Pith. sign in

REVIEW 3 minor 57 references

Reaction Mechanisms and Kinetics of CN and CCH with H2CS: Implications for Interstellar Sulfur Chemistry

T0 review · 0 major / 3 minor · reviewed 2026-06-26 · grok-4.3

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗

Pith's one-line read CN adds to H2CS through two barrierless paths that funnel flux mainly to HCSCN plus H.

desk verdict Gives usable low-T rates and branching for CN + H2CS and CCH + H2CS from standard ab initio + master equation work, with no obvious internal problems. read the letter →

arxiv 2606.21730 v1 pith:OKZEWNJU submitted 2026-06-19 astro-ph.GA

Boutheïna Kerkeni , Ghofrane Ouerfelli , Nicole Feautrier , Christian Balança This is my paper · ORCID
classification astro-ph.GA
keywords thioformaldehydeCNradicalCCHreactionkineticsinterstellarchemistrymasterequationsulfurcoldmolecularclouds
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper maps the potential energy surfaces and solves the kinetics for the reactions of thioformaldehyde with CN and CCH radicals. For CN the calculations show orientation-dependent barrierless additions that open a connected multi-well network whose dominant exit channel is HCSCN + H, while abstraction routes remain minor. The overall rate is capture-controlled at low temperature, leading the authors to recommend that this channel be added to models of sulfur chemistry in cold clouds. The CCH reaction splits flux between HCSCCH + H and HCCH + HCS because a late barrier blocks the most exothermic cyclic product. These results illustrate how small changes in radical structure alter branching in sulfur radical-molecule systems.

What carries the argument

Orientation-dependent barrierless addition pathways that open a connected multi-well network, treated with an energy-grained master equation on CCSD(T)-F12a//DSD-PBEP86 surfaces.

What would settle it

A low-temperature laboratory experiment that measures the HCSCN branching fraction for CN + H2CS and finds it much smaller than the computed value, or finds abstraction channels dominant.

Watch

Extended reading notes

Core claim

The CN + H2CS reaction proceeds through orientation-dependent entrance channels. Two barrierless addition pathways lead to a connected multi-well network that preferentially forms cyano thioformaldehyde, HCSCN + H, whereas abstraction-type channels leading to HNC + HCS or HCN + HCS contribute only marginally. The calculated kinetics indicate capture-controlled low-temperature reactivity and a strong preference for HCSCN formation.

Load-bearing premise

The chosen level of theory and master-equation treatment correctly locate the barrierless entrance channels, well depths, and product branching ratios without large errors from method choice or missing dynamical effects.

Editorial extensions

If this is right

  • The CN + H2CS reaction should be included in astrochemical models of cold molecular clouds.
  • HCSCN formation is strongly preferred over the abstraction products at low temperature.
  • The CCH + H2CS reaction shares flux between HCSCCH + H and HCCH + HCS, with the latter favored.
  • Subtle differences in radical structure and entrance-channel topology control product branching in these S-containing systems.
  • The computed rate coefficients supply direct input for interpreting detections of HCSCN and HCSCCH in sources such as TMC-1.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same addition-dominated pattern may appear in reactions of other carbon-centered radicals with H2CS, altering predicted sulfur speciation in molecular clouds.
  • Temperature-dependent branching ratios from these networks could be tested by varying the collision energy in future crossed-beam or flow-tube experiments.
  • The preference for linear addition products over cyclic isomers may help explain why certain S-bearing molecules are detected while others remain undetected in cold sources.
  • Extending the same computational protocol to larger S-containing molecules could reveal whether capture control is general for radical + thioformaldehyde reactions.
Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

0 major / 3 minor

Summary. The manuscript reports an ab initio and master-equation study of the CN + H2CS and CCH + H2CS reactions using DSD-PBEP86/aug-cc-pVTZ geometries, CCSD(T)-F12a single-point energies, and energy-grained master-equation kinetics. It concludes that CN + H2CS proceeds via orientation-dependent barrierless addition channels in a multi-well network that strongly favors HCSCN + H over abstraction channels (HNC + HCS or HCN + HCS), with capture-controlled low-T reactivity. For CCH + H2CS, barrierless capture leads to branching between HCSCCH + H and HCCH + HCS (the latter favored), despite the cyclic product being most exothermic. The results are positioned as input for astrochemical models of sulfur chemistry in cold clouds such as TMC-1.

