From the journal Environmental Science: Atmospheres Peer review history

21-Jul-2023

Dear Dr Watson:

Manuscript ID: EA-ART-06-2023-000102
TITLE: Bimolecular sinks of Criegee intermediates derived from Hydrofluoroolefins – A Computational Analysis

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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

In this manuscript, the authors have performed theoretical calculations to obtain rate coefficients for halogen substituted Criegee intermediates (CIs, CF3CHOO and CF3CFOO) and various atmospheric relevant molecules (CH2O, SO2, HNO3, CF3COOH, HF, HCl, H2S, H2O, (H2O)2 CH3OH). They have utilized transition state theory as well as dipole-dipole capture models to obtain the rate coefficients. Furthermore, for selected reactions, they performed a kinetics modeling study to obtain the branching ratio of the products. The paper contains rates for various reactions, and in terms of atmospheric impact, they find that the reaction with water molecules will be the likely decay pathway for these halogenated CIs. In terms of chemical perspective, they are able to see some interesting reactivity trends by the F-substitution, which is different from the previous trends determined for alkyl substituted CIs. The manuscript contains many detailed analysis of halogenated CIs and their impact on the atmosphere, so it fit the theme of the journal, but I would like the authors to clarify the following before accepting the manuscript for publication.
First, it seems like the main decay pathway in atmospheric conditions will be the reaction with water vapor, so it will be important to understand the products of this reaction. In the present calculation, the authors stop at the formation of hydroxy hydroperoxide. I am just curious, will this product be stabilized in the atmosphere, or would the OO bond break to make OH radicals?
Second, from an atmospheric perspective, how efficient is the ozonolysis of HFO compared to reactions with OH or other radicals in the atmosphere? In Table 1, the authors list CIs that can be produced from HFO but do not mention how effectively these halogenated CIs can be produced in the atmosphere. It would be good to mention this in the introduction.
Third, in terms of chemical perspective, previous studies, such as those by Anglada and coworkers Phys. Chem. Chem. Phys. 13, 13034- (2011) have mentioned the effect of zwitterionic and diradical character to evaluate the reactivity by evaluating the OO and CO bond lengths. Did the authors see differences in the OO or CO bond length for the halogen CIs compared to CH2OO?
Lastly, the work by Vereecken and coworkers, Phys. Chem. Chem. Phys. 19, 31599- (2017) have mentioned the importance of unimolecular decomposition. Is the unimolecular decomposition of these halogen substituted CIs slow so that the bimolecular reaction dominates in atmospheric conditions?
Minor things are:
There is a recent publication by Luo et al. on COMMUNICATIONS CHEMISTRY 6, 130 https://doi.org/10.1038/s42004-023-00933-2 reporting experimental studies for CH2OO+CH2O. This can be added to Table 2.
How were the concentration of various co-reactants in Table 1 selected? What is the justification for using those conditions? Are these all at the same temperature?

Reviewer 2

This comprehensive computational study investigates the atmospheric reactivity of the ozonolysis products of HFOs. This is a very topical and timely study. Society is currently debating the future of these compounds and this work comes at a good time. I think it will be impactful and a well-cited article in Environmental Science: Atmospheres. It reports a rigorous and well executed programme of work. The current state of knowledge of the chemistry is reviewed well. The methods are described thoroughly and the authors provide references for where it has performed well for similar compounds. The chemical and physical consequences of each outcome are discussed. There is a nice summary at the end of each section, making this long paper very readable. The abbreviations being listed in the ESI helped me a lot. The results are comprehensive and, in my opinion, reliable. I believe this work will encourage experiments, which the authors acknowledge are important and they even suggest areas to target. This paper is suitable for publication in Environmental Science: Atmospheres. I feel privileged being able to read it. I have one minor suggestion below that I would encourage the authors and editor to consider.

Introduction could use some revision. The journal and interest in this manuscript would benefit from a more thorough (correct) description of HFO proliferation. These synthetic compounds are engineered for more than use as “coolants”. For example, they are working fluids in heat pumps, which is perhaps more important than anything as, particularly Europe, moves away from gas heating. They are used for blowing foam and as propellants, etc. Their zero ODP does not come from their short lifespans, as the article implies, but from their inability to contribute reactive halogen species to the upper atmosphere. They are a direct response to the surface-heating implications of emitting large volumes of HFCs. Furthermore, it is not their high IR activity alone that makes fluorocarbons hazardous greenhouse gases, it is compounded by the fact that this occurs in a region of the spectrum where CO2 does not contribute. Lastly, the authors should refine the review of inbound regulations, particularly the F-gas discussion in the EU. HFOs are fighting a much more uphill battle than the reader would believe at this stage. The review of the state-of-the-art of the atmospheric chemistry of HFOs and their descendant Criegee intermediates is thorough and excellent.

