REVIEW 2 major objections 6 minor 1 references
Rate constants and product yields for the C + CH3CHO reaction at low temperatures
T0 review · 2 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read C(3P) + CH3CHO is fast and temperature independent at 50-296 K, with CH3CH/C2H4+CO as main products
desk verdict Solid first low-temperature kinetics for C + CH3CHO; the low-T clustering cut is a real but minor caveat, not a fatal flaw. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the ground triplet potential energy surface (PES) of the C3H4O system, computed at the M06-2X/aug-cc-pVTZ level with DLPNO-CCSD(T)/aug-cc-pVTZ single-point refinements for critical stationary points. This surface carries the argument because it shows that two entrance channels—carbon attacking the aldehyde oxygen—form stable CH3CHOC intermediates with no barrier above the reactants, and that these intermediates connect through low transition states (about 18-20 kJ/mol above the intermediate) to the products CH3CH + CO or C2H4 + CO. The experimental counterpart is the pseudo-first-order kinetic analysis in a CRESU supersonic flow reactor, where C atoms are photolytically generated from CBr4 and the exponential decay of C-atom VUV LIF as a function of CH3CHO concentration yields second-order rate constants.
What would settle it
A direct measurement of the C + CH3CHO rate constant at 50 K using an independent reagent-delivery method that quantifies the true gas-phase CH3CHO concentration—for example mass spectrometry or a calibrated photolytic source of CH3CHO—would settle whether the reported values are distorted by cluster formation; if such a measurement differed from 4.0 × $10^{-10}$ cm3 s-1 by more than the combined uncertainties, the recommended temperature-independent value would be wrong. Alternatively, a direct product detection experiment at low temperature that quantifies CH3CH and C2H4 yields in a 1:1 ratio with CO would confirm the predicted dominant channel.
Extended reading notes
Core claim
The paper's central claim is that the C(3P) + CH3CHO reaction is fast and essentially temperature independent, with a recommended rate constant k = (4.0 ± 0.8) × $10^{-10}$ cm3 s-1 valid from 50 K to 296 K. The supporting theoretical claim is that the ground triplet C3H4O potential energy surface has two barrierless entrance channels that form CH3CHOC intermediates, which then dissociate over low barriers to give primarily CH3CH + CO or C2H4 + CO, while H-atom producing channels are minor (experimentally estimated at roughly 3% at room temperature and qualitatively very low elsewhere). The paper also demonstrates, through gas-grain astrochemical modeling, that including this reaction in a dense-cloud network reduces gas-phase CH3CHO abundance by more than an order of magnitude at cloud ages near $10^{3}$-$10^{5}$ years, while having little effect on C2H4 abundance and only small influence at ages typical of well-studied clouds like TMC-1.
Load-bearing premise
The rate constants rest on the assumption that the acetaldehyde concentration in the reactor is exactly what is delivered by the bubbler and cold-trap system, and that at 50-75 K the remaining low-concentration data are unaffected by cluster formation or secondary chemistry.
Editorial extensions
If this is right
- If the recommended rate constant is correct, astrochemical networks should adopt k_C+CH3CHO = 4.0 × 10^-10 cm3 s-1 as a temperature-independent value down to interstellar cloud temperatures, making C atoms a major destruction route for gas-phase acetaldehyde whenever C is still abundant.
- The product outcome CH3CH/C2H4 + CO implies that the net effect of the reaction is to shorten the carbon chain of acetaldehyde, so it is a degradation path for an interstellar complex organic molecule rather than a growth path to larger organics.
- In dense-cloud models, the reaction lowers CH3CHO abundances by more than an order of magnitude at early and intermediate ages (roughly 10^3-10^5 years), so observations of acetaldehyde in young or less-evolved cores should be interpreted with this loss included.
- Because the measured H-atom yield is small (≈3% at 296 K), the reaction will not be a significant source of atomic hydrogen, and models that route it through H-producing channels would overestimate H abundances.
- The insensitivity of C2H4 abundance to the new reaction in the model means that treating CH3CH → C2H4 as the sole product does not introduce a large error in C2H4 predictions under dense-cloud conditions.
Reading between the lines
- The paper's temperature independence suggests the rate constant may stay at the collisional or capture-limited value even below 50 K; a direct measurement in the 10-30 K regime, or a capture-theory calculation, could test whether the recommended value holds in true cold-cloud conditions.
- By analogy with the C + acetone study cited in the paper, the triplet CH3CH/C2H4 products may undergo intersystem crossing to singlet ethene; if so, the reaction could be a minor chemical source of ground-state C2H4 in regions where gas-phase C is abundant, a channel the present model does not separately track.
- The cluster-formation concerns at 50-75 K imply that the low-temperature rate constants could be systematically low if undetected clusters survive at the lowest concentrations; an independent measurement using a different precursor or a mass-selected reagent delivery would clarify this.
