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arXiv · 2607.24656

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

Germán Molpeceres·Carlos Cabezas·Marcelino Agúndez·María Mallo·Yasuki Endo·Lucie Kolesniková·Gisela Esplugues·José Cernicharo
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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.

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Germán Molpeceres, Carlos Cabezas, Marcelino Agúndez, María Mallo, Yasuki Endo, Lucie Kolesniková, Gisela Esplugues, José Cernicharo. 2026-07-27. Laboratory spectroscopy, theoretical characterization, and astronomical search for syn-propenethial (CH2CHCHS). https://arxiv.org/abs/2607.24656

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