the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
Acid-catalyzed hydrolysis kinetics of organic hydroperoxides: Computational strategy and structure-activity relationship
Abstract. Organic hydroperoxides (ROOHs) are key components of atmospheric aerosols. Determining the acid-catalyzed hydrolysis rate constants (kA) of ROOHs is crucial for assessing their atmospheric fate and environmental impacts. However, available kA values are limited due to the difficulty in obtaining authentic ROOH standards. Herein, we solved this issue by developing a computational strategy and probing the structure-activity relationship of kA values. We screened the proton model, a critical prerequisite for density functional theory (DFT) calculations of kA, by comparing experimental kA values of four ROOHs with DFT-calculated values using different proton models. Results show the H3O+(H2O)1 model reliably predicts kA values with DFT method. Further investigation of 52 ROOHs reveals that substituents at the Cα site of the -OOH group, including -NH2, -N(CH3)2, -OH, -OCH3, -CH=CH2, -SH, and -PH2, can facilitate acid-catalyzed hydrolysis. Notably, the -NH2 and -N(CH3)2 substituents exhibit stronger facilitating effect than the well-documented -OH and -OCH3 substituents. Additionally, we clarified that not all nitrogen- or oxygen-containing substituents equally enhance kA, as their efficacy depends on the substituents attached to the O or N atoms. This study provides a reliable computational strategy and essential guidelines for predicting kA values of ROOHs, enabling accurate simulations in atmospheric chemistry models.
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RC1: 'Comment on egusphere-2025-1662', Anonymous Referee #1, 22 May 2025
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AC1: 'Reply on RC1', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC1-supplement.pdf
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AC1: 'Reply on RC1', Hongbin Xie, 10 Jul 2025
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RC2: 'Comment on egusphere-2025-1662', Anonymous Referee #2, 22 May 2025
Organic hydroperoxides are formed during the photochemical oxidation of organic compounds and represent a non-negligible fraction of the atmospheric aerosol mass. This study introduces a computational strategy to evaluate the hydrolysis rate constants of organic hydroperoxides and uses this strategy to explore the dependence of hydrolysis rate on molecular structure of hydroperoxides and also on medium acidity. The study is well executed, the results are mostly sound, and the manuscript is well written. I believe that it can be published subject to several changes as described below.
The introduction section must be expanded a bit to make it easier to understand the chemistry of the processes under consideration. The two possible hydrolysis mechanisms, concerted single-step and two-step depicted in Figure 2b, must be introduced in the introduction section. Adding a variant of Figure S7 to the introduction section would also be beneficial. The meaning of the proton model needs to be defined on the first occurrence. Is it “proton model” or “proton donor model”?
A highly non-monotonic dependence of the hydrolysis rate constant on the size of the proton model (the number of water molecules clustered around hydronium ion) is observed. One and three water molecules produce a nearly similar effect while with two water molecules the rate is significantly slower. This effect needs to be discussed, and its origin must be established.
The largest enhancement in the hydrolysis rate is reported for the nitrogen-containing hydroperoxides, and it is assumed that the amino groups remain unprotonated. This is grossly incorrect, as in the pH range considered in this study the fraction of the free, unprotonated form of -NH2 and -N(CH3)2 will be exceedingly small. To calculate the correct lifetime of these N-containing hydroperoxides, the fraction of the unprotonated form must be evaluated based on pKb. The latter can be estimated based on the data available for similar compounds in the literature or calculated explicitly from the Gibbs free energy of the amino group protonation evaluated by DFT.
A free carbocation is shown in Figure S7. Is using an implicit solvent model sufficient to stabilize this carbocation? How much would the reaction energetics change if this carbocation is stabilized explicitly, e.g., by hydration?
Having read the title, I assumed that the paper will eventually present some kind of quantitative structure–reactivity relationship. It did not and it is a pity, as a pretty large dataset has been produced. Is it possible to relate the rate constant with some parameters of the substituents, e.g., similar as in the Hammett equation? This would be very beneficial for the modeling studies.
Minor comments:
L35: more hydrophilic HYDROperoxide groups
L45: remove “extremely”
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC2 -
AC2: 'Reply on RC2', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC2-supplement.pdf
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AC2: 'Reply on RC2', Hongbin Xie, 10 Jul 2025
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RC3: 'Comment on egusphere-2025-1662', Anonymous Referee #3, 23 May 2025
The manuscript “Acid-catalyzed hydrolysis kinetics of organic hydroperoxides: Computational strategy and structure-activity relationship” presents an updated DFT-calculated proton model probing the structure activity relationships of organic peroxides, varied functional groups and acid catalyed hydrolysis. The manuscript incorporates empirical data for four initial compounds and once the model fits with observed values, expands the model to include several functional groups. The manuscript is clearly written and describes the work. However, there are several locations where clarification or additional information is needed before the manuscript is ready for publication.