Significance. If the reported PES topology, branching ratios, and rate coefficients hold, the work supplies concrete, falsifiable inputs for updating sulfur networks in cold-cloud models, directly addressing recent detections of HCSCN and HCSCCH. The parameter-free first-principles approach (no fitted rates or ad-hoc assumptions) and explicit treatment of connected multi-well networks constitute a clear strength for the field.

minor comments (3)
  1. The abstract states that abstraction channels 'contribute only marginally' for CN + H2CS; a quantitative table or figure showing the branching fractions versus temperature would strengthen the claim.
  2. The treatment of orientation-dependent entrance channels is mentioned but the specific capture-rate implementation (e.g., variable reaction coordinate or phase-space theory parameters) is not summarized in the provided abstract; a short methods paragraph clarifying this would improve reproducibility.
  3. For the CCH + H2CS system, the statement that the cyclic product is 'kinetically hindered by a high-lying late transition state' would benefit from an explicit energy value or TS label in the main text or SI.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our manuscript, including the recognition of its parameter-free approach and relevance to sulfur chemistry in cold clouds. We are pleased that the work is recommended for acceptance.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; derivation is self-contained first-principles computation

full rationale

The paper computes structures, frequencies, and energetics at DSD-PBEP86/aug-cc-pVTZ followed by CCSD(T)-F12a single points, then solves an energy-grained master equation for capture rates and branching. All quantities (entrance-channel topologies, well depths, transition-state barriers, product ratios) are obtained directly from electronic-structure methods and statistical rate theory without fitting to target observables, without self-citation load-bearing premises, and without renaming or smuggling ansatzes. The central claim that HCSCN + H is preferred follows from the reported PES connectivity and flux calculations; no step reduces by construction to its own input.

Assumptions & free parameters 0 free parameters · 2 assumptions · 0 invented entities

The work rests on standard quantum-chemical approximations for open-shell systems and the validity of the energy-grained master equation for barrierless capture reactions at low temperature. No free parameters or invented entities are introduced.

assumptions (2)
  • domain assumption DSD-PBEP86/aug-cc-pVTZ geometries and CCSD(T)-F12a energies are sufficiently accurate for the relative energetics of the multi-well networks
    Invoked for all structures, frequencies, and barrier heights in the abstract.
  • domain assumption The energy-grained master equation with capture-controlled entrance channels correctly describes product branching at interstellar temperatures
    Central to the kinetic predictions.

how reviews work

0 comments
Cite this review

Pith. "Pith review of Reaction Mechanisms and Kinetics of CN and CCH with H2CS: Implications for Interstellar Sulfur Chemistry." pith.science (2026). https://pith.science/paper/OKZEWNJU

@misc{pith2026260621730,
  author       = {Pith},
  title        = {Pith review of: Reaction Mechanisms and Kinetics of CN and CCH with H2CS: Implications for Interstellar Sulfur Chemistry},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OKZEWNJU}},
  note         = {Machine review of arXiv:2606.21730}
}
read the original abstract

