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., (2022), in press.

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