Christa Fittschen

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.

Allylic H-Abstraction Mechanism: The Potential Energy Surface of the Reaction of Propene with OH Radical.

The allylic H-atom abstraction reaction plays a more dominant role, especially at lower temperature, than addition reactions in the case of the CH2 [Formula: see text] CH-CH3 + •OH system. Different computational methods including ab initio as well as density functional methods have been used to examine allylic H-abstraction. Both the energetically less favorable direct H-abstraction and the more favorable indirect H-abstractions have been investigated. Using first principles computations, for the indirect abstraction, a stable π- or reactantlike as well as a late productlike complex were found on the potential energy surface. Based on higher level single point calculations (QCISD(T)/6-311+G(3df,2p)), a new activation enthalpy value, Δ(⧧)H° = 0.3 ± 2 kJ/mol, is suggested for the title reaction. The computed reaction enthalpy ΔrH° = -124.7 ± 2 kJ/mol is in good agreement with the experimental value. The stability of the initial π-complex was found to be ΔH°π-complex = -7.1 kJ/mol. The product complex between the transition state and the product was found with the stability of -127.2 kJ/mol.

The β C–C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323–383 K. The low and the high pressure limiting rate constants as well as the broadening factor Fc have been extracted from a complete falloff analysis of the experimental results: k0=[He]×1.5×10−8 exp(−38.5 kJ mol−1/RT) cm3 s−1, k∞=1.0×1014 exp(−60.5 kJ mol−1/RT) s−1, and Fc=0.87−T/870 K. We anticipate an uncertainty for these rate constants of ±30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C–C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C–C bond scission rate constants of all alkoxy radicals of A=1014±0.3 s−1. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k∞=1.0×1014 exp(−78.2 kJ mol−1/RT) and 1.0×1014 exp(−63.1 kJ mol−1/RT) s−1, respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)2CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol−1, respectively.

Experimental and Modeling Investigation of the Low-Temperature Oxidation of Dimethyl Ether

The oxidation of dimethyl ether (DME) was studied using a jet-stirred reactor over a wide range of conditions: temperatures from 500 to 1100 K; equivalence ratios of 0.25, 1, and 2; residence time of 2 s; pressure of 106.7 kPa (close to the atmospheric pressure); and an inlet fuel mole fraction of 0.02 (with high dilution in helium). Reaction products were quantified using two analysis methods: gas chromatography and continuous wave cavity ring-down spectroscopy (cw-CRDS). cw-CRDS enabled the quantification of formaldehyde, which is one of the major products from DME oxidation, as well as that of hydrogen peroxide, which is an important branching agent in low-temperature oxidation chemistry. Experimental data were compared with data computed using models from the literature with important deviations being observed for the reactivity at low-temperature. A new detailed kinetic model for the oxidation of DME was developed in this study. Kinetic parameters used in this model were taken from literature or calculated in the present work using quantum calculations. This new model enables a better prediction of the reactivity in the low-temperature region. Under the present JSR conditions, error bars on predictions were given. Simulations were also successfully compared with experimental flow reactor, jet-stirred reactor, shock tube, rapid compression machine, and flame data from literature. The kinetic analysis of the model enabled the highlighting of some specificities of the oxidation chemistry of DME: (1) the early reactivity which is observed at very low-temperature (e.g., compared to propane) is explained by the absence of inhibiting reaction of the radical directly obtained from the fuel (by H atom abstraction) with oxygen yielding an olefin + HO2·; (2) the low-temperature reactivity is driven by the relative importance of the second addition to O2 (promoting the reactivity through branching chain) and the competitive decomposition reactions with an inhibiting effect.

Quantification of hydrogen peroxide during the low-temperature oxidation of alkanes

The first reliable quantification of hydrogen peroxide (H(2)O(2)) formed during the low-temperature oxidation of an organic compound has been achieved thanks to a new system that couples a jet stirred reactor to a detection by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared. The quantification of this key compound for hydrocarbon low-temperature oxidation regime has been obtained under conditions close to those actually observed before the autoignition. The studied hydrocarbon was n-butane, the smallest alkane which has an oxidation behavior close to that of the species present in gasoline and diesel fuels.

