Molecular size dependent falloff rate constants for the recombination reactions of alkyl radicals with O₂ and implications for simplified kinetics of alkylperoxy radicals

Abstract

The recombination reactions of prototypical alkyl radicals (R) with O 2 , R + O 2 → RO 2 , have been investigated theoretically by using the variational transition state theory and Rice-Ramsperger-Kassel-Marcus theory/master equation calculations based on the CASPT2(7,5)/aug-cc-pVDZ//B3LYP/6-311G(d,p) potential energy curves and B3LYP/6-311G(d,p) geometries and vibrational frequencies. The calculated high-pressure limiting rate constants well reproduced the experimental room temperature rate constants for ethyl, i -propyl, n-butyl, s -butyl, and t-butyl radicals and were nearly the same for the same class of alkyl radicals, namely for primary (ethyl and n-butyl) and for secondary (i -propyl and s -butyl) radicals. Hypothetical falloff calculations ignoring the subsequent dissociation/isomerization reactions of RO 2 indicated that the low-pressure limiting (LPL) rate constants increase monotonically and systematically with the size of the molecule and are almost identical for the same-sized C 4 alkyl radicals, n-butyl, s -butyl, and t-butyl. With the extended calculations for 1-hexyl (C 6 H 13 ) and 2,4,4-trimethylpentyl (C 8 H 17 )+ O 2 based on the B3LYP/6-311G(d,p) calculations and estimated potential energy curves, class-specific molecular size dependent falloff rate expression was proposed. The analysis of the unexpected behavior of the LPL rate constants for large alkyl

A comprehensive experimental and modeling study of isobutene oxidation

“…The rate constants for the reaction channels are taken by analogy to rate constants from Miyoshi [56] for the entrance channel of the second (hydroperoxyl-alkyl) radical addition to molecular oxygen and Goldsmith et al [57] for the following isomerization and decomposition reaction channels. …”

Revisiting the Kinetics and Thermodynamics of the Low-Temperature Oxidation Pathways of Alkanes: A Case Study of the Three Pentane Isomers

“…Rate rules made for this purpose have been developed by calculation of high-pressure limit rate coefficients for many reactions relevant to the low-temperature oxidation of alkanes (straight-and branched-chained) in several recent studies. [12][13][14]16,17 However, in this work we aim to develop new rate rules, taking into account the rate coefficients calculated in all of these studies, as well as from others, which have used various levels of quantum theory to obtain their respective results. The rate coefficients considered for development of the rate rules were compared over a temperature range of 500-1250 K (in 50 K increments).…”

“…[12][13][14][15][16][17] These studies have calculated high-pressure limit rate coefficients for sets of training reactions so that they can be used directly in chemical kinetic mechanisms for combustion relevant applications. To our knowledge, these rate coefficients have not been applied to all of the important reaction classes in the low-temperature regime in a hydrocarbon oxidation mechanism.…”

Importance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

“…istry to the formation of tropospheric pollutant ozone (O 3 ). Because high levels of OH radicals were observed in field experiments in mainly forested environments with large concentrations of isoprene (Tan et al, 2001;Ren et al, 2008;Hofzumahaus et al, 2009;Kubistin et al, 2010;Whalley et al, 2011), a large number of investigations over the last decade focused on OH-initiated isoprene chemistry, including laboratory and chamber studies (Crounse et al, 2011;Berndt, 2012;Wolfe et al, 2012;Fuchs et al, 2013;Teng et al, 2017;Berndt et al, 2019), theoretical calculations (Peeters et al, 2009(Peeters et al, , 2014Da Silva et al, 2010;Peeters and Müller, 2010;Wang et al, 2018;Møller et al, 2019), and global model impact (Lelieveld et al, 2008;Taraborrelli et al, 2012;Bates and Jacob, 2019;Møller et al, 2019;Müller et al, 2019). The observed OH levels could only be explained if an OH radical regeneration mechanism exists independently of NO and thus without the formation of O 3 .…”