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REVIEW 1 major objections 8 minor 80 references

The missing high-energy form of propenethial is scarce because formation itself strongly prefers the low-energy isomer and tunneling erases the rest within a few thousand years.

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2026-07-31 08:53 UTC pith:2Y64YHCP

load-bearing objection Solid lab + non-detection paper with a useful steric explanation for isomer selectivity; the “even lower” theoretical limit is over-sold but the core claim holds. the 1 major comments →

arxiv 2607.24656 v1 pith:2Y64YHCP submitted 2026-07-27 astro-ph.GA

Laboratory spectroscopy, theoretical characterization, and astronomical search for syn-propenethial (CH2CHCHS)

Germ\'an Molpeceres , Carlos Cabezas , Marcelino Ag\'undez , Mar\'ia Mallo , Yasuki Endo , Lucie Kolesnikov\'a , Gisela Esplugues , Jos\'e Cernicharo This is my paper
classification astro-ph.GA
keywords propenethialsyn-anti isomerismTMC-1rotational spectroscopygas-phase kineticsquantum tunnelingsulphur chemistryastrochemistry
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper reports the first laboratory microwave and millimeter-wave spectrum of syn-propenethial, the higher-energy conformational isomer of the sulphur analogue of acrolein. With those constants in hand the authors search the deep QUIJOTE survey of the cold cloud TMC-1 and find nothing, setting a 3-sigma column-density upper limit of 1.5 imes10^10 cm^-2 (syn/anti < 0.34). Quantum-chemical mapping of the most plausible gas-phase route (allyl radical + atomic sulphur) shows that steric repulsion at the H-elimination transition state makes the path to the syn isomer ~6 kcal mol^-1 higher, so the reaction produces ~94 % anti at 40 K. Independently, small-curvature-tunneling rate calculations show that any residual syn molecules convert to anti on ~10^3-year timescales at 10 K. Together these two kinetic effects explain the non-detection and imply that syn-propenethial will be difficult to observe in both cold and warm interstellar gas.

Core claim

Syn-propenethial is not detected in TMC-1 because its dominant gas-phase formation channel is strongly isomer-selective (~94 % anti at cloud temperatures) and because quantum tunneling converts any leftover syn molecules into the anti form on astronomically short timescales of order 10^3 years.

What carries the argument

The reduced potential-energy surface of CH2CHCH2 + S, in which two H-elimination transition states from a gauche adduct differ by ~6 kcal mol^-1 because of steric repulsion; master-equation branching ratios and SCT-CVT tunneling lifetimes are derived from that surface.

Load-bearing premise

The barrierless allyl-plus-sulphur reaction is assumed to dominate formation of propenethial in TMC-1, so that its computed anti-to-syn branching ratio can be scaled directly against the observed anti column density.

What would settle it

A secure detection of syn-propenethial (or a much tighter upper limit) in a source whose chemistry is known to be grain-surface dominated, or laboratory measurement of a competing formation route that yields a substantially higher syn fraction.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Syn-propenethial is expected to remain below current detection thresholds in both cold dark clouds and warmer cores.
  • Observed aldehyde/thione abundance ratios need not be corrected upward for an undetected high-energy conformer of propenethial.
  • Steric control at the transition state of radical–atom additions can systematically suppress high-energy conformers of other sulphur-bearing organics.
  • Tunneling-mediated conformational equilibration on ~10^3 yr timescales must be included when predicting isomer ratios of molecules with comparable barriers.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If steric discrimination at H-elimination steps is common, many other undetected high-energy conformers of S- and O-bearing organics may share the same dual suppression (selective formation + tunneling erasure).
  • The same LED analysis used here could be applied a priori to rank which conformational isomers of newly detected COMs are worth deep astronomical searches.
  • A full gas-grain network that folds in the computed branching ratio and tunneling rates would give a quantitative prediction for the anti column density itself, testable against the already-measured value.