We report an ab initio and master-equation investigation of the gas-phase reactions of thioformaldehyde (H2CS) with CN and CCH radicals, motivated by the recent detection of the S-containing species HCSCN and HCSCCH in cold interstellar environments. Structures and frequencies were obtained at the DSD-PBEP86/aug-cc-pVTZ level, with energetics refined by CCSD(T)-F12a calculations and kinetics treated using an energy-grained master equation. The CN + H2CS reaction proceeds through orientation-dependent entrance channels. Two barrierless addition pathways lead to a connected multi-well network that preferentially forms cyano thioformaldehyde, HCSCN + H, whereas abstraction-type channels leading to HNC + HCS or HCN + HCS contribute only marginally. The calculated kinetics indicate capture-controlled low-temperature reactivity and a strong preference for HCSCN formation, suggesting that this reaction should be considered in astrochemical models of cold clouds. For CCH + H2CS, barrierless capture gives access to two connected entrance adducts. Although the cyclic product is the most exothermic channel, its formation is kinetically hindered by a high-lying late transition state. The flux is shared between propynethial formation, HCSCCH + H, and the HCCH + HCS channel, with the latter being favored. These results show that subtle differences in radical structure, bonding preferences, and entrance-channel topology strongly affect product branching in S-containing radical-molecule reactions. The computed mechanisms and rate coefficients provide useful input for astrochemical models of sulfur chemistry in cold molecular clouds and for interpreting recent molecular detections in sources such as TMC-1.

Figures

Figures reproduced from arXiv: 2606.21730 by the authors.

Figure 1
Figure 1. Reaction mechanism for the CN + H2CS system computed at the CCSD(T)-F12a+ZPE//DSD￾PBEP86/aug-cc-pVTZ level. Relative energies are given in kcal mol−1 with respect to the separated reac￾tants [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. Reaction mechanism for the CCH + H2CS system computed at the CCSD(T)-F12a+ZPE//DSD￾PBEP86/aug-cc-pVTZ level. Relative energies are given in kcal mol−1 with respect to the separated reac￾tants. 7 [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Short-range entrance-channel CCSD(T)-F12a potential scans for the CN + H [PITH_FULL_IMAGE:figures/full_fig_p013_3.png] view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Short-range entrance-channel CCSD(T)-F12a potential scans for the CCH + H [PITH_FULL_IMAGE:figures/full_fig_p014_4.png]
Figure 5
Figure 5. Figure 5: Temperature-dependent phenomenological rate coefficients for the CN + H [PITH_FULL_IMAGE:figures/full_fig_p016_5.png]
Figure 6
Figure 6. Figure 6: Temperature-dependent phenomenological rate coefficients for the CCH + H [PITH_FULL_IMAGE:figures/full_fig_p017_6.png]
Figure 7
Figure 7. Figure 7: Temperature dependence of the relative kinetic indicator [PITH_FULL_IMAGE:figures/full_fig_p018_7.png]
Figure 8
Figure 8. Figure 8: Temperature dependence of the summed product branching fractions for the dominant product [PITH_FULL_IMAGE:figures/full_fig_p019_8.png]
Figure 9
Figure 9. Figure 9: Comparison between the MESS product-forming rate coefficients and a simple low-temperature [PITH_FULL_IMAGE:figures/full_fig_p020_9.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

57 extracted references · 1 canonical work pages

  1. [1]

    J.; Scott, P.Annu

    Asplund, M.; Grevesse, N.; Sauval, A. J.; Scott, P.Annu. Rev. Astron. Astrophys.2009,47, 481–522

    2009

  2. [2]

    et al.Astron

    Fuente, A. et al.Astron. Astrophys.2023,670, A114

    2023

  3. [3]

    Vidal, T. H. G.; Loison, J.-C.; Jaziri, A. Y.; Ruaud, M.; Gratier, P.; Wakelam, V.Mon. Not. R. Astron. Soc.2017,469, 435–447

    2017

  4. [4]

    C.; Caselli, P.Astron

    Laas, J. C.; Caselli, P.Astron. Astrophys.2019,624, A108

    2019

  5. [5]

    M.Astron

    Esplugues, G.; Fuente, A.; Navarro-Almaida, D.; Rodríguez-Baras, M.; Majumdar, L.; Caselli, P.; Wake- lam, V.; Roueff, E.; Bachiller, R.; Spezzano, S.; Rivière-Marichalar, P.; Martín-Doménech, R.; Muñoz Caro, G. M.Astron. Astrophys.2022,662, A52