Point-by-point response to the comments made by the reviewers:
We thank the reviewers for their diligent review of this manuscript and we appreciate their comments. We have tried to address all of the suggestions made, and these are highlighted below.

Edits recommended by Referee 1:
Edit Suggested by Referee 1: First, it seems like the main decay pathway in atmospheric conditions will be the reaction with water vapor, so it will be important to understand the products of this reaction. In the present calculation, the authors stop at the formation of hydroxy hydroperoxide. I am just curious, will this product be stabilized in the atmosphere, or would the OO bond break to make OH radicals?
Added the following lines to Pg 37 (Pg 38 on tracked changes document) (with references): As the yields of the HHP products are sizable, the atmospheric fate of these HHPs is the subject of many scientific studies. Under standard conditions, photolysis, deposition, and reaction with hydroxyl radicals are usually the main sinks for HHPs, but the excess internal energy within the HHP at the point of formation increases the significance of thermal unimolecular HHP decay. The primary unimolecular decay mechanism for hydroxymethyl hydroperoxide (HMHP), the product of CH2OO + H2O, is the elimination of H2O2 to produce formaldehyde, with some yields from minor channels that produce either H2O + HCOOH or OH + OCH2OH. Sheps et al. showed in a study of CH2OO + (H2O)2 that while ~55% of the HMHP remained stable, this excess energy led to ~40% of the HMHP fragmenting to produce H2O2 and formaldehyde, both of which are implicated in biogenic degradation. Halogenated HHPs produced from sCIs 2 – 5 are also likely to be produced with excess energy, and therefore exploring the breakdown of these products may be useful in the future.

Edit Suggested by Referee 1: Second, from an atmospheric perspective, how efficient is the ozonolysis of HFO compared to reactions with OH or other radicals in the atmosphere? In Table 1, the authors list CIs that can be produced from HFO but do not mention how effectively these halogenated CIs can be produced in the atmosphere. It would be good to mention this in the introduction.
Added the following lines to Pg 6 (Pg 6 on tracked changes document) (with references): The efficiency of ozonolysis as a tropospheric depletion mechanism for HFOs is disputed because while the presence of halogenated groups in the HFO reduces the overall effectiveness of most major alkene depletion pathways, such as via reaction with OH or Cl, this varies considerably depending on the HFO. The maximum ozonolysis yield for most HFOs reviewed are around 1%, but a few studies indicate that up to 10% of the removal of some haloalkenes occur via reaction with O3, a significant enough pathway that HFO ozonolysis is being investigated thoroughly at present. Furthermore, while direct CI branching fraction measurements for HFO ozonolysis are not found in the literature, indirect measurements of their aldehyde/ketone co-products show strong yields for CF3CHOO species and other similar CIs (>40%). This indicates that ozonolysis of haloalkenes in general are a major pathway for producing HFO-sCIs, even if these halogenated sCIs are low in abundance and ozonolysis is not the dominant pathway for removal of HFOs. This is a highly important topic and of relevance to the study of Criegee intermediates. Relative HFO ozonolysis percentages have been avoided here due to this need for a much more thorough re-investigation.
Edit Suggested by Referee 1: Third, in terms of chemical perspective, previous studies, such as those by Anglada and coworkers Phys. Chem. Chem. Phys. 13, 13034- (2011) have mentioned the effect of zwitterionic and diradical character to evaluate the reactivity by evaluating the OO and CO bond lengths. Did the authors see differences in the OO or CO bond length for the halogen CIs compared to CH2OO?
Added the following lines to Pg 39 (Pg 40 on tracked changes document) (with references): Criegee intermediates exhibit a mixed biradical and zwitterionic character and it has been shown that the more reactive zwitterionic dominated COO moieties have the larger ratio of OO and CO bond lengths, ROO/RCO or q. Given these observations, sCIs 2 & 3 (q = 1.064 & 1.071) appear to have a more biradical COO moiety than sCI 1 (q = 1.078), whereas sCIs 5 & 4 (q = 1.101 & 1.114) are more zwitterionic. Anglada et al. noted in a computational study of sCI + H2O reactions that the degree of this zwitterionic nature of the carbonyl oxide moiety, and consequentially it’s reactivity, can be tuned by modification of the substituents.68 Furthermore, in a computational study of sCI + alcohol reactions it is noted that the electron-withdrawing nature of F substituents in syn- & anti-FCHOO is likely to increase the unsaturated nature of the zwitterionic carbonyl oxide. This then generates a more electropositive central C atom, which is more vulnerable to nucleophilic attack by the alcohol’s electronegative O atom. This study shows the same positive relationship between electron-withdrawing groups, zwitterionic COO moiety and high reactivity seen in the literature, as shown by the fact that increases in q values in the HFO-sCI series mostly correlates with the growth in rate constant: kTHEO (sCI 2) < kTHEO (sCI 3) ≈ kTHEO (sCI 1) < kTHEO (sCI 5) < kTHEO (sCI 4).