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

1 major / 8 minor

Summary. The manuscript reports the laboratory rotational characterization of syn-propenethial, syn-CH2CHCHS, using FTMW spectroscopy of discharge products and previously recorded millimeter-wave spectra. A global fit to 164 lines gives precise rotational and centrifugal-distortion constants, with the assignment supported by observation of the 34S isotopologue. The authors search the QUIJOTE TMC-1 survey and obtain a 3σ LTE upper limit N(syn-CH2CHCHS) < 1.5×10^10 cm−2, corresponding to syn/anti < 0.34. To interpret the non-detection, they calculate the CH2CHCH2 + S potential-energy surface, RRKM/master-equation branching ratios, and SCT-CVTST unimolecular isomerization rates. They find approximately 94% anti formation at 40 K and syn→anti tunneling on a nominal 10^3–10^4 yr timescale, and use the former to infer a conditional syn column density of 2.8×10^9 cm−2.

Significance. The principal result is a useful and technically strong laboratory catalog for a previously uncharacterized high-energy isomer, immediately enabling astronomical searches. Particular strengths are the complementary FTMW and millimeter-wave datasets, independent confirmation through 34S, a well-constrained global fit, explicit LTE assumptions and a rotational-temperature sensitivity check, and deposition of supporting data in a public repository. The observational upper limit and syn/anti ratio are model-independent of the subsequent chemical interpretation. The branching-ratio and tunneling calculations are first-principles rather than fitted to the non-detection and provide falsifiable chemical predictions. The inferred 2.8×10^9 cm−2 value is less secure because it assumes dominance of one formation channel, but this does not weaken the laboratory result or the observational upper limit.

major comments (1)
  1. [§3.3, §4, and Abstract] The value 2.8×10^9 cm−2 is repeatedly called a theoretical “upper bound” or “even lower upper limit.” It is obtained by dividing the observed anti column by the branching ratio of only the CH2CHCH2 + S reaction. That is a conditional prediction for that channel, not an upper limit on total syn-CH2CHCHS, especially because §3.3 states that exploratory grain-surface routes lack conformer selectivity and Appendix B's precursor model underpredicts the calibration radical CH2CCH by about two orders of magnitude. The core non-detection is unaffected: the observed syn/anti < 0.34 already limits nonselective channels. Nevertheless, the abstract, §3.3, §4, and conclusions should relabel 2.8×10^9 cm−2 as a channel-specific/conditional estimate, or provide a quantitative demonstration that alternative routes are minor.
minor comments (8)
  1. [§3.4 and Figure 7] The nominal syn→anti lifetime is exponentially sensitive to the barrier height, barrier width, and interpolated minimum-energy path. Since the text anticipates electronic-structure errors approaching 1 kcal mol−1, please provide a small sensitivity range for k_uni and τ, or state explicitly that the 10^3 yr value is an order-of-magnitude estimate rather than a chemically accurate lifetime.
  2. [§2.3.2 and §3.3] The MESS calculations should state the pressure or zero-pressure limit, bath-gas/collisional treatment, and whether stabilization of I1 is included. The claim that the assumed C6 coefficient cancels exactly in the syn/anti branching ratio is clearest under common-entrance, nonselective-stabilization conditions; those conditions should be made explicit.
  3. [§3.1 and Table 1] The text says that seven 34S lines were observed, whereas Table 1 lists N_e = 9 for the FTMW-34S fit. Please reconcile these counts or explain whether two measured frequencies were excluded or otherwise treated specially.
  4. [§2.3.2, §3.3, and §4] Several chemical formulae appear to be typographical errors: “formation of CH3CHCHS” in §2.3.2, “CH2CHSH + H” for the C3H5 + S product in §2.3.2/§3.3, and “CH2CHCSH” twice in §4. These should be checked systematically because they obscure atom balance and the identity of the product.
  5. [§3.1] The derivation of the stated 1110–1200 K formation temperature from the measured FTMW anti:syn ≈ 4.5:1 ratio is not explained. A simple Boltzmann estimate using the 1207 K energy separation would not obviously give that range, so please state the partition-function/degeneracy treatment or clarify that the temperature comes from an independent discharge diagnostic.
  6. [§3.3 and §4] The observational and conditional theoretical columns differ by 1.5×10^10 / 2.8×10^9 ≈ 5.4, or 0.73 dex. Calling this “nearly one order of magnitude” is somewhat inflated; “about a factor of five” would be more accurate. The anti-column uncertainty should also be propagated into the conditional syn estimate.
  7. [Figure 5] TS4 is shown but not explicitly identified in the surrounding discussion, presumably as H elimination from I2. Please label the reaction coordinate/energy units and describe TS4 in the text so that the 50 kcal mol−1 I2 elimination barrier is unambiguous.
  8. [General] There are several small wording and typographical issues, including “unaivailability” in §1, inconsistent hyphenation of anti/syn, and the use of “propenyl radical” for CH2CHCH2, which is more commonly called the allyl radical. A final copy-edit would improve readability.

