David Gutman

Kinetics of the reaction between oxygen atoms and ethyl radicals

The kinetics of the O + C2H5 reaction have been investigated experimentally in a heatable tubular reactor coupled to a photoionization mass spectrometer. The reactants were generated homogeneously in the reactor by the simultaneous in situ photolysis of SO2 and diethyl ketone. The rate constant was determined from time-resolved measurements of C2H5 decay profiles under conditions of [O] in express [O]/[C2H5]0 > 20. The value of the O + C2H5 rate constant [(2.2 ± 0.4)×10–10 cm3 molecule–1 s–1] was independent of temperature (295–600K) and density [(3–12)×1016 molecule cm–3]. The chemical branching was also characterized both experimentally and theoretically. In the experimental study, the absolute yields of products from three reactive routes were determined and found also to be independent of temperature (298–450K) and density (same range). The branching fractions are 0.32 ± 0.07 for CH2O + CH3, 0.40 ± 0.04 for CH3CHO + H and 0.23 ± 0.8 for C2H4+ OH. In the theoretical study, the master equation describing the many possible intramolecular dynamical processes of the excited C2H5O formed in the initial step of this reaction was solved. RRKM theory was used to obtain the transition probabilities for each step. Potential–energy surface information needed to obtain the transition probabilities was obtained using BAC–MP4 electronic structure calculations. Theoretical branching fractions are in good agreement with those measured for the two major routes. Far less C2H4+ OH is predicted than observed, suggesting that these products are primarily produced by another process, viz. direct metathesis.

Kinetics of the reactions of alkyl radicals (CH3, C2H5, i-C3H7, and t-C4H9) with molecular bromine

The gas-phase kinetics of the reactions of four alkyl radicals (CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with molecular bromine have been studied over the temperature range 296-532 K. The reactions were isolated for quantitative study in a heatable tubular rector coupled to a photoionization mass spectrometer. Radicals were homogeneously generated in the reactor by pulsed photolysis of suitable precursor molecules at 193 or 248 nm. The subsequent decays of the radical concentration in the presence of different Br{sub 2} concentrations were monitored in time-resolved experiments. Rate constants were obtained at five temperatures.

Kinetics of the reaction of the CCl3 radical with oxygen atoms

Abstract The kinetics of the reaction of CCl 3 with O( 3 P) have been investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The decay of CCl 3 was monitored as afunction of O atom concentration ([O] 0 /[CCl 3 ] 0 > 20) to determine the rate constant of the reaction as a function of temperature and pressure. The rate constant measured is independent of density and was correlated by the Arrhenius expression: (2.33 ± 0.74) · 10 −11 exp[(1.66 ± 1.05) kJ mol −1 / RT ] cm 3 molecule −1 s −1 over the temperature range 295–835 K. CCl 2 O was detected as a product of the CCl 3 + O( 3 P) reaction.

Kinetics of the reaction of CH2OH radical with oxygen atoms

Abstract The kinetics of the reaction of CH2OH with O(3P) has been investigated in a heatable tubular reactor coupled to a photoionization mass spectrometer. The decay of CH2OH was monitored as a function of O-atom concentration ([O]0/[CH2OH]0 &>; 20) to determine rate constants as a function of temperature and pressure. The rate constants are independent of density and were fit to an Arrhenius expression: (1.9 ± 0.37) x 10−10 exp [2.0 ± 0.9) kJ mol−1/RT] cm3 molecule−1 s−1 over the temperature range 300–508 K. Both formaldehyde and formic acid were detected as products of the CH2OH + O(3P) reaction.

Kinetics of the reaction between oxygen atoms and propargyl radicals

The kinetics of the reaction between C3H3 (propargyl radical) and O(3P) has been studied using a heatable tubular reactor coupled to a photoionization mass spectrometer. Rate constants were determined in the temperature range 295–750 K, and information on the reaction mechanism was obtained. The reactants were generated homogeneously in the reactor by the simultaneous in situ photolysis of SO2 (the O-atom source) and 1–3 butadiene or propargylbromide (the C3H3 source). Initial conditions were chosen to yield [O] in excess, [O]/[C3H3]o>20. The decay of C3H3 was monitored in time-resolved experiments. The rate constant was found to be independent of temperature, having the value 2.3×10−10 cm3 molecule−1 s−1, and also independent of density in the range used in this study, 6–12×1016 molecule cm−3. The reactivity of C3H3 in this reaction is compared with that of other polyatomic free radicals whose reactions with atomic oxygen have been investigated.

