Investigating Enols Chemistry in the Interstellar Medium: Rotational Spectroscopy and Interstellar Search of ( <i>E</i> )-1-Propenol

Abstract Enols represent highly reactive intermediates toward the synthesis of complex organic molecules in the interstellar medium (ISM) and serve as key tracers of nonequilibrium chemistry driven by Galactic cosmic rays. However, only few enols have been identified in the ISM, thus limiting the insights that can be derived from their observations. To expand the inventory of interstellar enols and improve our understanding of their chemistry, we provide the first spectroscopic characterization of 1-propenol, one of the simplest enols potentially observable in the ISM, which is a key step toward enabling its astronomical detection. 1-Propenol was generated through the retro Diels–Alder reaction of 3-methylnorborn-5-en-2-ol using a flash vacuum pyrolysis apparatus, and its rotational spectrum was recorded between 80 and 500 GHz using a millimeter/submillimeter-wave frequency-modulation spectrometer. High-level quantum chemical calculations supported the spectral analysis and provided accurate predictions of spectroscopic parameters. Over 2200 rotational transitions of ( E )-1-propenol were successfully assigned and analyzed with the XIAM code, yielding precise rotational and centrifugal distortion constants and the methyl internal rotation barrier. Despite being essentially isoenergetic with the E form, ( Z )-1-propenol could not be identified in our spectrum due to the experimental conditions. A targeted astronomical search was also carried out toward the Galactic center molecular cloud G+0.693-0.027 using ultrasensitive millimeter-wave surveys, which, however, did not reveal any hints of emission from ( E )-1-propenol. Only an upper limit for its column density could be derived, which is fully consistent with ratios observed for other keto-enol pairs, indicating that more sensitive observations are required.