Circularity Check

0 steps flagged

No significant circularity: observational upper limit, ab initio branching ratios, and tunneling lifetimes are independent inputs, not forced by construction.

full rationale

The paper’s load-bearing chain does not reduce any claimed prediction to its own inputs. (1) The 3σ column-density upper limit (1.5×10^10 cm^-2) is obtained directly from non-detection of predicted Q-band lines in QUIJOTE data under fixed LTE parameters; it does not depend on the formation or tunneling calculations. (2) The ~15.8 anti:syn branching ratio and the 2.8×10^9 cm^-2 ‘theoretical’ bound are computed from a new SCS-MP2/CCSD(T)-F12 PES plus RRKM/master-equation kinetics for CH2CHCH2+S, then multiplied by the independently measured anti column density from Cabezas et al. (2025); the ratio is not fitted to the non-detection. (3) Syn→anti lifetimes (~10^3 yr at 10 K) come from new SCT-CVTST rate constants on the same electronic-structure level; methodology citations (García de la Concepción et al. 2022; prior Molpeceres works) supply the algorithm, not the numerical result. Self-citations identify the candidate formation route and supply the anti abundance, but do not define or force the branching fractions or tunneling rates. The dominance assumption for the single gas-phase channel is a scientific vulnerability, not a circular reduction. No self-definitional loop, fitted-as-prediction step, uniqueness import, or renamed empirical pattern is present.

Axiom & Free-Parameter Ledger

2 free parameters · 4 axioms · 0 invented entities

The central non-detection explanation rests on standard quantum-chemistry and statistical-rate-theory machinery plus two domain-level assumptions: that the chosen gas-phase channel dominates and that unimolecular tunneling is the only relevant isomerization process on cloud timescales. No free parameters are fitted to the TMC-1 non-detection itself; the only numerical inputs taken from observation are the already-published anti column density and the kinetic temperature.

free parameters (2)
  • C6 capture coefficient = 100 a.u. (estimated)
    Estimated at 100 a.u. for the phase-space-theory entrance channel; absolute rates cancel in the branching-ratio ratio, so the value affects only the overall scale, not the anti:syn fraction.
  • Trot and source size for LTE upper limit = 9 K, 80"
    Fixed to the canonical TMC-1 values (9 K, 80 arcsec) used in prior QUIJOTE papers; a sensitivity check at 6 K changes the limit by only 10%.
axioms (4)
  • domain assumption CCSD(T)-F12/cc-pVTZ-F12//SCS-MP2/aug-cc-pVTZ energies are accurate to ≲1 kcal mol^-1 for the stationary points of interest.
    Stated in Section 2.3.2; underpins both the PES barriers and the tunneling lifetimes.
  • domain assumption The barrierless CH2CHCH2 + S reaction is the dominant formation route under TMC-1 conditions; grain-surface and ion-molecule channels are secondary.
    Section 3.3 and Appendix B; required to convert the computed branching ratio into a theoretical column-density ceiling.
  • domain assumption Unimolecular tunneling is the only process that interconverts the two conformers on astronomical timescales; bimolecular destruction rates are isomer-independent.
    Section 3.4; allows the computed syn lifetime of ~10^3 yr to be compared directly with cloud ages.
  • domain assumption LTE at a single rotational temperature adequately describes the excitation of both isomers in TMC-1.
    Standard assumption used to convert the non-detection into a column-density upper limit (Section 3.2).

pith-pipeline@v1.2.0-grok45-kimik3 · 23448 in / 2696 out tokens · 39890 ms · 2026-07-31T08:53:37.047512+00:00 · methodology

