Estimation of rate coefficients for the reactions of O<sub>3</sub> with unsaturated organic compounds for use in automated mechanism construction

Abstract

Abstract. Reaction with ozone (O3) is an important removal process for unsaturated volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of O3 with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of O3 with mono- and poly-unsaturated organic compounds. The methods are optimized using a preferred set of data including reactions of O3 with 221 unsaturated compounds. For conjugated dialkene structures, site-specific rates are defined, and for isolated poly-alkenes rates are defined for each double bond to determine the branching ratios for primary ozonide formation. The information can therefore guide the representation of the O3 reactions in the next generation of explicit detailed chemical mechanisms.

Gas-phase reactivity of acyclic α,β-unsaturated carbonyls towards ozone

“…Gallego-Iniesta and co-workers 36 predicted both to be 10.5 Â 10 À18 cm 3 molecule À1 s À1 using a SAR approach with group-reactivity factors given by Pfrang et al 37 While their predictions overestimate the rate coefficients for methyl 3-methyl-2-butenoate by a factor of 8 they underestimate k(O 3 + MTig) by a factor of 6. A similar divergence is observed using the recent SAR approach by Jenkin et al, 28 which yields a predicted rate coefficient of 6.5 Â 10 À18 cm 3 molecule À1 s À1 for both species. The reason for these differences will be tentatively explained in the following sections.…”

“…Thus none of these approaches captures the significant increase of reactivity compared to the alkene analogue (resulting from replacement of the carbonyl containing substituent by a hydrogen atom) as observed here for methyl vinyl ketone and 3-penten-2-one. By contrast, the very recent SAR method by Jenkin et al 28 predicts both ketones to be more reactive than their alkene analogues. However, the authors stated that, in contrast to the SAR modifications for alkenes, generic rate coefficients had to be assigned for a,b-unsaturated carbonyls (referred to as ''vinylic oxygenated compounds'') simply based on the experimental data of these ketones.…”

“…However, the authors stated that, in contrast to the SAR modifications for alkenes, generic rate coefficients had to be assigned for a,b-unsaturated carbonyls (referred to as ''vinylic oxygenated compounds'') simply based on the experimental data of these ketones. 28 In the liquid phase, a,b-unsaturated carbonyls are known to exhibit a reactivity other than the reactivity of an olefinic bond or a carbonyl group, respectively, but characteristic conjugate addition reactions (see for example: Rossiter and Swinger 45 and references therein). Due to conjugation, the olefinic bond and the carbonyl group are examined as a unit where p-electrons are delocalised thus lowering the electron density in the olefinic bond and reducing the reactivity towards electrophiles.…”

“…If not indicated, the observed rate coefficients are taken from Tables 3 and 4 Jenkin 28 approach. Huge discrepancies are found for substances where Z2-butene is the analogue structure according to method (a) as for ''vinylic esters and acids'' Jenkin and co-workers 28 only differentiate between two categories of substitution patterns, namely ethene and higher substituted alkenes. More experimental data are thus required.…”

“…[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] However, the systematic analysis of their reactivity towards O 3 is at best at the beginning. 28 It is quite common to relate a target compound to the reactivity of its core structure in order to explain and/or predict its behaviour towards one of the atmospheric oxidants. In literature, these core structures were obtained by replacing the substituent of interest with -H, -CH 3 or the elimination of the carbonyl group as the most common approaches.…”