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| 06 Jan 2025
The impact of unimolecular reactions on the possibility of acyl peroxy radical initiated isoprene oxidation
Abstract. The unimolecular H-shift and endoperoxide ring formation reactions were studied for several different acyl peroxy radicals (APRs) using quantum-mechanical methods. Also, for structures with slow unimolecular reactions, accretion reactions with isoprene were investigated. The reaction rate coefficients were calculated at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31+G* level using multi-conformer transition state theory. Unimolecular reactions of acyl peroxy radicals were shown to have rate coefficients up to 0.1 s−1 and bimolecular accretion reactions with isoprene up to 10−15 cm3 s−1. Both smaller and larger acyl peroxy radicals with rigid structures were observed to be more likely to initiate accretion reactions with isoprene because of their inability for fast unimolecular reactions. The pseudo-first-order reaction rates were calculated for accretion reactions of isoprene with OH and five APRs at four different temperatures. The significance of ARP-initiated isoprene oxidation was shown to increase with increasing temperature. APR-initiated oxidation could lead to dimeric products with atmospheric impact through new-particle formation.
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RC1: 'Comment on ar-2024-38', Anonymous Referee #1, 10 Jan 2025
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This manuscripts describes a quantum chemical and theoretical kinetic study of acylperoxy radicals (APR), examining both unimolecular reactions (H-migrations and ring closures), and addition reactions on the double bonds in isoprene. This topic is timely as there is significant interest at the moment for reactions forming low-volatility oxygenates in the atmosphere as contributors to aerosols. The accretion reactions of APR with olefines would lead to such low-volatility oxygenates with further options for autoxidation. The methodology is state of the art and expected to yield reliable results; the rate predictions are directly usable in modeling studies.
I support publication of this paper, but suggest the authors consider the comments below.
Main comments
The literature cited in this paper suggest that the reactions of APR are a recent topic. However, I feel the literature cited in this work does not do justice to the already existing body of data on APR reactions. The unimolecular and bimolecular reactions of APR have been studied for decades, and they were not included in atmospheric models because they have a negligible impact on the chemistry of RO2 and VOCs. The rekindled interest in these reactions stems mostly from the realization that organic aerosol formation and growth is governed by reactions with yields in the single-percentage range, and even less (see e.g. HOM yields from monoterpenes). See below for some suggestions of additional relevant literature; a comparison to earlier experimental data might be in order.
Modern measurements of total RO2 concentrations in the atmosphere yield values around 1E8 cm-3, of which CH3O2 is the main contributor. This study assigns a concentration of 1E8 cm-3 to all APR, even the more exotic ones like Ben-APR and Naf1-APR with the highest rate coefficients with isoprene. At the same time, the study uses a low rate coefficient for isoprene + OH, where the latest IUPAC recommendation, k(T) = 2.1E-11 exp(465/T), with [OH]=1E6 cm-3 yields a pseudo-first order rate k(323K) = 9E-5 s-1, roughly 3 times higher than the value used by the authors. Combined, these choices provide a very generous "upper limit" for the impact of APR accretion reactions of 1% at 323 K; my own estimates would be at least an order of magnitude lower. Both still confirm earlier conclusions that such accretion reactions have negligible impact on both RO2 and VOC in the atmosphere. However, that also misses the point of why these reactions are studied: what is interesting is whether these reactions can contribute meaningfully to low-volatility HOMs formation. To assess that, the authors need to look at the expected yields from the studied reactions, against the measured yield of HOMs from the most common VOCs (isoprene but also a-pinene, limonene,...) which typically is only a few % of the turnover even for the most favorable ones. I suggest the authors include such a comparison.
Minor comments
p. 1, Title: The subphrase "the possibility of" could be removed.
p. 1 line 12: "biomolecular reactions with hydroxyl radicals".
While RO2 + OH does contribute somewhat to RO2 loss (see work by Assaf and Fittschen), it is mostly the HO2 (hydroperoxyl) forming ROOH that is relevant in the atmosphere. When it comes to atmospheric chemistry of RO2, perhaps the overview paper by Jenkin et al. might be a good alternative reference (DOI 10.5194/acp-19-7691-2019)p. 1, line 21: "However, the direct APR formation route from aldehydes is the dominant one, since the removal of an aldehydic H atom is much faster than the removal of a non-aldehydic H atom (Barua et al., 2023)."
This has been known for a long time. Barua et al. is not the optimal reference for that statement.p. 1, line 23: "A recent study by Pasik et al. (2024a) showed that APRs..."
Rate data has been available for much longer. This needs a reference to Nozière and Fache 2021, as well as the rate data in Stark et al 1997 (irrespective of the expected products), and there is more literature available.p. 2, line 33: "...through endoperoxide ring formation reactions (Nozière and Vereecken, 2024)."
Another paper of interest could be DOI 10.1039/d1cp02758a, 2021, and perhaps other work cited therein. It would also be beneficial to compare somewhere the APR ring closure reactions described here against the olefinic RO2 ring closure described in the suggested paper to quantify the effect of the acyl group on these reactions.p. 3, line 63: "This is because the saddle point is very shallow or might not even exist (see Supplementary Information Figures S2 and S3 for PES graphs)."