0 comments
read the original abstract

We report the laboratory characterization of the higher-energy isomer of propenethial, syn-CH2CHCHS. While the lower-energy isomer, anti-CH2CHCHS, was detected in the interstellar medium during the QUIJOTE line survey of TMC-1, we report the non-detection of the syn isomer in the same source, deriving an upper limit to its column density of 1.5 $\times$ 10$^{10}$ cm$^{-2}$. A subsequent theoretical investigation into the origin of this non-detection reveals that the most plausible formation pathway for CH2CHCHS is highly isomer-specific, producing nearly 95\% of the anti isomer. This predicted branching ratio allows us to infer an even lower effective upper limit for the abundance of the syn isomer. In addition, calculations of tunneling-mediated unimolecular isomerization in the gas phase show that syn-CH2CHCHS converts to the lower-energy anti isomer on timescales of the order of 10$^{3}$ years. Overall, we conclude that the detection of syn-CH2CHCHS is highly challenging in both cold and warm interstellar environments. These results underscore the importance of electronic-structure and kinetic effects in determining isomer abundances across diverse interstellar environments.

Figures

Figures reproduced from arXiv: 2607.24656 by Carlos Cabezas, Germ\'an Molpeceres, Gisela Esplugues, Jos\'e Cernicharo, Lucie Kolesnikov\'a, Marcelino Ag\'undez, Mar\'ia Mallo, Yasuki Endo.

Figure 1
Figure 1. Figure 1: Molecular structure of the two conformational isomers that are con￾nected through a single bond rotation,syn and anti CH2CHCHS. The numbers represent the zero-point energy corrected energy separation in kcal mol−1 at the CCSD(T)-F12/cc-pVTZ-F12//SCS-MP2/aug-cc-pVTZ, described later in the text. dant sulphur bearing molecule detected in TMC-1. Propenethial is a molecule with two possible conformational isom… view at source ↗
Figure 2
Figure 2. Figure 2: Section of the FTMW spectrum of syn propenethial showing 20,2-10,1 pure rotational transition of the main isotopologue (left) and the same transition for the 34S isotopologue (right). The spectrum for the main isotopologue was achieved by 100-shots of accumulation and that for the 34S isotopologue was measured by 1000-shots of accumulation. The coaxial arrangement of the adiabatic expansion and the resonat… view at source ↗
Figure 3
Figure 3. Figure 3: Section of the mmw rotational spectrum of propenethial showing an example of a rotational transition of the syn isomer among the lines of the anti isomer (ground state and 𝑣18 = 1 excited vibrational state of the parent secies and the ground states of 13C and 34S isotopic species). Only the lower state quantum numbers for selected 𝑎-type R-branch transitions 𝐽𝐾𝑎,𝐾𝑐 are indicated for clarity. The top panel … view at source ↗
Figure 4
Figure 4. Figure 4: Spectra of TMC-1 in the Q band at the frequencies of the most favorable transitions of syn-CH2CHCHS. Black histograms show the observed spectra while red lines correspond to the computed synthetic spectra for a column density of 1.5 × 1010 cm−2 . The dotted pink line represent the 3𝜎 level. The abscissa corresponds to the rest frequency, assuming a local standard of rest velocity of 5.83 km s−1 . The ordin… view at source ↗
Figure 5
Figure 5. Figure 5: Simplified potential energy diagram for the shortest path in the formation of syn and anti propenethial. isomer from the S + C3H5 reaction, in agreement with the conclu￾sions drawn from the energetic analysis in the previous paragraphs. At 40 K, the lowest temperature considered in our master equation calculations (with the trend extrapolated down to 10 K), the forma￾tion of the anti isomer accounts for 94… view at source ↗
Figure 8
Figure 8. Figure 8: Equilibrium constants, K, defined in the anti→ syndirection as a function of temperature. The inset is a zoom in the 150–400 K region. rium, with Kanti−syn=7.1×10−53. We derived unimolecular rate constants, 𝑘uni,syn→anti(10 K)=5.3×10−12 s −1 , and, 𝑘uni,anti→syn(10 K)=3.7×10−64 s −1 , representing the latter an infinitely slow process. The rate constants and associated lifetimes as a function of temper￾atu… view at source ↗
Figure 7
Figure 7. Figure 7: Forward (upper panel) and backward (bottom panel) rate constants for unimolecular isomerization (kuni in inverse seconds) and lifetimes (𝜏 in years). C3H5 (CH2CHCH2, the lowest energy isomer), 0.05 D (Crabtree et al. 2025) makes the detection very challenging. Furthermore, the reaction network of C3H5 is currently less well constrained than that of CH2CCH. Nevertheless, preliminary astrochemical models (Br… view at source ↗

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