the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2025
Chlorine Radical-Initiated Atmospheric Oxidation of Imines: Implications for Structural Influence on the Nitrosamine Formation
Abstract. Chlorine radical (·Cl) initiated oxidation of organic nitrogen compounds (ONCs) plays an important role in carcinogenic nitrosamines formation. Imines are important constituents of ONCs, primarily formed from the atmospheric oxidation of amines. However, ∙Cl-initiated atmospheric oxidation of imines remains poorly understood. Here, we studied the reaction mechanisms and kinetics of ·Cl-initiated oxidation for five representative imines (CH2=NH, CH3CH=NH, CH3N=CH2, (CH3)2C=NH, HN=CHCH2OH) to elucidate their atmospheric fate and extend the limited available data of ONCs, thereby establishing a structure-activity relationship for the reactions. The calculated overall reaction rate constants (× 10–11 cm3 molecule−1 s−1) of ∙Cl + CH2=NH, ∙Cl + CH3CH=NH, ∙Cl + CH3N=CH2, ∙Cl + (CH3)2C=NH, and ∙Cl + HN=CHCH2OH are 4.5, 27.2, 7.32, 44.8 and 12.6, respectively, which are consistent with the available experimental values. Importantly, our results show that the ∙Cl-initiated reactions of the NH-containing imines mainly produce N-centered radicals. These N-centered radicals exhibit various fates under tropospheric conditions: mainly reacting with NO to form nitrosamines or with O2 to form cyanide compounds, which differs substantially from the behavior of previously reported amines. The various fates of the N-centered radicals formed from imines originates from the difference in direct hydrogen abstraction reaction rate constants (kO2) with O2 and the reaction rate (kNO) with NO, both of which are principally governed by the distinct molecular structure of N-centered radicals. The revealed reaction mechanism provides new insights into the atmospheric transformation and risks of imines, and enrich our understanding of ∙Cl/ONCs chemistry.
-
Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
-
Preprint
(1171 KB)
-
Supplement
(1941 KB)
-
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1171 KB) - Metadata XML
-
Supplement
(1941 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-4896', Anonymous Referee #1, 27 Oct 2025
-
AC2: 'Reply on RC1', Hongbin Xie, 18 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4896/egusphere-2025-4896-AC2-supplement.pdf
-
AC2: 'Reply on RC1', Hongbin Xie, 18 Dec 2025
-
RC2: 'Comment on egusphere-2025-4896', Anonymous Referee #2, 03 Dec 2025
Xu et al. systematically studied the reaction mechanisms and kinetics of the ·Cl + imines reactions using computational methods. The authors found that imine-derived N-centered radicals exhibits various fates under tropospheric conditions, which significantly differs from the behavior of previously reported amines. This is an interesting study and the paper is a nice addition to the literature on the oxidation of organic nitrogen compounds. I believe this paper is fitting well in ACP. I can recommend publication after the following comments have been addressed.
- I'm confused why two different kinetic simulation software programs are used for the initial and subsequent reactions.
- Not clear to this reviewer which reaction steps were simulated using MultiWell program, and which were simulated using the MESMER program.
- I am missing labels (A) ~ (E) in the Table 1.
- The branch ratio in Table 1 does not appear to be 1.
- The overall reaction rate constants in the abstract are reported with a unit factor of × 10–11 cm3 molecule−1 s−1, while the same constants in Table 1 use a factor of × 10–10 cm3 molecule−1 s−1. Please ensure consistency throughout the manuscript.
- Missing reference for Molclus program.
- The parameters used in the long-range transition-state theory were not shown in the manuscript.
- I suggest some important results can be moved from SI to the main text, for example, Figure S3 and S4.
Citation: https://doi.org/10.5194/egusphere-2025-4896-RC2 -
AC1: 'Reply on RC2', Hongbin Xie, 18 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4896/egusphere-2025-4896-AC1-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-4896', Anonymous Referee #1, 27 Oct 2025
This manuscript provides a comprehensive theoretical investigation into the ·Cl- -initiated reaction mechanisms of five representative imines. By integrating quantum chemical calculations and kinetic calculations, the study elucidates the reaction pathways, rate constants, and the fate of generated radicals, with particular emphasis on the formation of N-centered radicals and their potential role in nitrosamine formation. This work fills a gap in our understanding of ·Cl-initiated atmospheric oxidation of imines and has important implications for atmospheric chemistry. Overall, the manuscript is well written and clearly presented. In my opinion, it can be accepted for publication after the authors address the following issues.
