Ethanol, C₂H₅OH, occurs in two conformers. The anti conformer is the lowest one. The heavy atoms occupy a trans configuration; for that matter, this conformer is frequently, but not quite correctly refered to as the trans conformer. The gauche conformers are higher in energy by 0.47 kJ/mol, 39.5 cm^–1, or 56.8 K.

Ethanol in its lowest anti configuration was among the early molecules to be detected in space. Its first detection was reported by:
B. Zuckerman, B. E. Turner, D. R. Johnson, F. O. Clark, F. J. Lovas, N. Fourikis, P. Palmer, M. Morris, A. E. Lilley, J. A. Ball, C. A. Gottlieb, and H. Penfield,
Detection of Interstellar //trans//-Ethyl Alcohol
Astrophys. J. 196, L99–L102 (1975).
Three b-type transitions (6₀₆ – 5₁₅, 4₁₄ – 3₀₃, and 5₁₅ – 4₀₄) were detected with the 36-foot NRAO telescope in the 3 mm region toward the massive star-forming region Sagittarius B2(OH). Since then, anti-ethanol was detected in many massive star-forming regions, including W51M, G34.3+0.15, and Orion KL as well as other molecular clouds with somewhat elevated temperatures (around 100 K).

The molecule was also detected in some of the investigated Galactic center molecular clouds which have low dust temperatures (10–20 K), even lower rotational temperatures (around 10 K), which are moderately dense (a few tenthousand molecules per cubic centimeter), but which are kinetically moderately warm (about 100 K or more). Two or three transitions were detected with the IRAM 30 m telescope in the 3 mm range by
M. A. Requena-Torres, J. Martín-Pintado, A. Rodríguez-Franco, S. Martín, N. J. Rodríguez-Fernández, and P. de Vicente,
Organic Molecules in the Galactic Center – Hot Core Chemistry without Hot Cores
Astron. Astrophys. 455, 971–985 (2006).

S. E. Bisschop, J. K. Jørgensen, T. L. Bourke, S. Bottinelli, and E. F. van Dishoeck,
used the SMA to carry out
An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry
Astron. Astrophys. 488, 959–968 (2008).
They found evidence for the presence of ethanol in this low mass protostellar object.

J. C. Pearson, K. V. L. N. Sastry, E. Herbst, and F. C. De Lucia,
reported on the identification of 13 U-lines in Orion-KL as belonging to the higher lying gauche-conformer:
//gauche// Ethyl Alcohol: Laboratory Assignments and Interstellar Identification
Astrophys. J. 480, 420–L102 (1997).
All these lines are c-type Q-branch transitions with K_a = 1 ↔ 0 and J between 17 and 31; they occur between 97.5 and 98.0 GHz. Only one line appeared to be blended with a line different from gauche-ethanol. Additional U-lines in the 3 mm region were also assigned to this conformer.

H. S. P. Müller, A. Belloche, L.-H. Xu, R. M. Lees, R. T. Garrod, A. Walters, J. van Wijngaarden, F. Lewen, S. Schlemmer, and K. M. Menten,
reported on
Exploring molecular complexity with ALMA (EMoCA): Alkanethiols and alkanols in Sagittarius B2(N2)
Astron. Astrophys. 587, Art. No. A92 (2016).
A line survey of Sgr B2(N) was carried out with ALMA at 3 mm. Both ¹³C isotopomers were detected with a ¹²C/¹³C ratio of 25, which is usual in the Galactic Center.

J. K. Jørgensen, H. S. P. Müller, H. Calcutt, A. Coutens, M. N. Drozdovskaya, K. I. Öberg, M. V. Persson, V. Taquet, E. F. van Dishoeck, and S. F. Wampfler
reported on
The ALMA-PILS Survey: Isotopic Composition of Oxygen-containing Complex Organic Molecules toward IRAS 16293–2422B
Astron. Astrophys. 620, Art. No. A170 (2018).
The results were obtained in the course of a line survey of the solar-type protostellar binary IRAS 16293–2422 with ALMA covering the frequency range 329.1–362.9 GHz. The authors only investigated source B. They detected for the first time the mono-deuterated isotopomers (three different ones; in addition, CH₂DCH₂OH has two conformers). The D/H ratio per H atom is about 0.05 to about 0.06. Lines of the two ¹³C isotopomers were also detected, albeit not enough for a clear detection. The ¹²C/¹³C ratio appears to be about 25, much lower than the local ISM value of 68. A confirmation of this value is desirable. Ethanol is among the molecules with warmer rotational temperatures of around 300 K in that hot corino.

Contributor(s): H. S. P. Müller; 11, 2009; 07, 2012; 11, 2016; 01, 2019