- The model's large early-time effect on CH3CHO suggests that acetaldehyde observations in prestellar cores of differing ages could be used as a chemical clock probe, but only if the C-atom abundance itself is well constrained in those objects.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports a combined experimental and theoretical study of the C(3P) + CH3CHO reaction. Rate constants were measured with a CRESU supersonic flow reactor over 50-296 K using pulsed laser photolysis of CBr4 and VUV-LIF detection of C atoms, with CH3CHO delivered through a bubbler/cold-trap. The derived rate constants are large and approximately temperature independent, with a recommended value of (4.0 ± 0.8) × 10^-10 cm3 s^-1. H-atom product yields were also probed, giving a 296 K yield of (3.3 ± 1.1)% from 212 nm photolysis experiments, while 177 K measurements were not quantitatively interpretable. Supporting electronic structure calculations on the triplet C3H4O PES predict barrierless addition to form CH3CHOC intermediates, with CH3CH/C2H4 + CO as major products. Gas-grain astrochemical modeling with the Nautilus code shows that including this reaction can reduce gas-phase CH3CHO abundances by over an order of magnitude at early/intermediate cloud ages.
Significance. If the measured rate constant stands, this work provides an important destruction route for interstellar acetaldehyde and supports the emerging picture that ground-state carbon atoms react at near gas-kinetic rates with many oxygen-bearing complex organic molecules. The paper has clear strengths: rate constants were measured with two carrier gases at 296 K and with two photolysis wavelengths, the decay analysis is standard pseudo-first-order, and the theoretical PES is reported with stationary-point energies and frequencies in the SI. The astrochemical modeling quantifies the potential impact on CH3CHO abundances. The main weakness is that the two lowest-temperature rate constants (50 and 75 K) rest on a restricted CH3CHO concentration range after excluding high-concentration points attributed to cluster formation, with no direct measurement of the monomer concentration in the cold flow. The product-yield portion is also partially qualitative at low temperature, as acknowledged by the authors.
major comments (2)
- [§4.2, Table 1, Figure 3] The 50 K and 75 K rate constants depend on excluding the highest [CH3CHO] data points because of 'likely formation of CH3CHO clusters', so the second-order plots span only [CH3CHO] = 0-8.2 × 10^13 cm^-3 (50 K) and 0-6.1 × 10^13 cm^-3 (75 K). The paper provides no direct probe of the monomer concentration or cluster fraction in the supersonic flow, and the exclusion criterion is partly circular because points that break the assumed linear k1st versus [CH3CHO] relation are removed. If clustering depletes monomer even at the lower nominal concentrations, both slopes would be systematically low, biasing the recommended average value k = (4.0 ± 0.8) × 10^-10 cm3 s^-1. I request a sensitivity analysis (e.g., fits including the excluded points, or an upper-limit cluster-depletion correction) and/or an independent check of the CH3CHO monomer concentration, with the resulting additional systematic uncertainty propagated into the recommended value.
- [§4.3, Figure 6] The 177 K H-atom measurements cannot be reduced to a quantitative yield because the reactive H-atom signal grows monotonically with time, corresponding to a negative loss constant k_L(H) in the biexponential fit. Although the authors correctly state this limitation, the title promises 'product yields' and the astrochemical model treats C2H4 + CO as the sole products based on theory rather than on a measured branching at low temperature. Please either provide an upper bound to the 177 K H-atom yield that accounts for the secondary H-atom source, or state explicitly in the abstract and conclusions that product branching below 296 K is inferred from the PES, not measured.
minor comments (6)
- [References] Reference 20 (Kaifu et al. 2004) lists the DOI 10.3847/0067-0049/225/2/25, which appears to be the DOI of Reference 19 (Gratier et al. 2016) rather than of the PASJ article; please correct it.
- [§4.1] In the paragraph summarizing product channels, the text states 'with only low barriers for the formation of CH3C + CO', but the product P2 is defined as CH3CH + CO and CH3C + CO is not a listed channel; this should read 'CH3CH + CO'.
- [§5, Figure 8 caption] The sentence beginning 'The dashed lines in Figure 8 show the simulated abundances...' is garbled: the figure shows dashed lines for the standard network and solid lines with the C + CH3CHO reaction added, so the text should state that solid and dashed lines show the presence and absence of the reaction, respectively.
- [Abstract] The abstract gives the average rate constant as 4.0 × 10^-10 cm3 s^-1 without the uncertainty; for consistency with Section 4.2 it should read (4.0 ± 0.8) × 10^-10 cm3 s^-1.
- [Throughout] The abbreviation for interstellar complex organic molecules is spelled both 'iCOMS' and 'iCOMs'; please use one consistent form.
- [Table 1] The column header 'Flow density] / 10^17 cm^-3' contains a stray bracket, and the 'Nb' column would benefit from a footnote explaining that it is the number of individual decay measurements.
