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Overall Rate Constant Measurements of the Reactions of Alkene-Derived

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Overall Rate Constant Measurements of the Reactions of Alkene-Derived RESEARCH PAPER www.rsc.org/pccp Overall rate constant measurements of the reactions of PCCP alkene-derived hydroxyalkylperoxy radicals with nitric oxide Angela M. Miller, Laurence Y. Yeung, Annastassja C. Kiep and Matthew J. Elrod* Department of Chemistry, Oberlin College, Oberlin, OH 44074, USA. E-mail: [email protected]; Fax: þ1 801 697 4917; Tel: þ1 440 775 6583 Received 11th February 2004, Accepted 11th March 2004 F|rst published as an Advance Article on the web 16th April 2004 The overall rate constants of the NO reaction with hydroxyalkylperoxy radicals derived from the OH-initiated oxidation of several atmospherically abundant alkenes—ethene, propene, 1-butene, 2-butene, 2-methyl propene, 1,3-butadiene, and isoprene (2-methyl-1,3-butadiene)—were determined using the turbulent flow technique and pseudo first-order kinetic conditions with high pressure chemical ionization mass spectrometry for the direct detection of hydroxyalkylperoxy radical reactants. The individual 100 Torr, 298 K rate constants for each alkene system were found to be identical within the 95% confidence interval associated with each separate measurement. The average overall rate constant for the reaction of all alkene-derived hydroxyalkylperoxy radicals with NO was determined to be 9.1 Æ 0.5 (2s) Â 10À12 cm3 moleculeÀ1 sÀ1. Previous studies of this reaction for the ethene, 2-methyl propene and isoprene systems reported varying rate constant values that ranged from a factor of two lower to a factor of two higher than the present result. However, the invariant nature and value of the present rate constant determination closely parallels the results obtained in the measurement of rate constants for the reactions of several alkane-derived peroxy radicals with NO. Introduction which is generally followed by reaction with nitric oxide: 0 The reactions involved in the oxidation of alkenes are known R CHðOHÞ CHðO2Þ R þ NO 1 to be a major source of tropospheric ozone. Atmospheric 0 ! R CHðOHÞ CHðOÞ R þ NO2 ð3aÞ concentrations of alkenes are significantly impacted by both 0 anthropogenic and biogenic emissions. Alkenes are important ! R CHðOHÞ CHðONO2Þ R ð3bÞ constituents of fuels and automobile exhaust,2,3 as well as The formation of NO in reaction (3a) leads to ozone produc- other industrial and agricultural (biomass burning) processes.4 2 tion, while the formation of the hydroxynitrate in reaction (3b) Biogenic sources of alkenes include emissions from vegetation, terminates the oxidation cycle. soils and the oceans.5 In rural areas, isoprene (2-methyl-1, Despite the importance of alkenes to ozone formation in 3-butadiene), which is emitted by deciduous trees, is one of both urban and rural locations, the oxidation processes of the most abundant non-methane hydrocarbons (up to 1/3 of alkenes have received less detailed scrutiny than the corre- the total organic content of the atmosphere1). Due to the high sponding processes for the alkane family. There have been a chemical reactivity of isoprene, it plays a large role in the large number of experiments (mostly using the environmental production of ozone in rural locations, and is responsible for chamber technique) in which the end products of the OH- nearly 100% of ozone formation in certain environments.1 initiated oxidation of alkenes in the presence of NO have been The oxidation of alkenes in the troposphere is usually explored. Observations of carbonyl and hydroxynitrate pro- initiated by reaction with OH radicals, which is known to ducts have been interpreted as indicators of the importance proceed primarily through an addition mechanism: of the hydroxyalkylperoxy þ NO reaction in the overall oxida- R CHBCH R0 þ OH ! R CHðOHÞ CH R0 ð1Þ tion sequence. The recent monograph by Calvert et al. pro- vides an excellent review of these studies.1 There have also The two possible hydroxy adduct isomers—with OH adding to 0 been several direct studies of the formation and/or self-reac- either the R or R side of the C=C double bond—are both tion kinetics of hydroxyalkylperoxy radicals derived from the commonly observed. For example, it is observed that OH pre- small alkenes7–10 and some studies of hydroxyalkylperoxy ferentially adds to the terminal carbon of the C=C double radical formation from the oxidation of isoprene.11–14 bond in propene at about a 2:1 ratio as compared to OH addi- 6 To our knowledge, however, there have been only two direct tion at the central carbon. However, in order to simplify and measurements of the reaction kinetics of an alkene-derived clarify the subsequent chemical steps in the oxidation of the hydroxyalkylperoxy radical with NO: 