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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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 -
RC2: 'Comment on egusphere-2025-1662', Anonymous Referee #2, 22 May 2025
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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
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