I recommend for publication after addressing the following questions and corrections:
- The abstract (line 13) uses of the word solved and this is a broad assertion, since the model is built using a sparse set of empirically derived data points. Are you asserting that this method replaces the need for all authentic ROOH standards and can be used in lieu of authentic ROOH standards? If not, I recommend changing this word. Additionally, (line 17) define Cα prior to first use and clarify if the 52 ROOHs include the four ROOH stated in the previous sentence because it is unclear where the total 52 compounds originate.
- The introduction needs to be expanded to further detail the work this manuscript is building on. Specifics include: (line 31) clarify what is meant by lack of kinetic data in this sentence, there are many types of kinetics data beyond acid-catalyzed hydrolysis. Similarly to the abstract, (line 68) please clarify if 45 ROOH model compounds were used or if 52 compounds were used as is stated in the abstract and (line 68) define Cα and Cβ prior to first use. The methods section is well written and straight forward, however the empirical values are not referenced explicitly, (line 100-101) please clarify if C13 α-AH, C12 α-AH(1),C12 α-AH(2), and C10 α-HH were chosen based on a specific reference, i.e., Hu 2022. Figure 1 in the results and discussion section should be altered to clearly designate the empirically derived values. Currently, the experimental values are currently orange and difficult to see on the graph. Please consider changing to a different contrasting color, such as black, so the empirical data will be distinct from the model data. Section 3.3 Acid-catalyzed hydrolysis of atmospheric ROOHs needs to be expanded. Specifically, (lines 239-241) please clarify if these seven compounds have experimental kinetics data and if they were used in the model.
- The supplemental information needs to be more thoroughly explained. Figure S1 needs to have the chemical formulas and molecular weights for the molecules listed (a-d) beneath the molecules. For figures S3-S6, please clarify how the products were determined, i.e., the reactions including H2O, NO3, (SO4)2. Those products appear to be uniformly formed regardless of case. In addition, the SI must include a reference for the four compounds with empirical data need to be referenced here.
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC3 -
AC3: 'Reply on RC3', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC3-supplement.pdf
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RC4: 'Comment on egusphere-2025-1662', Anonymous Referee #4, 07 Jun 2025
This study focuses on improving our representation of acid-catalyzed hydrolysis of ROOHs. The authors appear to have a strong understanding of the computational models employed in this study and do a good job of explaining the chemical reasoning behind the modeled behavior. Additionally, the figures are clear, helpful, and well-made. I recommend this paper for publication after very minor changes.
Comments
- In Section 2.2, it would be helpful to have more background on why those 4 compounds were chosen. In the introduction you describe the importance of alpha-HHs and alpha-AHs, and it would be good in Section 2.2 to include a brief description and/or references to explain why these specific 4 were chosen.
- Consider expanding the discussion of future work in the Conclusions section. Do you think that more modeling studies, laboratory validation studies, or both would be helpful to expand on and utilize this work?
Technical Corrections
- There are some minor grammatical errors throughout the paper. I recommend having a native English speaker review the paper.
- Specify the pH that was used to calculate the enhancement factors in Figure 3 b-f. Or are these enhancement factors constant across pH?
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC4 -
AC4: 'Reply on RC4', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC4-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2025-1662', Anonymous Referee #1, 22 May 2025
The authors present a computational study investigating the acid-catalyzed hydrolysis rate constants of various hydroperoxides. In a first step, a proton model is screened and tested against experimentally derived hydrolysis rate constants and in a second step, the hydrolysis of a variety of hydroperoxides are investigated and discussed for typical atmospheric conditions. The study aims to overcome limitations of limited availability of authentic standards by using computational methods, which delivers an important contribution to the understanding of the fate of hydroperoxides in the atmosphere. As the reaction also leads to hydrogen peroxide formation, the findings of this study have large implications for the oxidant budget in the atmosphere. The study is well written with a clear flow and logic. I have, however, some concerns about the wide application of a system that was tested for quite narrow conditions. I recommend publication once a few issues have been addressed.