 
Edit Suggested by Referee 1: Lastly, the work by Vereecken and coworkers, Phys. Chem. Chem. Phys. 19, 31599- (2017) have mentioned the importance of unimolecular decomposition. Is the unimolecular decomposition of these halogen substituted CIs slow so that the bimolecular reaction dominates in atmospheric conditions?
Added the following lines to Pgs 47-48 (Pgs 48-49 on tracked changes document) (with references): After collisional stabilisation, sCIs are less energetic so fewer unimolecular fragmentation mechanisms have energy barriers low enough to compete with bimolecular reactions as sinks for sCIs under tropospheric temperature and pressure conditions. According to Guidry et al., the central mechanism for the fragmentation of both syn- & anti-CF3CHOO is via a 1,3-ring closure to produce a dioxirane ring, both of which produce energy barriers (~80 kJ mol-1) higher than anti-CH3CHOO (~70 kJ mol-1) and more similar to those of CH2OO (70-100 kJ mol.-1). The unimolecular rate constant (kUNI) of a reaction path and kEFF are both equivalent and effective tools to show the efficiency of the respective reaction path as an sCI sink but, while Wang et al. have identified syn- & anti-CF3CHOO experimentally, no kUNI values were identified in either the Wang et al. or Guidry et al. studies. Therefore, the kUNI for CH2OO (~0.3 s-1) is used here instead as it may be the closest analogue to that of syn- & anti-CF3CHOO. On the basis of these premises, while the kUNI value used here is greater than many of the kEFF value for syn- & anti-CF3CHOO found in Table 6, the kEFF value for other reactions, particularly syn- & anti-CF3CHOO + (H2O)2, are greater still. This indicates that bimolecular reactions can compete with unimolecular decomposition.
No equivalent kUNI analogue could be found for syn- & anti-CF3CFOO especially considering the role of the F substituent in CI fragmentation in isolation is also challenging. One study by Ljubić and Sabljić calculated that the antiF substituent induced a lower anti-FCHOO fragmentation barrier (TSUNI ~ 47.3 kJ mol-1), whereas syn-FCHOO saw no such effect (TSUNI ~ 83.5 kJ mol-1). It is quite possible that the antiF substituent could therefore theoretically induce a higher kUNI for syn-CF3CFOO, but this might be offset by the lack of any tunnelling effect and the steric hinderance of the synCF3 substituent. Nevertheless, no computational or experimental literature exists of the syn- & anti-CF3CFOO unimolecular fragmentation reactions, and an assessment of these reactions has been not been deliberated here, due to the need for a more in-depth analysis of these reactions than the kind of provisional evaluation that would be covered in this study.

 
Edit Suggested by Referee 1: There is a recent publication by Luo et al. on COMMUNICATIONS CHEMISTRY 6, 130 https://doi.org/10.1038/s42004-023-00933-2 [doi.org] reporting experimental studies for CH2OO+CH2O. This can be added to Table 2. .
Action: Added number and reference to Table 2 on Pg 16 (or Pg 17 on tracked changes document).
Also Added the following lines to Pg 19 (Pg 20-21 on tracked changes document) (with references):
Original Sentence: Although experimental evaluations of this system are sparing, the kTHEO value determined here for sCI 1 + HCHO is similar to the range of kEXP values seen in studies of the analogous sCI 1 + CH3CHO reaction (3.0 × 10-13 — 1.7 × 10-12 cm3 s-1).
Correction: The kTHEO value determined here is also similar to the kEXP values measured for sCI 1 + HCHO by Luo et al., (4.11±0.25) × 10−12, as well as those determined for studies of the analogous sCI 1 + CH3CHO reaction (3.0 × 10-13 — 1.7 × 10-12 cm3 s-1).


Edit Suggested by Referee 1: How were the concentration of various co-reactants in Table 1 selected? What is the justification for using those conditions? Are these all at the same temperature?
Original Sentence Pg 15 (Pg 16 for tracked changes document) this is actually for Table 2 however: Also included is the co-reactant abundance selected for each kEFF calculation (chosen from the literature survey of the tropospheric abundance of the co-reactants found in SI section S4.2).
Correction: The co-reactant abundances featured in Table 2 are selected from locations where either the pollutant has a high enough concentration to be a potentially significant sCI sink and/or an area where humans may face a significant degree exposure of either the pollutant or the potential product of this reaction. These values are then used to generate the kEFF values seen in Table 2. (To view the extended literature survey of the tropospheric abundances of the co-reactants, see SI section S4.2.) As most of these values are averages from single locations across a protracted period of time or a generalised upper-limit in certain urban environments from a variety of locations, most measurements occur over a range of temperatures that include 298 K.


