Theoretical Study of the Alkoxy Radicals Derived from Isoprene:  Pressure- and Temperature-Dependent Decomposition Rates

Abstract

Pressure-and temperature-dependent thermal decomposition rates for the primary alkoxy radicals derived from OH-initiated oxidation of isoprene have been calculated. The master-equation calculations are based on ab initio energetics and structures previously reported. We also provide analytical descriptions of the theoretical falloff curves using the semiempirical Troe formalism. Additional calculations are also provided for two isomers not previously reported and to further characterize basis set effects on the energetics. The results indicate that the decomposition rates for these radicals should not be considered to be in the high-pressure limit at 760 Torr. Under atmospheric conditions the alkoxy radicals will be activated, and we find that prompt reaction for several isomers may be significant. The results are compared to recent estimates of overall alkoxy radical decomposition rates derived from OH cycling studies.

Isoprene photooxidation: new insights into the production of acids and organic nitrates

“…The reaction of the peroxy radical with NO yields an alkoxy radical which undergoes a δ 5 1 isomerization (Atkinson, 1997;Park et al, 2004). The resulting β-hydroxy allyl radical can then react with O 2 and form a 1,4-hydroxycarbonyl, (2Z)-4-hydroxy-2-methylbut-2-enal (HC5 Z(1,4)) detected as a cluster at m/z=185 (Fig.…”

“…In this study, we assume that channels (2, 1) (respectively (3, 4)) yield MVK and MACR. Park et al (2003) proposed that the radicals formed in these channels undergo a cyclization, thus reducing the yield of MVK and MACR. The yield of the nitrates which should originate from the hydrocarbons proposed by Park et al (2003) is too small to provide conclusive experimental evidence in favor or against this mechanism.…”

“…Park et al (2003) proposed that the radicals formed in these channels undergo a cyclization, thus reducing the yield of MVK and MACR. The yield of the nitrates which should originate from the hydrocarbons proposed by Park et al (2003) is too small to provide conclusive experimental evidence in favor or against this mechanism. Given the small combined yield of these channels, this uncertainty remains small compared to the ones affecting the major channels.…”

OH/OD Initiated Oxidation of Isoprene in the Presence of O2 and NO

“…Simulations of the cycling data of Reitz et al 25 suggested that the majority of hydroxy alkoxy radicals underwent thermal decomposition, in contrast to the conclusion from the theoretical study. 36 The conclusions of the early cycling experiment suffered from an overly simplified chemical mechanism that did not include important termination reactions, notably the reaction of the isoprene-OH adduct with NO. Termination at high NO concentrations was interpreted by the authors as arising from the competition between alkoxy radical decomposition and reaction with NO.…”

Atmospheric Oxidation Mechanism of Isoprene

“…[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. [27,28] Recent studies investigated the NO 3 -isoprene reaction by combining experimental and theoretical studies.…”