the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Nov 2024
Monoterpene oxidation pathways initiated by acyl peroxy radical addition
Abstract. Monoterpenes are released into the atmosphere in significant quantities, where they undergo various oxidation reactions. Despite extensive studies in this area, there are still gaps that need to be addressed to fully understand the oxidation processes occurring in the atmosphere. Recent findings suggest that reactions between alkenes and acyl peroxy radicals can be competitive under atmospheric conditions. In this study, we investigate the oxidation reactions of seven monoterpenes with acyl peroxy radicals and the subsequent diverse reactions, including accretion, alkyl radical rearrangements, H-shift, and β-scissions reactions. The accretion reaction leads to the release of excess energy, which is sufficient to open small secondary rings in the alkyl radical structures. This reaction is most significant for sabinene (39 %) and α-thujene (20 %). A competing reaction is O2 addition, which the majority of alkyl radicals undergo, subsequently leading to the formation of peroxy radicals. These then react further, forming alkoxy radicals that can subsequently undergo β-scission reactions. Our calculations show that for β-pinene, camphene, and sabinene, β-scission rearrangements result in radicals capable of undergoing further propagation of the oxidative chain, while for limonene, α-pinene and α-thujene, scissions leading to closed-shell products that terminate the oxidative chain are preferred. The results indicate that if reactions of monoterpenes with acyl peroxy radicals are indeed competitive under atmospheric conditions, their oxidation would lead to more oxygenated compounds with a higher molar mass, potentially contributing to secondary organic aerosol yields. Moreover, this study highlights the significance of stereochemistry in controlling the oxidation of monoterpenes initiated by acyl peroxy radicals.
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RC1: 'Comment on egusphere-2024-3464', Anonymous Referee #1, 03 Dec 2024
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In this manuscript, the authors provide quantum chemical and theoretical kinetics data that demonstrate the diversity of monoterpene + acylperoxy radical reactions. The authors present a very clear exposition of the ring-opening, hydrogen-shift, and beta-scission reactions to expect for each of seven terpenes considered in the manuscript. The manuscript makes frequent, relevant references to previous mechanistic studies and provides solid evidence for which terpenes may be expected to generate significant secondary organic aerosol due to initial attack by acylperoxy. This is a valuable contribution to the atmospheric chemistry literature.
I have three minor scientific issues for the authors to consider. First, as presented in Table 1, the rates of ring-opening rearrangements as predicted by RRKM theory are higher than those predicted by transition state theory for beta-pinene, sabinene, and alpha-thujene. Clearly, then, these acylperoxy-terpene adducts are chemically activated. Thus, there is the possibility that the yields of ring-opened products, as depicted in Figure 4, may be pressure-dependent. The authors should address this possibility. Second, on p. 11, the authors compare the rates of 5-membered-ring vs. 6-membered-ring endoperoxide cyclizations. The fact that the 5-membered-ring process involves the formation of a presumably more stable tertiary carbon-centered radical should be considered as a reason for the lower activation barrier. Finally, the focus on the conversion of peroxy radicals to alkoxy radicals, starting on p. 11, is understandable and justified, but it would be good to acknowledge the competing pathway of RO2 + HO2 -> ROOH + O2.
Technical corrections: (1) In Figure 9, the rightmost structure should have two C=O double bonds, but the C=O next to the radical center is hard to discern. (2) In Figure 14, the rightmost block of text is almost too small to be legible.
Citation: https://doi.org/10.5194/egusphere-2024-3464-RC1 -
RC2: 'Comment on egusphere-2024-3464', Anonymous Referee #2, 06 Dec 2024
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The manuscript describes a theoretical study of the very complex reaction mechanisms arising from addition of the CH3C(O)OO radical (APR) to in total seven different monoterpenes (MT) in presence of O2. Particular emphasis is laid on the kinetics of the competition between unimolecular reactions (bond dissociation, hydrogen shift, and ring opening) of the intermediarily formed alkyl-type radicals and their addition of O2. The work seems in part to supplement and extend an experimental study published very recently by three of the authors jointly with three other researchers (Pasik 2024a). In the current study, advanced quantum chemical methods were used to calculate molecular and transition state properties, and statistical rate theory was applied to derive rate constants and branching ratios. In general, all the methods used are state-of-the-art and appear adequate. The mechanisms derived and the calculated kinetic data are discussed in great detail, including even stereochemical aspects. Conclusions about the atmospheric relevance of these oxidation pathways are also discussed.
So, all in all, this is a very laborious work exploring a special branch of atmospheric monoterpene oxidation. Whether this branch is really important has still to be shown and depends essentially on the actual atmospheric APR concentration, a point which is also mentioned by the authors. Because also important general mechanistic and kinetic information on atmospheric terpene oxidation is provided, the manuscript can generally be recommended for publication in ACP.
There is however one important point that should be addressed by the authors before final acceptance: The presentation of the kinetics calculations should be improved. From the current version of the manuscript, it is very difficult to detect, which method was applied to which reaction step: Obviously, the bimolecular reaction APR + MT was characterized by MC-TST, eq. (1). The rate constant of the subsequent ring-opening reaction was calculated by both TST-2, eq. (2), and RRKM (not specified, probably within MESMER). Here the APR-addition reaction (APR + MT?) was simulated using ILT (line 108). Does this rate coefficient grossly agree with the value obtained from eq. (1)? What was the rationale for choosing 1x10-12 cm3 molecule-1 s-1 for O2 addition? What does kTST-1 (Table 1) characterize: APR + MT or alkyl + O2 or ring-opening product + O2? This is difficult to find out. It is reasonable that kRRKM (should it not better read kMESMER?) in Table 1 is larger than kTST-2. But this is not always the case, why? In Figure 4, species profiles are plotted; what is the difference between RO-APR and ring-opened RO-APR?
All these points (which can also not be resolved by looking into the supplemental material or into reference Pasik 2024a) should be presented a bit more clearly and more conclusively, maybe within a graphical scheme. This would help the reader of this otherwise convincing paper very much.
Some minor, more formal points are:
general: The authors should consider using the term ‘acetyl peroxyl” instead of ‘acyl peroxyl’ because the latter may suggest RC(O)OO whereas only CH3C(O)OO was studied in the present work.
line 37: ‘unique’ should probably better read ‘specific’
line 44: insert [X] ‘… and [this] reaction … to lead [to] epoxides …’
line 45 and elsewhere: for elementary reactions, the term ‘accretion’ is very unusual, it should better read ‘association’ or ‘addition’
line 73: What was done with ‘squared rotational constant values’?
line 86: Different conformers of the acyl peroxyl radical were accounted for. What about possible conformers of the monoterpenes (alpha-thujene, sabinene)?
line 104: ‘reaction dynamics’ should read ‘reaction kinetics’
Figure 2: please check the meaning of RO2 above the third arrow.
Citation: https://doi.org/10.5194/egusphere-2024-3464-RC2
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