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
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 -
AC1: 'Reply on RC1', Dominika Pasik, 01 Jan 2025
We thank the reviewer for their comments and valuable insights. The manuscript has been revised to address the points raised, which have clarified the study.
Regarding the first comment (pressure dependence):
We fully agree with this observation, and the following has been added to the manuscript:
“It is worth noticing that the considered ring-opening reaction might be pressure dependent, as acetyl peroxy-terpene adducts are chemically activated. However, this was not the focus of the current study. ”
Regarding the second comment (tertiary carbon-centered radical):
We also agree with the insightful observation regarding the formation of a tertiary carbon-centered radical in endocyclization reactions as a reason for the lower barrier compared to the other reaction described on page 11. The following has been added to the manuscript:
“Comparing our results with those of \cite{moller2020double}, the formation of the 5-membered ring is generally faster than that of the 6-membered ring, which may be attributed to the formation of a stabilized tertiary carbon-centered radical in the 5-membered case.”
Lastly, we have updated the manuscript in Section 3.3 to include the following:
“Reaction with \ce{RO2} or \ce{HO2} could also lead to the formation of carboxylic acids and molecular oxygen, following \ce{RO2} + \ce{HO2} \rightarrow \ce{ROOH} + \ce{O2}. However, this process was not investigated in this study.”We also thank the reviewer for pointing out the technical errors, which have now been corrected.
Citation: https://doi.org/10.5194/egusphere-2024-3464-AC1
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AC1: 'Reply on RC1', Dominika Pasik, 01 Jan 2025
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RC2: 'Comment on egusphere-2024-3464', Anonymous Referee #2, 06 Dec 2024
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 -
AC2: 'Reply on RC2', Dominika Pasik, 01 Jan 2025
We thank you for your insightful comments and suggestions, which we have addressed with the following revisions.
Regarding the presentation of kinetic data, the manuscript has been modified to clarify the methodology used. Described variables have been added in brackets at the end of relevant descriptions to help readers follow the text more easily, and these updates have also been incorporated into the workflow figure. Additionally, a reference to the rate of O2-addition (1x10-12 cm3 molecule-1 s-1) has been added. Figure 4 caption has been modified as follows: The red line represents the reactant (monoterpene+CH3C(O)OO.) for ring-opening reaction, the green line shows the formation of the ring-opening reaction product, and the blue line represents the remaining non-ring-opened product that undergoes further oxidation with O2.
We are now using kMESMER instead of kRRKM. The MC-TST rate calculated for 𝛼-pinene is larger than the kMESMER value due to the identification of more 𝛼-pinene-TS conformers, which resulted in a larger partition function contribution. In contrast, the LC-TST rate (details provided in the SI) is smaller than the kMESMERvalue, aligning with expectations. Furthermore, the text has been modified to emphasize that acetyl peroxy radical was used in the studies.
To address the identification of duplicate conformers, squared rotational constant values, along with electronic energy and dipole moments, were used to differentiate and identify duplicates, which were subsequently removed. These variables were chosen because, together, they provide reliable identification of duplicates.
Regarding the use of the term "accretion," we have decided to retain it for consistency with our previous publications. Lastly, the MC-TST equation was modified to account for different conformers of monoterpene reactants in the partition function.
We thank you for pointing this out, and we hope that these revisions make the paper clearer and easier to follow.
Citation: https://doi.org/10.5194/egusphere-2024-3464-AC2
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AC2: 'Reply on RC2', Dominika Pasik, 01 Jan 2025
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