4±1.4)x 10-11 (T/300 K)(0.0±0.2) cm3 s-1 and gave a broadening factor FC(He) = exp(-T/590 K) + exp(-947 K/T). In separate experiments we monitored the non-resonant fluorescence yield of CN at 418.5 nm in the presence of NO after delayed excitation in the (1, 2)-band of the B2∑+ X2∑+ transition near 386.6 nm. The rate constant for collisional deactivation of vibrationally excited CN radicals in the electronic ground state CN(ν=1) + NO →CN(ν=0) + NO is strongly related to the high pressure limiting recombination rate constant. We determined its rate constant to be k2 = (5.3±1.3)x 10-11 (T/300 K)(-0.2±0.2) cm3 s-1 in excellent agreement with our high pressure limiting recombination rate constant.