The reaction of OH with double bonds is typically described as a 2-TS system where a pre-reactive complex is formed first (the H-atom facing the double bond). E.g. Greenwald et al, 2005, DOI 10.1021/jp058041a and especially 10.1021/jp071412y are relevant. Comparison to this earlier work, and subsequent work by other authors, is necessary. At least it needs to be stated explicitly whether wB98X-D gives a pre-reactive complex or not.p. 3, line 64: "the M06-2X functional"
This is a bit an unfortunate choice. For long-range interactions such as low-barrier additions, dispersion should have been included to get the lowest modes correct (as well as e,g, better integration grids), as these modes are the most influential on the partition function. I.e. M06-2X-D3 might have been a better choice, and methodologically better comparable to wB98X-D which also includes dispersion. I recognize it is too late to change this, but perhaps these shortcomings in the methodology choice could be mentioned somewhere.p. 4, line 92: "Only the partition function of the lowest energy conformer of isoprene was used due to the rigid structure."
This reasoning is not always valid. Example: Decker et al. 2017, DOI 10.1039/C6CP08602Kp. 7, line 120: "This result for APRs is possibly due to the π-bond being delocalized between the acyl group and the double bond, which may favor the 4-endoperoxide reaction."
In the TS, the double bonds seem to be not co-planar and thus little to no delocalization seems possible; if anything, the loss of the conjugation of the C=C-C=O bonds in the TS would increase the barrier. No speculation is necessary though: if there is delocalization it should be borne out by the population analysis in the calculations. However, the impact of the different geometric properties of the endocyclic C=O bond, and the presence of an oxygenated group adjacent to the addition site seem more likely contributors than delocalization.p. 9, line 158: "The value calculated in this study does not take into account the other two reaction pathways R2 and R3 which do also contribute to the rate coefficient."
Mention that these contributions are only in the % range (with appropriate reference)Citation: https://doi.org/10.5194/ar-2024-38-RC1 -
RC2: 'Comment on ar-2024-38', Anonymous Referee #2, 16 Jan 2025
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This manuscript investigated unimolecular H-shift and endoperoxide ring formation reactions for a variety of acyl peroxy radicals using quantum-mechanical methods. Bimolecular accretion reactions between isoprene and APRs were also investigated in this study. The temperature dependence of these reactions was also studied and compared to OH-initiated isoprene oxidation. The pseudo-first-order reaction rates were calculated for accretion reactions of isoprene with OH and six APRs at four different temperatures. The significance of APR-initiated isoprene oxidation was shown to increase with increasing temperature. APR-initiated oxidation could lead to dimeric products with atmospheric impact through new-particle formation. I would support this manuscript for publication at Aerosol Research if the authors could carefully address the following comments.
- Line 8: "ARP" should be "APR".
- In the introduction section, the authors introduced the research background and the current research progress, which is comprehensive and easy to follow. However, the difficulties and limitations of existing research are not shown well. Additionally, the significance of the study and its potential environmental impacts are not clearly articulated. These elements should be more explicitly stated to highlight the broader implications of the research.
- The M06-2X functional was used, and all reactant and TS conformers were optimized at the M06-2X/6-31+G* level for the reactions between OH and isoprene. Why was the M06-2X functional chosen for the reactions between OH and isoprene? It is recommended that the authors provide further explanation and clarify why this functional is more suitable.
- It is suggested that the authors analyze the uncertainty of the theoretical calculation results, including parameter and model errors, to evaluate the reliability of the results.
- Line 104: A decomposition of the product results in the formation of ethenone and hydroperoxy radical during the optimization, making it difficult to find the correct TS. How was the correct TS structure attempted to be found during the calculation process?
- This result for APRs is possibly due to the π-bond being delocalized between the acyl group and the double bond, which may favor the 4-endoperoxide reaction. Please consider citing other recent work or adding clarification.
- Line 126: The 1,5 H-shift in met-APR is two orders of magnitude faster than the 1,5 H-shift in iso-APR. How is the difference of two orders of magnitude obtained?
- Line 160: Is it possible to further quantify the relationship between barriers and reaction rates for different APR structures? For example, is there any quantitative barrier difference or other data to support this trend?
- The discussion on the changes in reaction rates at different temperatures was very interesting, especially that the APR reaction rate increases with increasing temperature, while the OH-initiated isoprene reaction rate decreases with increasing temperature. However, the mechanism of the effect of temperature on the reaction rate was not sufficiently elaborated.
Citation: https://doi.org/10.5194/ar-2024-38-RC2 -
RC3: 'Comment on ar-2024-38', Anonymous Referee #3, 20 Jan 2025
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This manuscript uses electronic structure theory and multi-conformation transition state theory to explore the possibility that acyl peroxy radicals (APRs) can undergo bimolecular accretion reactions with isoprene to form adducts that may contribute to atmospheric new particle formation. This study thus has high potential relevance to atmospheric chemistry. The crux of the study is to compare the pseudo-first-order rate constants for accretion to the rate constants for unimolecular APR reactions and to the pseudo-first-order rate constants for isoprene-OH reactions. The electronic structure methods used are reasonable, with the need for, and consequences of, using both ωB97X-D and M06-2X density functionals clearly explained. The results are presented very clearly. I have only two suggestions about the interepretation of the results:
- On p. 7, explain why a very large tunneling factor (~104) may make the Eckart prediction not reliable.
- On p. 8, the trend of decreasing activation barrier with larger APR needs more consideration. First, what is relevant is the stability of the transition state (TS) structure, not the stability of the reaction product, as stated in line 150. Second, what controls the activation barrier is the energetic cost of starting to break a pi bond in isoprene. A bigger APR may indeed be more stable, but that additional stability is present in both the reactant and the TS structure--the difference in reactant and TS energy, which is the activation barrier--should be roughly the same.
Citation: https://doi.org/10.5194/ar-2024-38-RC3
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