    2022

  6. [6]

    Astrophys.2021,650, L14

    Cernicharo, J.; Cabezas, C.; Endo, Y.; Agúndez, M.; Tercero, B.; Pardo, J.-R.; Marcelino, N.; de Vi- cente, P.Astron. Astrophys.2021,650, L14

    2021

  7. [7]

    Kaifu, N.; Ohishi, M.; Kawaguchi, K.; Saito, S.; Yamamoto, S.; Miyaji, T.; Miyazawa, K.; Ishikawa, S.-i.; Noumaru, C.; Harasawa, S.; Okuda, M.; Suzuki, H.Publ. Astron. Soc. Jpn.2004,56, 69–173

    2004

  8. [8]

    R.; Marcelino, N.; Gallego, J

    Cernicharo, J.; Cabezas, C.; Agúndez, M.; Tercero, B.; Pardo, J. R.; Marcelino, N.; Gallego, J. D.; Tercero, F.; López-Pérez, J. A.; de Vicente, P.Astron. Astrophys.2021,648, L3

    2021

Show all 57 references
  1. [9]

    Cernicharo, J.; Cabezas, C.; Agúndez, M.; Tercero, B.; de Vicente, P.; Marcelino, N.; Pardo, J. R. A Sensitive Line Survey of TMC-1: the Chemical Complexity of a Cold Dark Cloud. 2021 International Symposium on Molecular Spectroscopy. 2021

    2021

  2. [10]

    R.; Fuentetaja, R.; de Vicente, P.EPJ Web Conf.2022,265, 00041

    Cernicharo, J.; Agúndez, M.; Cabezas, C.; Marcelino, N.; Tercero, B.; Pardo, J. R.; Fuentetaja, R.; de Vicente, P.EPJ Web Conf.2022,265, 00041

    2022

  3. [11]

    R.; de Vicente, P.; Cernicharo, J.Astron

    Cabezas, C.; Agúndez, M.; Marcelino, N.; Tercero, B.; Endo, Y.; Fuentetaja, R.; Pardo, J. R.; de Vicente, P.; Cernicharo, J.Astron. Astrophys.2022,657, L4

    2022

  4. [12]

    Cernicharo, J.; Cabezas, C.; Agúndez, M.; Fuentetaja, R.; Tercero, B.; Marcelino, N.; de Vicente, P. Astron. Astrophys.2024,688, L13

    2024

  5. [13]

    Esplugues, G.; Agúndez, M.; Molpeceres, G.; Tercero, B.; Cabezas, C.; Marcelino, N.; Fuentetaja, R.; Cernicharo, J.arXiv e-prints2025, arXiv:2506.12974. 25

  6. [14]

    R.; de Vicente, P.; Cernicharo, J.Astron

    Fuentetaja, R.; Agúndez, M.; Cabezas, C.; Tercero, B.; Marcelino, N.; Pardo, J. R.; de Vicente, P.; Cernicharo, J.Astron. Astrophys.2022,667, L4

    2022

  7. [15]

    Cabezas,C.; Agúndez,M.; Endo,Y.; Tercero,B.; Lee,Y.P.; Marcelino,N.; deVicente,P.; Cernicharo,J. Astron. Astrophys.2024,686, L3

    2024

  8. [16]

    G.; Lique, F.; Tercero, B.; Marcelino, N.; de Vicente, P.; Cernicharo, J.Astron

    Fuentetaja, R.; Cabezas, C.; Endo, Y.; Agúndez, M.; Palluet, A. G.; Lique, F.; Tercero, B.; Marcelino, N.; de Vicente, P.; Cernicharo, J.Astron. Astrophys.2025,702, A23

    2025

  9. [17]

    Astrophys.2019,627, A1

    Agúndez, M.Astron. Astrophys.2019,627, A1

    2019

  10. [18]

