Pascal Devolder

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.

Experimental and Numerical Analysis of a Low Pressure Stoichiometric Methanol-Air Flame

Abstract A stoichiometric methanol-air flame stabilized at low pressure was investigated by coupling gas chromatography and electron spin resonance in order to verify a postulated chemical mechanism for methanol combustion. To interpret experimental investigation two codes, based on the CHEMKIN code package, are used: CALFLA, a code calculating the net reaction rates and PREM1X, the flame modeling code from SANDIA. The experimental and modeled results are compared in terms of species mole fraction profiles and, more significantly, in terms of net reaction rate profiles. Furthermore, ihe formation and consumption rates of different species, as well as a first order sensitivity analysis are used in order to describe the importance of the different reactions throughout the flame. The experimental results are in good agreement with the computed values analyzed in terms of the Dove-Warnatz mechanism.

Absolute rate constants for elementary reactions between chlorine atoms and CHFzC1, CH3CFCI:, CHaCFaC1 and CH2FCF3 at 297± 2 K

The reactions of chlorine atoms with CHF 2 CI, CH 3 CFCI 2 , CH 3 CF 2 CI and CH 2 FCF 3 have been investigated at 297 ± 2 K and 3–6 Torr under pseudo-first-order conditions with an excess of chlorine atoms by means of the discharge flow technique coupled to a mass spectrometer (DF/MS). The following absolute rate constants were obtained in units of cm 3 molecule −1 s −1 : HCFC-22, (1.7±0.2)×10 −5 ; HCFC-141b, (2.1+0.2)×10 −15 ; HCFC-142b, (5.6±2.0)×10 −16 and HFC-134a, (1.6 ± 0.3) × 10 −15 . Quoted errors are ± 2σ. These values are in good agreement with those determined previously by relative or absolute methods. The role of these reactions under tropospheric conditions is discussed.

Rate constant for the reaction OH + H2S in the range 243–463 K by discharge-flow laser-induced fluorescence

The absolute rate constant k1 of the reaction of OH with H2S has been measured in helium in the range 243–363 K by discharge-flow laser-induced fluorescence. A few measurements have also been performed with the same flow tube by resonance fluorescence in the range 294–463 K. The parabolic behaviour of k1vs. temperature, with a minimum value of (33 ± 5)× 10–13 cm3 molecule–1 s–1 at 294 K, is confirmed. Our data above 294 K can be fitted with an Arrhenius form: k1= 1.32 × 10–11 exp [–(394 + 190)/T] cm3 molecule–1 s–1.

Flow tube/LIF measurements of the rate constants for the reactions with NO of CF3O and CF3O2 radicals

Abstract The rate constants for the reactions CF 3 O 2 + NO → CF 3 O + NO 2 (1) and CF 3 O + NO → products (2) have been measured at room temperature using a fast flow reactor associated with a monitoring of CF 3 O radicals by laser induced fluorescence (LIF); for the measurement of k 1 , CF 3 O radicals are prepared by the discharge flow technique in the reactive system F/CF 3 H/O 2 /NO; the average value is k 1 = (1.76 ± 0.35) × 10 −11 cm 3 molecule −1 s −1 . For reaction (2), CF 3 O radicals are prepared by another source: the thermolysis of the dimer CF 3 OOCF 3 and the derived rate constant is k 2 = (4.7 ± 0.9) × 10 −11 cm 3 molecule −1 s −1 . Our results are in reasonable agreement with recent measurements using different techniques.

Reaction of OH Radicals with Acetone: Determination of the Branching Ratio for the Abstraction Pathway at 298 K and 1 Torr

We have determined the 2-oxo-propyl CH3C(O)CH2 (sometimes called 1-methylvinoxy or acetonyl) radical yield for the reaction of acetone with OH radical relative to the 2-oxo-propyl yields for the reactions of F- and Cl atoms with acetone using the Discharge Flow technique. The 2-oxo-propyl radical has been monitored by Laser Induced Fluorescence LIF at short reaction times in the systems: OH + acetone (R1), F + acetone (R2), and Cl + acetone (R3). From these measurements we have deduced the branching ratio for the 2-oxo-propyl radical formation in the title reaction to be in the range 0.8 ≤ R ≤ 1.