- The authors cited many references to introduce the significance sources of ·Cl. I think some significant works also need to read and properly cited in the revised manuscript. (For example, Sci. Technol.2025, 59, 12775; Environ. Sci. Technol. 2025, 59, 9167; Environ. Sci. Technol. 2025, 59, 6155)
- Missing references for the sentence ‘Daytime ∙Cl concentrations have been reported to reach up to 106 molecule cm-3’.
- Computational Details: Missing reference for Molclus program.
- They should make a bit more clear (in section 2.2 and 2.3) why two different methods (MP2 and M06-2X) and programs (MultiWell and MESMER) were used.
- Computational Details: please give the parameters used in the long-range transition-state theory with a dispersion force potential.
- ‘CCSD(T)/6-311+g(2df,2p)’ should be ‘CCSD(T)/6-311+G(2df,2p)’
- Please mention the pressure in the figure caption.
- The branching ratio in Table 1 does not appear to be 1. Please check carefully.
- Page 6, line 170: Missing Figure in SI.
- Page7, line 220: ‘intramolecular’ should be ‘unimolecular’.
- Page 10, line 305-309, Missing references.
Citation: https://doi.org/10.5194/egusphere-2025-4896-RC1 -
AC2: 'Reply on RC1', Hongbin Xie, 18 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4896/egusphere-2025-4896-AC2-supplement.pdf
-
RC2: 'Comment on egusphere-2025-4896', Anonymous Referee #2, 03 Dec 2025
Xu et al. systematically studied the reaction mechanisms and kinetics of the ·Cl + imines reactions using computational methods. The authors found that imine-derived N-centered radicals exhibits various fates under tropospheric conditions, which significantly differs from the behavior of previously reported amines. This is an interesting study and the paper is a nice addition to the literature on the oxidation of organic nitrogen compounds. I believe this paper is fitting well in ACP. I can recommend publication after the following comments have been addressed.
- I'm confused why two different kinetic simulation software programs are used for the initial and subsequent reactions.
- Not clear to this reviewer which reaction steps were simulated using MultiWell program, and which were simulated using the MESMER program.
- I am missing labels (A) ~ (E) in the Table 1.
- The branch ratio in Table 1 does not appear to be 1.
- The overall reaction rate constants in the abstract are reported with a unit factor of × 10–11 cm3 molecule−1 s−1, while the same constants in Table 1 use a factor of × 10–10 cm3 molecule−1 s−1. Please ensure consistency throughout the manuscript.
- Missing reference for Molclus program.
- The parameters used in the long-range transition-state theory were not shown in the manuscript.
- I suggest some important results can be moved from SI to the main text, for example, Figure S3 and S4.
Citation: https://doi.org/10.5194/egusphere-2025-4896-RC2 -
AC1: 'Reply on RC2', Hongbin Xie, 18 Dec 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4896/egusphere-2025-4896-AC1-supplement.pdf
Peer review completion
Post-review adjustments
Journal article(s) based on this preprint
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 321 | 141 | 42 | 504 | 48 | 39 | 43 |
- HTML: 321
- PDF: 141
- XML: 42
- Total: 504
- Supplement: 48
- BibTeX: 39
- EndNote: 43
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Qian Xu
Fangfang Ma
Chang Liu
Qiaojing Zhao
Jingwen Chen
Hong-Bin Xie
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1171 KB) - Metadata XML
-
Supplement
(1941 KB) - BibTeX
- EndNote
- Final revised paper
This manuscript provides a comprehensive theoretical investigation into the ·Cl- -initiated reaction mechanisms of five representative imines. By integrating quantum chemical calculations and kinetic calculations, the study elucidates the reaction pathways, rate constants, and the fate of generated radicals, with particular emphasis on the formation of N-centered radicals and their potential role in nitrosamine formation. This work fills a gap in our understanding of ·Cl-initiated atmospheric oxidation of imines and has important implications for atmospheric chemistry. Overall, the manuscript is well written and clearly presented. In my opinion, it can be accepted for publication after the authors address the following issues.