Measurements of the kinetics of the OH‐initiated oxidation of isoprene

Abstract

[1] The mechanism of the OH-initiated oxidation of isoprene has been studied at 300 K and 100 and 150 Torr total pressure using a turbulent flow technique coupled with laserinduced fluorescence detection of the OH radical. The rate constant for the reaction of OH with isoprene was measured to be (10.8 ± 0.5) Â 10 À11 cm 3 molecule À1 s À1 at 150 Torr total pressure and independent of pressure between 100 and 150 Torr, in excellent agreement with most previous absolute measurements. In the presence of O 2 and NO, propagation of OH radicals and loss of OH through radical termination resulting from the formation of organic nitrates were measured at 150 Torr total pressure and compared to simulations of the kinetics of this reaction system. The results of these experiments are consistent with an overall rate constant of (1.1 ± 0.8) Â 10 À11 cm 3 molecule À1 s À1 for the reaction of NO with isoprene-based hydroxyalkyl peroxy radicals, with branching ratios of 0.85 ± 0.10 for the bimolecular channel (oxidation of NO to NO 2 ) and 0.15 ± 0.10 for the termolecular channel (formation of organic nitrates). Although the organic nitrate yield reported here is the result of indirect measurements, it suggests that isoprene may be a more significant sink of NO x than previously estimated.

Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

“…The absolute rate constant measurements carried out over the temperature range 231-509 K by Schmidt et al (1985), Droege and Tully (1986b), Abbatt et al (1990), Schiffman et al (1991, Talukdar et al (1994), Donahue et al (1998) and Chuong and Stevens (2002) are in good agreement, with earlier absolute rate measurements of Greiner (1970), Perry et al (1976) and Paraskevopoulos and Nip (1980) at room temperature being ∼10-15% higher than these more recent studies. Figure 7 shows an Arrhenius plot of the absolute rate constants of Droege and Tully (1986b), Abbatt et al (1990), Talukdar et al (1994) and Donahue et al (1998) together with the relative rate data of Baker et al (1970) (as re-evaluated by Baldwin and Walker, 1979), Hucknall et al (1975) and DeMore and Bayes (1999).…”

The OH-Initiated Oxidation of 1,3-Butadiene in the Presence of O2 and NO: A Photolytic Route To Study Isomeric Selective Reactivity

“…The figure demonstrates that the OD concentration profiles depend sensitively on few rate constants, in particular, the relative branching between R-hydroxyalkyl and -hydroxyalkyl radicals (k 1 and k 2 ), 61 the rate of O 2 reaction with R-hydroxyalkyl radicals (k 3 ), the rate constant for the reaction between NO and DO 2 (k 13 ), and the rate between OD and 1-iodo-3-buten-2-ol (k 16 ). The rate constant between NO and DO 2 (k 13 ) is well established as 1.1(0.4 × 10 -11 cm 3 molecules -1 s -1 , 62,63 and we have determined a rate constant of 1.5(0.1 × 10 -11 cm 3 molecules -1 s -1 for the reaction between OD and the photolytic precursor (k 16 ) in the present work. The simulations are relatively insensitive to the remaining rate constants at short times.…”

“…These simulations exhibit an exponential decay of the initial OD, due to the reaction with the precursor, and level off at much longer times due to OD cycling. 10,13,14 Figure 9 shows normalized sensitivity coefficients for all the rate constants in the abridged reaction mechanism evaluated at 30 µs and NO and O 2 concentrations of 6.49 × 10 14 and 8.40 × 10 14 molecules cm -3 , respectively. The figure demonstrates that the OD concentration profiles depend sensitively on few rate constants, in particular, the relative branching between R-hydroxyalkyl and -hydroxyalkyl radicals (k 1 and k 2 ), 61 the rate of O 2 reaction with R-hydroxyalkyl radicals (k 3 ), the rate constant for the reaction between NO and DO 2 (k 13 ), and the rate between OD and 1-iodo-3-buten-2-ol (k 16 ).…”

Experimental and Ab Initio Dynamical Investigations of the Kinetics and Intramolecular Energy Transfer Mechanisms for the OH + 1,3-Butadiene Reaction between 263 and 423 K at Low Pressure

“…The addition of oxygen appears to reduce the reagent-catalyzed loss of OH by inhibiting active wall sites. 30,33,36,37 Pseudo-first-order conditions were maintained during all experiments, and the OH concentrations were kept below 3 × 10 11 molecules cm -3 . 1,3-Butadiene (Sigma-Aldrich, 99%) was used to prepare reagent mixtures (0.08-0.4% of 1,3-butadiene) in approximately 760 Torr of helium using either a 5.5 or 10 L calibrated reservoir fitted with a capacitance manometer.…”