Experimental and Computational Study of the OH−Isoprene Reaction:  Isomeric Branching and Low-Pressure Behavior

Abstract

The kinetics of the isoprene−OH reaction have been studied both experimentally and computationally. Experimental rate constants at pressures in the range 0.5−20 Torr have been determined at 295 K using pulsed photolysis/laser-induced fluorescence detection of the OH radical. A rate constant of (0.99 ± 0.05) × 10-10 molecules-1 cm3 s-1 at 20 Torr in argon was determined, which is consistent with previous results for the high-pressure limiting rate constant. We present the first experimental observation of the falloff region for this reaction and have modeled the pressure dependence of the rates using the Troe formalism. Canonical variational transition state theory calculations were performed on the basis of recent ab initio calculations to determine the relative branching among the four possible isoprene−OH adducts in the high-pressure limit. We find OH addition to the outer carbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure dependence of the overall rate and the rates for the individual isomers in the pressure range 0.25−1000 Torr. The excellent agreement between the calculated and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mechanisms for oxidation of isoprene in the troposphere.

Atmospheric Oxidation Mechanism of Isoprene

“…1). The reaction rate constant of 1.0 × 10 −10 cm 3 molec −1 s −1 has been measured experimentally and calculated theoretically, [9,10,17] and the relative isomeric branching ratios have been predicted as 0.56:0.02:0.05:0.37 for ISOA:ISOB:ISOC:ISOD (Table 2). [17] Under atmospheric conditions, ISOA and ISOD react primarily with O 2 , each forming a β-and a δ-hydroxyperoxy radical, because of the presence of two addition centres (β or δ to the OH position) in those two hydroxyalkyl radicals.…”

“…Experimental studies have investigated the kinetics and product yields of the OHisoprene reactions. [9][10][11][12][13][14][15] Theoretical calculations employing quantum chemical methods and kinetic rate theories have predicted the reaction rate constants and isomeric branch ratios of the OH-isoprene reaction and several intermediate steps. [16][17][18][19][20][21][22][23][24][25] The peroxy radical kinetics, especially the 'self 'reactions between the peroxy radicals, were experimentally studied by Jenkin et al [26] The work by Zhang et al investigated the detailed mechanism of isoprene ozonolysis employing theoretical calculations.…”

Photodegradation of SF6 on polyisoprene surface: Implication on elimination of toxic byproducts

“…The H 2 O is dissociated into hydrogen atom and hydroxyl while irradiated [25]. Because hydroxyl has a tendency to obtain electrons, it may react with reductive radicals through hydroxylation [26][27][28]. The inhibition of degradation can be explained by the combined effect of the competition of H 2 O for UV energy and the interaction between hydroxyl and reductive radicals.…”

Kinetics in Cold Laval Nozzle Expansions: From Atmospheric Chemistry to Oxidation of Biomolecules in the Gas Phase

“…Several laboratory experiments have reported temperature-dependent rate constants of the reaction of isoprene with OH and have identified the major reaction products. [56][57][58] Recent laboratory measurements of the rate constant of the reaction displayed a negative temperature dependence between 250 and 400 K, which continues down to temperatures around 100 K and then decreases somewhat. [17,59] The reaction of OH radicals with propene was investigated by Tully and Goldsmith at high temperatures [60] and recently by Leone's group [16] and the Gçttingen group in the temperature range 60 and 296 K. [17] The negative temperature dependence for both reactions, namely OH + isoprene and OH + propene can be rationalised by inspecting Figure 6 and following the same argumentation as for the reaction of OH + ethylene, highlighted in some detail above.…”