The gaseous reaction of vinyl radical with oxygen

Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O2 Reaction

The kinetics of (CH₃)₂CCH + O₂ (1) and (CH₃)₂CCCH₃ + O₂ (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k₁(298 K) = (4.5 ± 0.5) × 10^-12 cm³ s^-1 and k₂(298 K) = (8.9 ± 1.0) × 10^-12 cm³ s^-1. The observed products for reactions 1 and 2 were CH₃COCH₃ and CH₃ + CH₃COCH₃, respectively. Substituting both β-hydrogens in the vinyl radical (CH₂CH) with methyl groups decreases the rate coefficient of the CH₂CH + O₂ reaction by about 50%. However, the rate coefficient of the triply substituted (CH₃)₂CCCH₃ radical reaction with O₂ is almost identical to the CH₂CH + O₂ rate coefficient under the covered temperature range.

Kinetics of the Methyl-Vinyl Radical + O2 Reactions Associated with Propene Oxidation

The bimolecular rate coefficients of reactions CH3CCH2 + O2 (1) and cis/ trans-CH3CHCH + O2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures ( T < 420 K) and appear to be independent of temperature at high temperatures ( T > 420 K). Observed products of the reaction CH3CCH2 + O2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O2. Methyl substitution of the vinyl radical changes its reactivity toward O2 upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the β-positions, respectively.

Multichannel Reaction of C2Cl3 + O2 Studied by Time-Resolved Fourier Transform Infrared Emission Spectroscopy

The multichannel reaction of the C(2)Cl(3) radical with O(2) has been studied thoroughly by step-scan time-resolved Fourier transform infrared emission spectroscopy. Vibrationally excited products of Cl(2)CO, CO, and CO(2) are observed and three major reaction channels forming respectively ClCO + Cl(2)CO, CO + CCl(3)O, and CO(2) + CCl(3) are identified. The vibrational state distribution of the product CO is derived from the spectral fitting, and the nascent average vibrational energy of CO is determined to be 59.9 kJ/mol. A surprisal analysis is applied to evaluate the vibrational energy disposal, which reveals that the experimentally measured CO vibrational energy is much more than that predicted by statistical model. Combining previous ab initio calculation results, the nonstatistical dynamics and mechanism are characterized to be barrierless addition-elimination via short-lived reaction intermediates including the peroxy intermediate C(2)Cl(3)OO* and a crucial three-member-ring COO intermediate.

Rate coefficients for the reaction of OH with (E)-2-pentenal, (E)-2-hexenal, and (E)-2-heptenal

Rate coefficients for the gas-phase reaction of the OH radical with (E)-2-pentenal (CH(3)CH(2)CH[double bond]CHCHO), (E)-2-hexenal (CH(3)(CH(2))(2)CH[double bond]CHCHO), and (E)-2-heptenal (CH(3)(CH(2))(3)CH[double bond]CHCHO), a series of unsaturated aldehydes, over the temperature range 244-374 K at pressures between 23 and 150 Torr (He, N(2)) are reported. Rate coefficients were measured under pseudo-first-order conditions in OH with OH radicals produced via pulsed laser photolysis of HNO(3) or H(2)O(2) at 248 nm and detected by pulsed laser-induced fluorescence. The rate coefficients were independent of pressure and the room temperature rate coefficients and Arrhenius expressions obtained are (cm(3) molecule(-1) s(-1) units): k(1)(297 K)=(4.3 +/- 0.6)x 10(-11), k(1)(T)=(7.9 +/- 1.2)x 10(-12) exp[(510 +/- 20)/T]; k(2)(297 K)=(4.4 +/- 0.5)x 10(-11), k(2)(T)=(7.5 +/- 1.1)x 10(-12) exp[(520 +/- 30)/T]; and k(3)(297 K)=(4.4 +/- 0.7)x 10(-11), k(3)(T)=(9.7 +/- 1.5)x 10(-12) exp[(450 +/- 20)/T] for (E)-2-pentenal, (E)-2-hexenal and (E)-2-heptenal, respectively. The quoted uncertainties are 2sigma(95% confidence level) and include estimated systematic errors. Rate coefficients are compared with previously published room temperature values and the discrepancies are discussed. The atmospheric degradation of unsaturated aldehydes is also discussed.

Reaction of vinyl radical with oxygen: A matrix isolation infrared spectroscopic and theoretical study

The reaction of vinyl radical with molecular oxygen in solid argon has been studied using matrix isolation infrared absorption spectroscopy. The vinyl radical was produced through high frequency discharge of ethylene. The vinyl radical reacted with oxygen spontaneously on annealing to form the vinylperoxy radical C(2)H(3)OO with the O-O bond in a trans position relative to the C-C bond, which is characterized by O-O stretching and out-of-plane CH(2) bending vibrations at 1140.7 and 875.5 cm(-1). The vinylperoxy radical underwent visible photon-induced dissociation to the CH(2)OH(CO) complex or CH(2)OH+CO, which has never been considered in previous studies. The CH(2)OH(CO) product was predicted to be more thermodynamically accessible than the previously reported major HCO+H(2)CO channel, and is most likely produced by hydrogen atom transfer from the first-formed H(2)CO-HCO pair in solid argon.