Circularity Check
No significant circularity: the measured rate constants are independent of the PES and the astrochemical model; minor self-citations in the H-atom calibration are not load-bearing.
full rationale
The recommended k = (4.0 ± 0.8) × 10^-10 cm3 s^-1 is an average of second-order rate constants obtained by weighted linear fits of k1st versus [CH3CHO]; the slopes are measured observables, and no parameter is fitted to, or back-derived from, the triplet PES or the astrochemical model. The PES calculation is an independent electronic-structure result, and the agreement with the measured temperature independence is a consistency check, not an input. The H-atom yield analysis uses the C + C2H4 reference yield of 0.92 from Bergeat and Loison, which includes a present co-author, and assumes temperature independence based on prior work by the same group; this is supporting and qualitative only, and the calibration is an external measurement, so it is not a load-bearing self-citation that forces the conclusions. The low-temperature cluster-exclusion step (Section 4.2, p. S16) is a data-selection and systematic-uncertainty concern: excluding high-[CH3CHO] points attributed to clustering could bias the 50 K and 75 K slopes, but this is not a circular derivation because the remaining points still define an independently measured slope and the rate constant is not an input to those experiments. The astrochemical model's abundance decrease is an application of the measured rate, explicitly labeled as such, not a validation that is folded back into the rate constant. No equation in the paper reduces to its own input, and no fitted parameter is renamed as a prediction. Minor self-citations appear only in calibration and setup contexts, so the circularity score is low.
Assumptions & free parameters
free parameters (1)
- Recommended rate constant k_C+CH3CHO =
4.0 x 10^-10 cm3 s^-1 (average of six values over 50-296 K)
assumptions (5)
- domain assumption Pseudo-first-order isolation: C-atom loss follows k1st = k_C+CH3CHO[CH3CHO] + k_C+CBr4[CBr4] + k_diff, with [CH3CHO] constant and known.
- domain assumption CH3CHO concentration from the bubbler and cold trap at 275 K is saturated and accurate, with no downstream condensation.
- ad hoc to paper At 50-75 K the higher [CH3CHO] points are invalid because clusters form, and the excluded points leave a valid linear region.
- domain assumption M06-2X/aug-cc-pVTZ geometries with DLPNO-CCSD(T)/aug-cc-pVTZ single-point energies correctly describe the low-energy triplet pathways.
- domain assumption The Nautilus gas-grain model with the kida.uva.2014 network and the paper's updates represents dense cloud chemistry well enough for the abundance predictions.
Cite this review
Pith. "Pith review of Rate constants and product yields for the C + CH3CHO reaction at low temperatures." pith.science (2026). https://pith.science/paper/2QDKW24X
@misc{pith2026250604794,
author = {Pith},
title = {Pith review of: Rate constants and product yields for the C + CH3CHO reaction at low temperatures},
year = {2026},
howpublished = {\url{https://pith.science/paper/2QDKW24X}},
note = {Machine review of arXiv:2506.04794}
}
read the original abstract
Reactions involving atomic carbon in its ground electronic state, C(3P), play an important role in astrochemistry due to high C-atom abundance levels. Here we performed a kinetic investigation of the reaction between C(3P) and acetaldehyde, CH3CHO, determining rate constants for this process over the 50-296 K range. Measurements of the formation of atomic hydrogen, H(2S), were also performed to provide insight into product formation. Experiments were conducted using a supersonic flow reactor coupled with pulsed laser photolysis for C-atom generation and pulsed laser induced fluorescence in the vacuum ultraviolet range for the detection of both C(3P) and H(2S) atoms. Quantum chemical calculations of the ground triplet state potential energy surface of C3H4O were also performed to provide theoretical support for the measurements. The rate constants were large and temperature independent with an average value of 4.0 x 10-10 cm3 s-1. This result is consistent with the theoretical results which predict either very low barriers or none at all on the underlying potential energy surface. Although experimental difficulties prevented the quantitative determination of H-atom formation, qualitatively, H-atom yields were very low with CH3CH/C2H4 + CO as the major products based on the calculations. The influence of this reaction on interstellar chemistry was tested using a gas-grain model of dense interstellar clouds. These simulations predict that the C(3P) + CH3CHO reaction decreases gas-phase CH3CHO abundances by more than an order of magnitude at early and intermediate cloud ages, with a lower influence at typical dense cloud ages.
Reference graph
Works this paper leans on
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[1]
(S1) Hickson, K. M.; Loison, J. -C.; Lique, F.; Kłos, J. An Experimental and Theoretical Investigation of the C(1D) + N2 → C(3P) + N2 Quenching Reaction at Low Temperature. J. Phys. Chem. A 2016, 120, 2504-2513. DOI: 10.1021/acs.jpca.6b00480. S40 Frequencies (in cm-1) of the stationary points involved in the C(3P) + CH3CHO reaction obtained at the M06-2X/...
Reviewed August 7, 2026 · model on record in the stance chip above.
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