On Vinyl Alcohol in the ISM

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.

At present, there has only been one report on vinyl alcohol observations by B. E. Turner and A. J. Apponi,
Microwave Detection of Interstellar Vinyl Alcohol, CH2CHOH
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 the detection with some caution.


Contributor(s): H. S. P. Müller; 04, 2012