Consecutive Chemical Activation Steps in the OH‐Initiated Atmospheric Degradation of Isoprene: An Analysis with Coupled Master Equations

Abstract

The influence of a two-step chemical activation on 1,5-H and 1,6-H shift reactions of hydroxyl-peroxy radicals formed in the atmospheric photooxidation of isoprene was investigated by means of a master equation analysis. To account for multiple chemical activation processes, three master equations were coupled. The general approach of this coupling is described, and consequences for steady-state regimes are examined. The specific calculations show that chemical activation has no substantial influence on the rate coefficients of the above-mentioned reactions under tropospheric conditions. However, it is demonstrated that high-pressure limits of the thermal rate coefficients instead of the falloff-corrected values have to be used for kinetic modeling. This is a consequence of the continuous population of the high-energy part of the isoprene-OH-O 2 adduct distribution by the forming reactions under steady-state conditions. The rate coefficients of the isomerization reactions at T = 298 K were calculated to be k 3a ∞ = 1.5 × 10 −3 s −1 (1,5-H-shift of the 1,2-isomer) and k 4a ∞ = 6.5 s −1 (1,6-H-shift of the (Z)-1,4-isomer). The calculated value of k 4a ∞ is three orders of magnitude larger than a recently reported experimentally observed rate coefficient for the hydrogen shift reactions of the hydroxyl-peroxy intermediates. It is shown that this discrepancy is in part due to the fact that the experiment does not distinguish between different structural isomers. A comparison of the experimentally determined isotope effect with the calculated value shows a reasonable agreement. C

From theoretical reaction dynamics to chemical modeling of combustion

“…Another such example involves the low temperature oxidation sequence corresponding to the addition of OH to an alkene or alkyne followed by the addition of O 2 to the adduct, which has been shown to have important non-thermal effects under atmospheric conditions [227][228][229][230]…”

Harnessing the Combined Power of Theoretical and Experimental Data through Multiscale Informatics

“…Active parameters were also included to describe uncertainties in the product branching ratios due to model structural uncertainties in the treatment of reaction sequences proceeding through multiple chem-ically activated steps-creating potential for non-Boltzmann reactant distributions (e.g., [17,[78][79][80][81][82][83][84][85][86]). For the remaining reactions, Arrhenius parameters, ln(A), n, and E a , were used as the active parameters.…”

“…In addition to the uncertainties in the calculation of molecular properties discussed above, there are additional uncertainties that can arise in some situations from inadequacy of the employed elementary kinetics model, e.g., the breakdown of standard assumptions in cases where non‐Boltzmann reactant distributions , non‐RRKM , nonadiabatic , coupled torsions , multidimensional tunneling , weak‐collision‐in‐ J energy transfer , or nonlinear mixture rules are important. For example, in previous implementations of the multiscale informatics approach for Cl‐initiated C 3 H 8 oxidation , the structural uncertainty parameter describing uncertainty due to non‐Boltzmann reactant distributions was identified to play a key role in the interpretation of some of the experimental data (γ n‐B in Fig.…”

Temperature- and pressure-dependent kinetics of the competing C–O bond fission reactions of dimethoxymethane

“…45,46 For more details of our specific implementation see ref. [47][48][49] and the literature cited therein.…”