- The authors test their proton model on four compounds that are structurally very similar and all derived from the aqueous-phase ozonolysis of α-pinene with different alcohols as reaction partners of the corresponding Criegee-intermediates. The compounds tested however, span a much wider variety including functional groups such as -NH2, -PH2, -SH and -CH=CH2 and compounds from much different precursors, such as isoprene and DMS. This might lead to significant uncertainties that should be discussed in more detail. Furthermore, hydroperoxides formed in the gas phase might be structurally quite different, as isomerization reactions are expected to be more pronounced.
- More to this point, there are some hydroperoxides also commercially available and synthetic procedures have been published for others. Although I recognize that determining the hydrolysis rate constants for these compounds might not be within the scope of this study, the limitations of this procedure should be discussed.
- The authors apply the model to atmospheric ROOHs described in the literature. The cited study corresponding to DMS oxidation shows in fact, that instead of CH3SCH2OOH discussed in this study, a pronounced isomerization step mainly leads to the formation of CHOSCH2OOH, again a more functionalized compound. I suggest including this compound in the list of tested compounds.
- In Line 193 ff., the authors discuss the effect of functional groups in their findings and trace it back to a stabilization of the intermediate step. Although I can support the analysis, I want to point out, that the reaction pathway and the nature of the intermediate step is determined by the method the authors applied. This trend is caused by the input parameters for this analysis, which I think should be reflected by the discussion.
- Similarly, in lines 254, the authors claim, that the results prove that the SAR can be applied to atmospheric ROOHs, but as far as I understand there is no proof that the hydrolysis constants derived are valid for atmospheric conditions. I suggest to rephrase that section to better reflect that point.
- In the conclusion, the authors introduce the reaction with sulfate of the intermediate carbocation that was previously not mentioned. This might have large implications in the atmosphere and I would suggest including this reaction pathway as well as potentially the reaction with nitrate in the discussion section.
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC1 -
AC1: 'Reply on RC1', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-1662', Anonymous Referee #2, 22 May 2025
Organic hydroperoxides are formed during the photochemical oxidation of organic compounds and represent a non-negligible fraction of the atmospheric aerosol mass. This study introduces a computational strategy to evaluate the hydrolysis rate constants of organic hydroperoxides and uses this strategy to explore the dependence of hydrolysis rate on molecular structure of hydroperoxides and also on medium acidity. The study is well executed, the results are mostly sound, and the manuscript is well written. I believe that it can be published subject to several changes as described below.
The introduction section must be expanded a bit to make it easier to understand the chemistry of the processes under consideration. The two possible hydrolysis mechanisms, concerted single-step and two-step depicted in Figure 2b, must be introduced in the introduction section. Adding a variant of Figure S7 to the introduction section would also be beneficial. The meaning of the proton model needs to be defined on the first occurrence. Is it “proton model” or “proton donor model”?
A highly non-monotonic dependence of the hydrolysis rate constant on the size of the proton model (the number of water molecules clustered around hydronium ion) is observed. One and three water molecules produce a nearly similar effect while with two water molecules the rate is significantly slower. This effect needs to be discussed, and its origin must be established.
The largest enhancement in the hydrolysis rate is reported for the nitrogen-containing hydroperoxides, and it is assumed that the amino groups remain unprotonated. This is grossly incorrect, as in the pH range considered in this study the fraction of the free, unprotonated form of -NH2 and -N(CH3)2 will be exceedingly small. To calculate the correct lifetime of these N-containing hydroperoxides, the fraction of the unprotonated form must be evaluated based on pKb. The latter can be estimated based on the data available for similar compounds in the literature or calculated explicitly from the Gibbs free energy of the amino group protonation evaluated by DFT.
A free carbocation is shown in Figure S7. Is using an implicit solvent model sufficient to stabilize this carbocation? How much would the reaction energetics change if this carbocation is stabilized explicitly, e.g., by hydration?
Having read the title, I assumed that the paper will eventually present some kind of quantitative structure–reactivity relationship. It did not and it is a pity, as a pretty large dataset has been produced. Is it possible to relate the rate constant with some parameters of the substituents, e.g., similar as in the Hammett equation? This would be very beneficial for the modeling studies.
Minor comments:
L35: more hydrophilic HYDROperoxide groups
L45: remove “extremely”
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC2 -
AC2: 'Reply on RC2', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Hongbin Xie, 10 Jul 2025
-
RC3: 'Comment on egusphere-2025-1662', Anonymous Referee #3, 23 May 2025
The manuscript “Acid-catalyzed hydrolysis kinetics of organic hydroperoxides: Computational strategy and structure-activity relationship” presents an updated DFT-calculated proton model probing the structure activity relationships of organic peroxides, varied functional groups and acid catalyed hydrolysis. The manuscript incorporates empirical data for four initial compounds and once the model fits with observed values, expands the model to include several functional groups. The manuscript is clearly written and describes the work. However, there are several locations where clarification or additional information is needed before the manuscript is ready for publication.
