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

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 in the course of the Quijote line survey of TMC-1, we report the non-detection of the syn-isomer in the same source, with a derived upper limit to its column density of 1.5× 1010 cm−2. A subsequent theoretical investigation for the causes of the non-detection reveals that the most plausible route for the formation of CH2CHCHS is isomer specific, with nearly a 95% of formation of the anti-isomer. This ratio allows us to derive an even lower upper limit for the molecule. Besides, studies of tunneling mediated unimolecular isomerization in the gas reveal that syn-CH2CHCHS converts to the low energy isomer in timescales of the order of 103 years. Overall, we conclude that the detection of syn-CH2CHCHS is very challenging on cold and warm environments alike. Our results underscore the importance of electronic and kinetic effects affecting isomer abundances in diverse interstellar environments.