    F.; Jiménez-Serra, I.; Rivilla, V

    Rodríguez-Almeida, L. F.; Jiménez-Serra, I.; Rivilla, V. M.; Martín-Pintado, J.; Zeng, S.; Tercero, B.; de Vicente, P.; Colzi, L.; Rico-Villas, F.; Martín, S.; Requena-Torres, M. A.Astrophys. J.2021,912, L11

    2021

  11. [19]

    Astrophys.2025,693, L20

    Agúndez, M.; Molpeceres, G.; Cabezas, C.; Marcelino, N.; Tercero, B.; Fuentetaja, R.; de Vicente, P.; Cernicharo, J.Astron. Astrophys.2025,693, L20

    2025

  12. [20]

    et al.Astrophys

    Sanz-Novo, M. et al.Astrophys. J.2025,980, L37

    2025

  13. [21]

    C.; Enrique-Romero, J.; Lamberts, T.; Linnartz, H.; Chuang, K.-J.ACS Earth Space Chem

    Santos, J. C.; Enrique-Romero, J.; Lamberts, T.; Linnartz, H.; Chuang, K.-J.ACS Earth Space Chem. 2024,8, 1646–1660

    2024

  14. [22]

    S.; Zeng, X.Nat

    Li, X.; Lu, B.; Wang, L.; Xue, J.; Zhu, B.; Trabelsi, T.; Francisco, J. S.; Zeng, X.Nat. Commun.2022, 13, 7150

    2022

  15. [23]

    M.; Wang, J.; Marks, J

    Herath, A.; McAnally, M.; Turner, A. M.; Wang, J.; Marks, J. H.; Fortenberry, R. C.; Garcia- Alvarez, J. C.; Gozem, S.; Kaiser, R. I.Nat. Commun.2025,16, 5571

    2025

  16. [24]

    J.; Hua, L.; Li, A.Astrophys

    Yang, X. J.; Hua, L.; Li, A.Astrophys. J.2024,974, 30

    2024

  17. [25]

    Smith, I. W. M.Mon. Not. R. Astron. Soc.2004,350, 323–330

    2004

  18. [26]

    Chang, Y.-W.; Wang, N. S.Chem. Phys.1995,200, 431–437

    1995

  19. [27]

    V.; Klippenstein, S

    Vuitton, V.; Yelle, R. V.; Klippenstein, S. J.Astrophys. J.2012,744, 11

    2012

  20. [28]

    J.2020,900, 85

    Tonolo, F.; Lupi, J.; Puzzarini, C.; Barone, V.Astrophys. J.2020,900, 85

    2020

  21. [29]

    A.; Li, L

    West, N. A.; Li, L. H. D.; Millar, T. J.; Van de Sande, M.; Rutter, E.; Blitz, M. A.; Lehman, J. H.; Decin, L.; Heard, D. E.Phys. Chem. Chem. Phys.2023,25, 7719–7733

    2023

  22. [30]

    Heitkämper, J.; Suchaneck, S.; García de la Concepción, J.; Kästner, J.; Molpeceres, G.Front. Astron. Space Sci.2022,9, 1020635

    2022

  23. [31]

    J.; Farquhar, P

    Millar, T. J.; Farquhar, P. R. A.; Willacy, K.Astron. Astrophys. Suppl. Ser.1997,121, 139–185. 26

    1997

  24. [32]

    Alessandrini, S.; Ye, H.; Puzzarini, C.ACS Earth Space Chem.2025,9, 1217–1226

    2025

  25. [33]

    A.; Burke, M

    Georgievskii, Y.; Miller, J. A.; Burke, M. P.; Klippenstein, S. J.J. Phys. Chem. A2013,117, 12146– 12154

  26. [34]

    Grimme, S.; Goerigk, L.; Fink, R. F.Phys. Chem. Chem. Phys.2011,13, 6670–6688

    2011

  27. [35]

    Kozuch, S.; Martin, J. M. L.Phys. Chem. Chem. Phys.2011,13, 20104–20107

    2011

  28. [36]