2-methyl propene alkenes, the notation used in this article will arbitrarily specify À12 3 À1 À1 7 (k3 ¼ 4.9 Â 10 cm molecule s ) and isoprene one isomer, with the implicit understanding that the other iso- À12 3 À1 À1 15 (k3 ¼ 9 Â 10 cm molecule s ). There have also been mer is usually present and can undergo the same chemical two indirect measurements of the hydroxyalkylperoxy þ NO transformations (since the final oxidation products are not reaction for the important isoprene system: Stevens and the subject of this article, this notation simplification does co-workers12,16 and North and co-workers17 used indirect not present any ambiguities). The OH adduct rapidly reacts OH-cycling measurements to estimate the rate constant for with O2 to form a b-hydroxyalkylperoxy radical, reaction (3) and derived values of 11 and 25 Â 10À12 cm3 À1 À1 0 0 molecule s , respectively. There is only one other alkene 10.1039/b402110j R CHðOHÞ CH R þ O ! R CHðOHÞ CHðO Þ R 2 2 for which there is even an indirect estimate of the rate constant ð2Þ DOI: for reaction (3). Becker et al. used indirect OH-cycling 3402 Phys. Chem. Chem. Phys., 2004, 6, 3402–3407 This journal is Q The Owner Societies 2004 measurements to estimate the rate constant for reaction (3) for injected at the rear of the flow tube. The gases necessary to ethene-derived hydroxyalkylperoxy radicals and arrived at a generate the hydroxyalkylperoxy radicals were introduced value of 9 Â 10À12 cm3 moleculeÀ1 sÀ1.18 though a 20 cm long 12.5 mm diameter sidearm located at In contrast to the alkene situation, there have been numer- the rear of the tube. NO was added via an encased movable ous direct kinetics studies of the overall rate constant for the injector. The encasement (made from corrugated Teflon tub- reaction of alkane-based peroxy radicals with NO. Based on ing) was used so that the injector could be moved to various these studies, Howard and co-workers concluded that for the injector positions without breaking any vacuum seals. A fan- C1–C5 unsubstituted alkanes, an overall peroxy þ NO rate shaped Teflon device was placed at the end of the injector in constant of about 8 Â 10À12 cm3 moleculeÀ1 sÀ1 at 298 K is order to enhance turbulent mixing. The polonium-210 a-emit- observed, regardless of the length or branching of the alkyl ting ionization source was placed between the temperature chain.19 However, it is not clear from the limited rate constant regulated flow tube and the inlet to the CIMS. Most of the values available thus far for the reaction of alkene-based flow tube gases were removed at the CIMS inlet by a 31 L hydroxyalkylperoxy radicals with NO (which, as described sÀ1 roughing pump. All gas flows were monitored with cali- above, range in value from 4.9 to 25 Â 10À12 cm3 moleculeÀ1 brated mass flow meters. The flow tube pressure was measured sÀ1) that the alkenes possess a similarly unvarying rate upstream of the ionization source using a 0–1000 Torr capa- constant value. Therefore, it is important to perform direct citance manometer. The temperature in the flow tube was measurements of the atmospherically relevant alkene-derived determined using Cu-constantan thermocouples. hydroxyalkylperoxy radicals with NO in order to better define the role of alkenes in tropospheric ozone formation. In this article an investigation of the kinetics of the reaction Overall rate constant determination of several alkene-derived hydroxyalkylperoxy radicals with The hydroxyalkylperoxy species were generated using the NO is described. The measurements were conducted at 100 following reactions: Torr and 298 K under pseudo first-order kinetics conditions using a turbulent flow (TF) tube coupled to a high pressure CF4 ! CF4Àx þ xF ð4Þ chemical ionization mass spectrometer (CIMS), which allowed F þ H2O ! HF þ OH ð5Þ for direct detection of the hydroxyalkylperoxy radicals. The 0 0 NO reaction kinetics of hydroxyalkylperoxy radicals obtained OH þ R CHBCH R ! R CHðOHÞ CH R ð6Þ from the OH-initiated oxidation of ethene, propene, 1-butene, 0 0 R CHðOHÞ CH R þ O2 ! R CHðOHÞ CHðO2Þ R 2-butene, 2-methyl propene, 1,3-butadiene, and isoprene were investigated in this work. These parent alkenes were chosen to ð7Þ allow a systematic investigation of the possible structural and The kinetics measurements were carried out at 100 Æ 1 Torr electronic effects of varying alkene functionality on the rate and 298 Æ 2 K. A dilute mixture of He/CF4 was obtained by constant. In addition, these species include the most atmo- combining a 5.0 STP L minÀ1 flow of helium (99.999%), which spherically relevant alkenes (as a group, the seven alkenes had passed through a silica gel trap immersed in liquid nitro- listed above represent 77% of the total alkene concentration gen, with a 1.0–1.5 STP mL minÀ1 flow of a 2% CF 1 4 typically observed in Los Angeles ).
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