Ethenol, C2H3OH, aka vinyl alcohol, is a tautomer of ethanal, also known as acetaldehyde. Ethenol is very much higher in energy than ethanal. Under thermal conditions, ethenol has an abundance more than 4 orders of magnitude lower than that of ethanal, decreasing at lower temperatures. There is, however, a very high isomerization barrier which may prevent rapid isomerization, in particular at lower densities.
Ethenol has two conformers; the syn-conformer is lower in energy, the anti-conformer is considerably higher in energy, by about 550 K according to relative intensity measurements in the microwave region. Quantum-chemical calculation suggest the energy difference to be even higher, at least a factor of 2.

The first report on vinyl alcohol observations was made by B. E. Turner and A. J. Apponi,
Microwave Detection of Interstellar Vinyl Alcohol, CH<sub>2</sub>CHOH
Astrophys. J. 561, L207–L210 (2001).
Several b-type transitions J+11,J+1J0,J between 71 and 155 GHz were searched for with the Kitt Peak 12 m telescope toward the famous massive star-forming region Sagittarius B2(N) near the Galactic Center. Transitions with J = 0 to 4 have been detected for the energetically higher lying anti-conformer, whereas only J = 1 and 2 were found for the syn-conformer because of the less favorable dipole moment component.
The low rotational temperature of 11.6 K (with a possible range from 6.9 to 25.1 K because of uncertainty) together with the large beam sizes of the telecope point to an occurance in cold and less dense regions of the source which are known to be far away from local thermodynamic equilibrium.
There are very many lines of similar or larger intensity even in the proximity of the observed lines. Therefore, the number of transitions appears to be somewhat small to view the detection as fully secure, even more so as several transitions are (at least) partially blended. Moreover, the derived column density has been found to be basically identical to that of ethanal, which appears to be rather peculiar, though is maybe not impossible. All in all, it is recommended to view this report with some caution.

M. Agúndez, N. Marcelino, B. Tercero, C. Cabezas, J. D. Gallego, P. de Vicente, and J. Cernicharo
reported on
O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: Detection of C<sub>2</sub>H<sub>3</sub>CHO, C<sub>2</sub>H<sub>3</sub>OH, HCOOCH<sub>3</sub>, and CH<sub>3</sub>OCH<sub>3</sub>
Astron. Astrophys. 649, Art. No. L4 (2021).
The lower enerhy syn-conformer was identified through 2 a-type transitions with J = 2 − 1 and Ka = 0 and 1 and one b-type transition (404 − 313) in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 32.8 and 49.2 GHz; a third J = 2 − 1 a-type transition is blended. This is by far the clearest indication of the presence of ethenol in the ISM. TMC-1 is not a line-poor source at the intensities of these lines. Therefore, even this detection should be viewed with slight caution. The column density is about a factor of 9 higher than that of propenal and only marginally less than its ethanal (acetaldehyde) tautomer.

I. Jimenez-Serra, L. F. Rodríguez-Almeida, J. Martín-Pintado, V. M. Rivilla, M. Melosso, S. Zeng, L. Colzi, Y. Kawashima, E. Hirota, C. Puzzarini, B. Tercero, P. de Vicente, F. Rico-Villas, M. A. Requena-Torres, and S. Martín
reported on
Precursors of Fatty Alcohols in the ISM: Discovery of //n//-Propanol
Astron. Astrophys. 663 (2022), Art. No. A181.
Using data from line surveys of the quiescent giant molecular cloud G+0.693−0.027, about 1' north-east of Sagittarius (Sgr) B2(N) carried out with the Yebes 40 m dish at 7 mm and with the IRAM 30 m dish at 3, 2, and 1 mm, the authors also identified syn- and anti-ethenol in a column density ratio of about 10 : 1. Interestingly, ethanol and ethanal are only about a factor of 5 and 4, respectively, more abundant than ethenol.

Contributor(s): H. S. P. Müller; 04, 2012; 05, 2021; 08, 2022

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