    Santra, G.; Sylvetsky, N.; Martin, J. M. L.J. Phys. Chem. A2019,123, 5129–5143

  29. [37]

    Frisch, M. J. et al. Gaussian 16, Revision C.01. Gaussian, Inc.: Wallingford, CT, 2016

    2016

  30. [38]

    J.; Knizia, G.; Manby, F

    Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; others MOLPRO, version 2012.1. MOLPRO, 2012; MOLPRO quantum chemistry package

    2012

  31. [39]

    Werner, H.-J. et al.J. Chem. Phys.2020,152, 144107

    2020

  32. [40]

    J.; Hampel, C.; Werner, H.-J.J

    Knowles, P. J.; Hampel, C.; Werner, H.-J.J. Chem. Phys.1993,99, 5219–5227

    1993

  33. [41]

    J.; Hampel, C.; Werner, H.-J.J

    Knowles, P. J.; Hampel, C.; Werner, H.-J.J. Chem. Phys.2000,112, 3106–3107

    2000

  34. [42]

    B.; Werner, H.-J.J

    Knizia, G.; Adler, T. B.; Werner, H.-J.J. Chem. Phys.2009,130, 054104

    2009

  35. [43]

    B.; Knizia, G.; Werner, H.-J.J

    Adler, T. B.; Knizia, G.; Werner, H.-J.J. Chem. Phys.2007,127, 221106

    2007

  36. [44]

    Dunning, T. H. J.J. Chem. Phys.1989,90, 1007–1023

    1989

  37. [45]

    E.; Dunning, T

    Woon, D. E.; Dunning, T. H. J.J. Chem. Phys.1993,98, 1358–1371

    1993

  38. [46]

    A.; Dunning, T

    Kendall, R. A.; Dunning, T. H. J.; Harrison, R. J.J. Chem. Phys.1992,96, 6796–6806

    1992

  39. [47]

    A.; Adler, T

    Peterson, K. A.; Adler, T. B.; Werner, H.-J.J. Chem. Phys.2008,128, 084102

    2008

  40. [48]

    Marcus, R. A.J. Chem. Phys.1952,20, 359–364

    1952

  41. [49]

    W.; Klippenstein, S

    Jasper, A. W.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B.J. Phys. Chem. A2007,111, 3932–3950

  42. [50]

    R.; Liang, C.-H.; Morley, C.; Pilling, M

    Glowacki, D. R.; Liang, C.-H.; Morley, C.; Pilling, M. J.; Robertson, S. H.J. Phys. Chem. A2012, 116, 9545–9560

  43. [51]

    Georgievskii, Y.; Klippenstein, S. J.J. Chem. Phys.2003,118, 5442–5455

    2003

  44. [52]

    Rev.1930,35, 1303–1309

    Eckart, C.Phys. Rev.1930,35, 1303–1309

    1930

  45. [53]

    Löhle, A.; Kästner, J.J. Chem. Theory Comput.2018,14, 5489–5498

    2018

  46. [54]

    A.; Whittet,D.C.B.Annu

    Boogert,A.C.A.; Gerakines,P. A.; Whittet,D.C.B.Annu. Rev. Astron. Astrophys.2015,53, 541–581. 27

    2015

  47. [55]

    Boogert, A. C. A.; Brewer, K.; Brittain, A.; Emerson, K. S.Astrophys. J.2022,941, 32

    2022

  48. [56]

    M.; West, N

    Douglas, K. M.; West, N. A.; Lucas, D. I.; Van de Sande, M.; Blitz, M. A.; Heard, D. E.ACS Earth Space Chem.2024,8, 2428–2441

    2024

  49. [57]

    J.2026,1001, 179

    Molpeceres, G.; Enrique-Romero, J.Astrophys. J.2026,1001, 179. 28

    2026

Pith tools

Reviewed June 26, 2026 · model on record in the stance chip above.