 
Edit Suggested by Referee 2: Introduction could use some revision. The journal and interest in this manuscript would benefit from a more thorough (correct) description of HFO proliferation. These synthetic compounds are engineered for more than use as “coolants”. For example, they are working fluids in heat pumps, which is perhaps more important than anything as, particularly Europe, moves away from gas heating. They are used for blowing foam and as propellants, etc. Their zero ODP does not come from their short lifespans, as the article implies, but from their inability to contribute reactive halogen species to the upper atmosphere. They are a direct response to the surface-heating implications of emitting large volumes of HFCs. Furthermore, it is not their high IR activity alone that makes fluorocarbons hazardous greenhouse gases, it is compounded by the fact that this occurs in a region of the spectrum where CO2 does not contribute.
Replaced this line on Pg 3 (Pg 3 for tracked changes document) (with references): HFOs are relatively non-toxic, have minimal impact on stratospheric ozone (O3) or global warming due to their short lifespans, and have high ignition energies compared to the first generation of refrigerants used in the 19th century (e.g. NH3, SO2 & CH3CHO).1–3,13–17
Added the following lines to Pg 3 (Pg 3 for tracked changes document) (with references): Furthermore, as well as being used as blowing foam and as propellants, HFOs are very important working fluids for waste heat recovery applications in heat pumps, as they are stable, have favourable toxicity profiles and are compatible with many plastics and elastomers used in such set-ups. As haloalkenes breakdown it is the Cl & Br radicals that are primarily implicated in ozone destruction in the upper atmosphere and therefore only HFOs containing Cl & Br atoms (e.g. HCFO-1233zd(E)) appear to marginally affect stratospheric ozone levels. But their compact lifetime means HFOs are often depleted in the troposphere and contribute to only minimal ozone depletion. Additionally, they are relatively non-toxic, have minimal impact on global warming due to their short lifespans, and have high ignition energies compared to the first generation of refrigerants used in the 19th century (e.g. NH3, SO2 & CH3CHO). Moreover, whereas CO2 dominates the lower IR wave numbers (600 – 750 cm-1), HFCs/HCFCs are also strongly IR active in the higher wavenumbers (1000 – 1400 cm-1), increasing their heat trapping ability.


 
Edit Suggested by Referee 2: Lastly, the authors should refine the review of inbound regulations, particularly the F-gas discussion in the EU.
Replaced this lines to Pgs 3-4 (Pgs 3-4 for tracked changes document) (with references): This has led to the European Union (EU) attempting to phase in HFO coolants to replace the HCFCs and HFCs with high GWP100 values. In parallel to this the United States of America (USA) is attempting to replace the automobile HFC-134a refrigerant in the same way.
Added the following lines to Pgs 3-4 (Pgs 3-4 for tracked changes document) (with references): In areas as widespread as China, the USA and the EU, the phasing in of HFOs has been increasingly integrated into regional regulations, to try and eliminate HCFC & HFC emissions, and this has led to HFOs seeing increasing use in refrigerators, insulation and vehicle cooling units. For example, a UK government 2022 report on the “F gas regulation in Great Britain” documents that HFO-1234yf has dominated total refrigerant usage in new small vehicles since 2017 and HFO-HFC blends use could eliminate high GWP HFCs from transport refrigeration by 2050. Modest concerns about both the tropospheric implications of HFO breakdown (e.g. TFA formation) and about the carbon-intensity of HFO production has meant that both the UK and the EU are currently assessing the role of HFOs in future climate regulations. Many refrigerant experts and industrial leaders continue to strongly support HFO use and are consulting on such regulations with governments currently, especially as the alternatives to HFOs also have trade-offs in the areas of toxicity, flammability and/or high-pressure requirements.

The following references numbers have also been added to accommodate made above: 7, 8, 11 – 12, 18, 24, 29 – 32, 52, 54, 177, 178, 180, 181, 200, 201. Plus some rearrangements.

15-Aug-2023

Dear Dr Watson:

Manuscript ID: EA-ART-06-2023-000102.R1
TITLE: Bimolecular sinks of Criegee intermediates derived from Hydrofluoroolefins – A Computational Analysis

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 1

This is a revised manuscript concerning the theoretical rate coefficients for halogen-substituted Criegee intermediates (CIs, CF3CHOO, and CF3CFOO) and various atmospheric relevant molecules (CH2O, SO2, HNO3, CF3COOH, HF, HCl, H2S, H2O, (H2O)2 CH3OH). The authors have answered the issues raised by the referees.

Reviewer 2

I'm satisfied that the authors have addressed the reviewers' concerns.