Kinetics and thermochemistry of the oxidation of unsaturated radicals: C4H5+O2

The kinetics and mechanism of the reaction of C 4 H 5 (methylpropargyl radical) with O 2 were investigated from 296 to 900 K in a tubular reactor coupled to a photoionization mass spectrometer. At room temperature the reaction proceeds by a simple pressure-dependent addition reaction. Between 369 and 409 K the equilibrium C 4 H 5 + O 2 ⇄ C 4 H 5 O 2 was clearly observable and equilibrium constants were measured as a function of temperature. These measurements yielded the values of ΔH o 298 (−78±3 kJ mol −1 ) and ΔS o 298 (−122±9 J mol −1 K −1 ). Above 600 K the rate of reaction of methylpropargyl with O 2 is independent of density and increases with temperature with a phenomenological rate constant equal to 6.9×10 −14 exp(−10.5 kJ mol −1 /RT) cm 3 molecule −1 s −1 . A mechanism of the C 4 H 5 +O 2 reaction is proposed which involves initial formation of a C 4 H 5 O 2 adduct. At temperatures above 600 K, decomposition of the chemically activated adduct competes with redissociation to C 4 H 5 +O 2 . The role of elementary reactions between unsaturated radicals and molecular oxygen in combustion processes is briefly reviewed.

Experimental and theoretical investigation of the product channels of the O + CH{sub 3} reaction

The product channels of the O({sup 3}P)+CH{sub 3} reaction was investigated. In the experimental part, the branching fraction for formaldehyde production (O+CH{sub 3}{r_arrow}H{sub 2}CO+H) was measured at room temperature in a tubular flow reactor coupled to a photoionization mass spectrometer. The reactants (CH{sub 3} and O) were generated homogeneously in the reactor by simultaneous {ital in}{ital situ} 193-nm photolysis of acetone and SO{sub 2}. Formaldehyde yield relative to the methyl radicals consumed (branching fraction) was determined to be 1.0{+-}0.15. In the theoretical part, calculations of the energetics of possible decomposition pathways of the energy-rich methoxy radical initially formed in the O+CH{sub 3} reaction indicate that the dominant channel for decomposition is C-H bond cleavage leading to atomic hydrogen and formaldehyde. A possible, minor, secondary channel is hydrogen migration, followed by O-H bond cleavage, leading to the same final products. No energetically competitive pathways leading to H{sub 2}, HCO, HOC, or CO could be found.

Detailed chemical kinetic modeling of the pyrolysis and oxidative pyrolysis of C2H5Cl

A detailed chemical kinetic mechanism describing the pyrolysis and oxidative pyrolysis of C2H5Cl has been developed and tested using experimental data acquired in an atmospheric pressure, one dimensional flow reactor operating at 600–630°C and 0.7–1.17 s residence time. The mechanism involves the participation of 31 species in 73 reversible elementary reactions, and accounts for all the experimental data measured in a one-dimensional flow reactor. The mechanism predicts the increased production of C2H3Cl in the presence of O2, which is caused by the preferential removal of α-H from C2H5Cl by radical attack followed by the abstraction of β-H from CH3CHCl by O2. The major reaction pathways responsible for the formation and destruction of species have been identified via the calculation of reaction rates.

Kinetics and thermochemistry of the equilibria CH2Cl+O2⇔CH2ClO2 and CHCl2+O2⇔CHCl2O2

The equilibria, CH 2 Cl+O 2 ⇔CH 2 ClO 2 and CHCl 2 +O 2 ⇔CHCl 2 O 2 , were studied between 562 and 664 K and 498 and 562 K respectively. Equilibrium constants were measured as a function of temperature. The chlorinated methyl radicals were produced by pulsed UV excimer laser photolysis. The decay of the radicals to their equilibrium concentrations in the presence of O 2 was monitored in time-resolved experiments using photoionization mass spectrometry. The measured equilibrium constants were combined with calculated entropy and heat capacity changes in Third Law calculations to obtain the enthalpy changes (ΔH 298 o ) for the two reactions: −121(±11) kJ mol −1 for the CH 2 Cl+O 2 →CH 2 ClO 2 reaction and −106(±6) kJ mol −1 for the CHCl 2 +O 2 →CHCl 2 O 2 reaction. The results indicate a considerable weakening of the C−O bond in the methylperoxy radical (which is 136 kJ mol −1 ) due to chlorine substitution in the CH 3 group. This effect of chlorine substitution on bond strengths is compared with that found for other compounds of the type CH 3 -X. The implications of weaker R-O 2 bond strengths caused by chlorine substitution in the oxidation chemistry of chlorinated hydrocarbons is briefly discussed. Heats of formation of CH 2 ClO 2 (ΔH f .298 o =−8(±12) kJ mol −1 ) and CHCl 2 O 2 (ΔH f .298 o =−21(±10) kJ mol −1 ) were also obtained using the experimental and theoretical results.