The reaction of CH3O2 radicals with OH radicals: a neglected sink for CH3O2 in the remote atmosphere.

for CH3O2 in the Remote Atmosphere Christa Fittschen,*,† Lisa K. Whalley,‡,§ and Dwayne E. Heard‡,§ †Universite ́ Lille 1, PhysicoChimie des Processus de Combustion et de l’Atmospher̀e PC2A, Cite ́ Scientifique, Bat. C11, 59655 Villeneuve d’Ascq, France ‡School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, U.K. National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, U.K.

Experimental and modeling study of the oxidation of n-butane in a jet stirred reactor using cw-CRDS measurements

The gas-phase oxidation of n-butane has been studied in an atmospheric jet-stirred reactor (JSR) at temperatures up to 950 K. For the first time, continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared has been used, together with gas chromatography (GC), to analyze the products formed during its oxidation. In addition to the quantification of formaldehyde and water, which is always difficult by GC, cw-CRDS allowed as well the quantification of hydrogen peroxide (H2O2). A comparison of the obtained mole fraction temperature profiles with simulations using a detailed gas-phase mechanism shows a good agreement at temperatures below 750 K, but an overestimation of the overall reactivity above this temperature. Also, a strong overestimation was found for the H2O2 mole fraction at higher temperatures. In order to improve the agreement between model and experimental results, two modifications have been implemented to the model: (a) the rate constant for the decomposition of H2O2 (+M) ↔ 2OH (+M) has been updated to the value recently proposed by Troe (Combust. Flame, 2011, 158, 594-601) and (b) a temperature dependent heterogeneous destruction of H2O2 on the hot reactor walls with assumed rate parameters has been added. The improvement (a) slows down the overall reactivity at higher temperatures, but has a negligible impact on the maximal H2O2 mole fraction. Improvement (b) has also a small impact on the overall reactivity at higher temperatures, but a large effect on the maximal H2O2 mole fraction. Both modifications lead to an improved agreement between model and experiment for the oxidation of n-butane in a JSR at temperatures above 750 K.

Microcontroller based resonance tracking unit for time resolved continuous wave cavity-ringdown spectroscopy measurements

We present in this work a new tracking servoloop electronics for continuous wave cavity-ringdown absorption spectroscopy (cw-CRDS) and its application to time resolved cw-CRDS measurements by coupling the system with a pulsed laser photolysis set-up. The tracking unit significantly increases the repetition rate of the CRDS events and thus improves effective time resolution (and/or the signal-to-noise ratio) in kinetics studies with cw-CRDS in given data acquisition time. The tracking servoloop uses novel strategy to track the cavity resonances that result in a fast relocking (few ms) after the loss of tracking due to an external disturbance. The microcontroller based design is highly flexible and thus advanced tracking strategies are easy to implement by the firmware modification without the need to modify the hardware. We believe that the performance of many existing cw-CRDS experiments, not only time-resolved, can be improved with such tracking unit without any additional modification to the experiment.

Quantification of OH and HO2 radicals during the low-temperature oxidation of hydrocarbons by Fluorescence Assay by Gas Expansion technique

Significance The design of internal combustion engines relies on a good understanding of the kinetic mechanism of the autoignition of hydrocarbons. •OH and •HO2 radicals are known to be the key species governing all stages of the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions has been achieved by coupling the fluorescence assay by gas expansion technique, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. •OH and •HO2 radicals are known to be the key species in the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions (T = 550–1,000 K) has been achieved by coupling a technique named fluorescence assay by gas expansion, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. Calibration allows conversion of relative fluorescence signals to absolute mole fractions. Such radical mole fraction profiles will serve as a benchmark for testing chemical models developed to improve the understanding of combustion processes.

Synchrotron-based double imaging photoelectron/photoion coincidence spectroscopy of radicals produced in a flow tube: OH and OD

We present a microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation. The system has been applied to the study of the photoelectron spectroscopy of the well-known radicals OH and OD. The coincidence imaging scheme provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts. The photoelectron spectra encompassing the X(3)Σ(-) ground state of the OH(+) and OD(+) cations have been extracted and the vibrational constants compared satisfactorily to existing literature values. Future advantages of this approach include measurement of high resolution VUV spectroscopy of radicals, their absolute photoionization cross section, and species/isomer identification in chemical reactions as a function of time.