I recommend for publication after addressing the following questions and corrections:
- The abstract (line 13) uses of the word solved and this is a broad assertion, since the model is built using a sparse set of empirically derived data points. Are you asserting that this method replaces the need for all authentic ROOH standards and can be used in lieu of authentic ROOH standards? If not, I recommend changing this word. Additionally, (line 17) define Cα prior to first use and clarify if the 52 ROOHs include the four ROOH stated in the previous sentence because it is unclear where the total 52 compounds originate.
- The introduction needs to be expanded to further detail the work this manuscript is building on. Specifics include: (line 31) clarify what is meant by lack of kinetic data in this sentence, there are many types of kinetics data beyond acid-catalyzed hydrolysis. Similarly to the abstract, (line 68) please clarify if 45 ROOH model compounds were used or if 52 compounds were used as is stated in the abstract and (line 68) define Cα and Cβ prior to first use. The methods section is well written and straight forward, however the empirical values are not referenced explicitly, (line 100-101) please clarify if C13 α-AH, C12 α-AH(1),C12 α-AH(2), and C10 α-HH were chosen based on a specific reference, i.e., Hu 2022. Figure 1 in the results and discussion section should be altered to clearly designate the empirically derived values. Currently, the experimental values are currently orange and difficult to see on the graph. Please consider changing to a different contrasting color, such as black, so the empirical data will be distinct from the model data. Section 3.3 Acid-catalyzed hydrolysis of atmospheric ROOHs needs to be expanded. Specifically, (lines 239-241) please clarify if these seven compounds have experimental kinetics data and if they were used in the model.
- The supplemental information needs to be more thoroughly explained. Figure S1 needs to have the chemical formulas and molecular weights for the molecules listed (a-d) beneath the molecules. For figures S3-S6, please clarify how the products were determined, i.e., the reactions including H2O, NO3, (SO4)2. Those products appear to be uniformly formed regardless of case. In addition, the SI must include a reference for the four compounds with empirical data need to be referenced here.
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC3 -
AC3: 'Reply on RC3', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC3-supplement.pdf
-
RC4: 'Comment on egusphere-2025-1662', Anonymous Referee #4, 07 Jun 2025
This study focuses on improving our representation of acid-catalyzed hydrolysis of ROOHs. The authors appear to have a strong understanding of the computational models employed in this study and do a good job of explaining the chemical reasoning behind the modeled behavior. Additionally, the figures are clear, helpful, and well-made. I recommend this paper for publication after very minor changes.
Comments
- In Section 2.2, it would be helpful to have more background on why those 4 compounds were chosen. In the introduction you describe the importance of alpha-HHs and alpha-AHs, and it would be good in Section 2.2 to include a brief description and/or references to explain why these specific 4 were chosen.
- Consider expanding the discussion of future work in the Conclusions section. Do you think that more modeling studies, laboratory validation studies, or both would be helpful to expand on and utilize this work?
Technical Corrections
- There are some minor grammatical errors throughout the paper. I recommend having a native English speaker review the paper.
- Specify the pH that was used to calculate the enhancement factors in Figure 3 b-f. Or are these enhancement factors constant across pH?
Citation: https://doi.org/10.5194/egusphere-2025-1662-RC4 -
AC4: 'Reply on RC4', Hongbin Xie, 10 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1662/egusphere-2025-1662-AC4-supplement.pdf
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The authors present a computational study investigating the acid-catalyzed hydrolysis rate constants of various hydroperoxides. In a first step, a proton model is screened and tested against experimentally derived hydrolysis rate constants and in a second step, the hydrolysis of a variety of hydroperoxides are investigated and discussed for typical atmospheric conditions. The study aims to overcome limitations of limited availability of authentic standards by using computational methods, which delivers an important contribution to the understanding of the fate of hydroperoxides in the atmosphere. As the reaction also leads to hydrogen peroxide formation, the findings of this study have large implications for the oxidant budget in the atmosphere. The study is well written with a clear flow and logic. I have, however, some concerns about the wide application of a system that was tested for quite narrow conditions. I recommend publication once a few issues have been addressed.