Kinetic and mechanistic study of the pressure and temperature dependence of the reaction CH3O + NO

New kinetic measurements for the CH3O + NO reaction have been performed using two different techniques. The discharge flow (DF) technique has been used to investigate the 0.5–5 Torr and 248–473 K pressure and temperature ranges and pulsed laser photolysis (PLP) has been used for the 30–500 Torr and 284–364 K ranges. These new results represent an extension of the pressure and temperature ranges investigated previously. This reaction is known to present two reaction pathways, the association pathway yielding CH3ONO and the disproportionation pathway yielding CH2O+HNO. Based on literature and present experimental data, using the results of ab initio calculations, a multichannel RRKM analysis was developed to interpret the experimental results. This analysis has shown that the disproportionation reaction occurs simultaneously by both a direct hydrogen abstraction reaction, and via the formation of energized CH3ONO* complex in competition with the association reaction. The RRKM analysis, fitted to present and previous data, has yielded a second-order limiting low-pressure value of 2.5 × 10−12 cm3 molecule−1 s−1 at 298 K, with a complex temperature dependence. The limiting high-pressure rate constant derived in the same way is k∞ = (3.4 ± 0.4) × 10−11(T/298)−0.75. The model allows the prediction of CH3O loss rate constants and of the branching ratios in the 1–760 Torr and 220–600 K ranges. For a convenient presentation of the overall rate constant, an analytical expression using the conventional Troe expression with a temperature-dependent addition constant, has been fitted to the results of the RRKM analysis.

Rate constant determinations by laser photolysis/diode laser infrared absorption: examples of HCO+O2→HO2+CO and CH2OH+O2→HCH(O)+HO2 reactions at 294 K

Abstract The reactions of formyl (HCO) and hydroxymethyl (CH 2 OH) radicals with O 2 have been studied at room temperature (294±2 K ) using pulsed laser photolysis (PLP)/tunable diode laser (TDL) absorption spectroscopy. The formation of the stable molecular product, CO or HCH(O), has been monitored. The derived values, k 1 =(5.0±0.7)×10 −12 cm 3 molecule −1 s −1 and k 2 =(10±1.4)×10 −12 cm 3 molecule −1 s −1 , are in excellent agreement with previously published values obtained with different techniques.

Rate constant for the reaction of OH with nitric acid: A new investigation by discharge flow resonance fluorescence

Abstract The absolute rate constant of the title reaction has been measured using the technique of discharge flow resonance fluorescence over the temperature range 253–373 K. An Arrhenius plot of the data below room temperature can be fitted by a negative activation energy E/R = −843 K. The room-temperature value of the rate constant k 1 = (0.93 ± 0.10) × 10 −13 cm 3 molecule −1 s −1 is in excellent agreement with previous discharge flow studies using optical detection but lies significantly below the results from flash photolysis investigations.

Pressure and temperature dependence of the rate constants for the association reactions of vinoxy and 1-methylvinoxy radicals with nitric oxide

The reactions of vinoxy, CH2CHO and 1-methylvinoxy (acetonyl) CH2C(CH3)O radicals with NO and He as a bath gas have been investigated by experimental and theoretical methods as a function of temperature and pressure (0.8–920 mbar). Two classical techniques have been used for the experiments: the discharge flow (DF) and the laser flash photolysis (LFP) techniques, both associated with a monitoring of the radicals by laser induced fluorescence. Calculations of the potential energy surface were performed using ab initio molecular orbital theory at the G2 level. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reactions under investigation. The nitroso-adducts, ON–CH2CHO and ON–CH2C(CH3)O are predicted as the major addition products. Fall-off behavior of the reaction system was analyzed within the Troe formalism. The constructed fall-off curves allow a description of the reaction kinetics in wide ranges of pressure and temperature. The following high and low pressure limiting rate constants have been obtained: