There are two isomers of propanol, the straight-chain normal- or just n-propanol, also known as 1-propanol, and the branched iso- or just i-propanol, also known as 2-propanol. There is another structural isomer, ethyl methyl ether, also known as methoxyethane (CH₃OC₂H₅), whose detection was already reported.

To make matters more complicated, n-propanol exists in five conformers which are descrobed by the orientation of the terminal CH₃ group and by the OH group. The anti conformation refers to a dihedral angle of 180^o with respect to the CCO plane, whereas the gauche conformations refer to a dihedral angle of ±60^o. A and G refer to the orientations of the methyl group, a, g, and g' to that of the OH group. Ga is the lowest energy conformer, Gg and Gg' are higher in energy, and the energy differences are known very well from rotational spectroscopy. Aa is probably between Ga and Gg while Ag may be highest in energy. Tunneling within Ag leads to Ag⁺ and Ag⁻ substates which are separated in energy by roughly 100 GHz.

2-Propanol occurs in two conformers a and g, respectively, with a about 85 cm⁻¹ higher in energy. Tunneling within g leads to g⁺ and g⁻ substates which are separated in energy by 46.8 GHz.

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.
Transitions of the Ga and Aa conformers of 1-propanol were observed in the course of molecular 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 identified transitions cover the 2 to 7 mm regions 2 ≤ J ≤ 11 and K_a ≤ 6 for Ga and 2 ≤ J ≤ 12 and K_a ≤ 3 for Aa. The number of clean, slightly or partly blended transitions is sufficiently large for both conformers together and for Ga alone. The excitation temperature is about 13 K. n-Propanol is about a factor of 15 less abundant than ethanol which, in turn, is about a factor of 25 less abundant than methanol.

A. Belloche, R. T. Garrod, O. Zingsheim, H. S. P. Müller, and K. M. Menten
reported on the
Interstellar Detection and Chemical Modeling of iso-Propanol and its normal Isomer
Astron. Astrophys. 662 (2022), Art. No. A110.
2-Propanol was detected in the course of the ReMoCA 3 mm line survey of Sgr B2(N) carried out with ALMA toward a position Sgr B2(N2b) slightly offset from the dust continuum of Sgr B2(N2) and described here for the first time. Four (largely) unblended transitions were detected for the gauche conformer with 12 ≤ J ≤ 16 and K_a ≤ 1 and three with 8 ≤ J ≤ 11 and K_a ≤ 2 for anti.

The authors also reported the first detection of n-propanol in a star-forming region. Interestingly, eight transitions of the higher lying Gg' conformer with 8 ≤ J ≤ 12 and K_a ≤ 4 and five transitions of the higher lying Ag conformer with 12 ≤ J ≤ 15 and K_a ≤ 6 were detected. Transitions of the lower lying conformers were also present, but all were blended more or less severely. Rotational temperatures of 135 K were inferred. The iso to normal ratio is about 0.6, fairly similar to the 0.4 ratio in the case of the two propyl cyanide isomers. In addition, n-propanol is about a factor of 18 less abundant than ethanol which, in turn, is about a factor of 19 less abundant than methanol.

The Max-Planck-Institut für Radioastronomie issued a press release in English and in German. Similar press releases were issued by the Max Planck Society and by the Universität zu Köln.

Contributor(s): H. S. P. Müller; 06, 2022