Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

Peroxyl radicals (ROO.) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established ROO. sink reactions. We assume that the photolysis channel is always ROO. + hν => RO + O(3P). Our results show that the maximal value of the cross-section for this channel is σ = 1.3 × 10−18 cm2 at 240 nm for five atmospherically representative peroxyl radicals: CH3OO., C(O)HCH2OO., CH3CH2OO., HC(O)OO. and CH3C(O)OO.. These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10−5 s−1 for all five ROO.. As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D1) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH3OO. to formation of O.H and CH2O or HOO. and CH2 products. While the activation barriers for H-shifts in the D1 state may be lower than in the ground state (D0), we find that H-shifts are unlikely to be competitive with decay back to the D0 state through internal conversion, as this has a rate of the order of 1013 s−1 for all studied systems.


Comments to the Author(s)
This is a logical study that addresses a potentially important process for atmospheric chemistry. The manuscript is generally well written but the conclusions are rather disappointing. Nonetheless, it is important to consider the possible fate of peroxyl radicals and the authors have provided a convincing account of the likely photochemistry. Minor points to consider are: (i) Peroxide radicals in the title should read peroxyl radicals and (ii) the Strickler-Berg expression was not developed for such weakly absorbing species. This might not be a valid application. Finally, peroxyl radicals are good oxidants and are able to abstract hydrogen atoms from many substrates. Are such reactions important here as competing processes?

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
This is a nice and interesting study of the fate of peroxyl radicals in the troposphere, carried out using highly correlated post-HF methods. Particularly, the authors investigate the photolysis channel and come to the conclusion that it is not a competitive process. Subsequently, they study H shift reactions in the D1 excited state, but also in this case they come to the conclusions that they are unlikely to occur due to the fast decay to D0. The paper is well written and the scientific contents is sound. I have no reason to suspect about the correctness of the results, which are rigorously described and all computational strategies are fully and properly justified. Few minor points: -I would use the symbol of radicals in the formulas in the text. Although it is boring to add, it is chemically more correct -I would introduce the H shift investigation in the title too, since it is an important aspect of the work.

Decision letter (RSOS-200521.R0)
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Dear Dr Valiev:
Title: Is Photolysis of Peroxide Radicals a Competitive Pathway in the Troposphere? Manuscript ID: RSOS-200521 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Supplementary files will be published alongside the paper on the journal website and posted on the online figshare repository (https://figshare.com). The heading and legend provided for each supplementary file during the submission process will be used to create the figshare page, so please ensure these are accurate and informative so that your files can be found in searches. Files on figshare will be made available approximately one week before the accompanying article so that the supplementary material can be attributed a unique DOI. ********************************************** RSC Associate Editor: Comments to the Author: The research presented in this draft is original and might be of interest to RSOS audience. Although conclusions are supported by experimental data, the length of the manuscript should be shortened. Some of the material (about the various peroxide radicals) should be presented as an electronic appendix, while the use of radical symbols in the formulas in the text is kindly requested. Additionally, the title should be modified according to the Reviewers suggestions.

RSC Subject Editor:
Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This is a logical study that addresses a potentially important process for atmospheric chemistry. The manuscript is generally well written but the conclusions are rather disappointing. Nonetheless, it is important to consider the possible fate of peroxyl radicals and the authors have provided a convincing account of the likely photochemistry. Minor points to consider are: (i) Peroxide radicals in the title should read peroxyl radicals and (ii) the Strickler-Berg expression was not developed for such weakly absorbing species. This might not be a valid application. Finally, peroxyl radicals are good oxidants and are able to abstract hydrogen atoms from many substrates. Are such reactions important here as competing processes?
Reviewer: 2 Comments to the Author(s) This is a nice and interesting study of the fate of peroxyl radicals in the troposphere, carried out using highly correlated post-HF methods. Particularly, the authors investigate the photolysis channel and come to the conclusion that it is not a competitive process. Subsequently, they study H shift reactions in the D1 excited state, but also in this case they come to the conclusions that they are unlikely to occur due to the fast decay to D0. The paper is well written and the scientific contents is sound. I have no reason to suspect about the correctness of the results, which are rigorously described and all computational strategies are fully and properly justified. Few minor points: -I would use the symbol of radicals in the formulas in the text. Although it is boring to add, it is chemically more correct -I would introduce the H shift investigation in the title too, since it is an important aspect of the work.

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
The revision of the manuscript is satisfactory. Please notice that there is a mistake in the title, which should read '... A Pathway....' and not '..A Pathways...'

Decision letter (RSOS-200521.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below. Comments to the Author(s) The revised manuscript addresses the concerns raised earlier and is now suitable for publication.

Reviewer: 2
Comments to the Author(s) The revision of the manuscript is satisfactory. Please notice that there is a mistake in the title, which should read '... A Pathway....' and not '..A Pathways...'