There are two isomers of propyl cyanide, the straight-chain normal- or just n-propyl cyanide, also known as 1-cyanopropane or butyronitrile, and the branched iso- or just i-propyl cyanide, also known as 2-cyanopropane or iso-butyronitrile. In addition, n-propyl cyanide exists in two conformers, anti- or a-n-propyl cyanide with a planar heavy atom frame, and gauche- or g-n-propyl cyanide in which the CH₃-group, or equivalently the CN-group, is rotated by 120^o.

A. Belloche, R. T. Garrod, H. S. P. Müller, K. M. Menten, C. Comito, and P. Schilke
reported on the
Increased Complexity in Interstellar Chemistry: Detection and Chemical Modeling of Ethyl Formate and //n//-Propyl Cyanide in Sagittarius B2(N)
Astron. Astrophys. 499, (2009) 215–232.
See also the MPIfR press release (in English; German version also available).
In the group of the complex molecules detected by radio astronomical means, n-propyl cyanide, n-C₃H₇CN, and ethyl formate are among the largest. They have been detected in the famous massive star-forming region Sagittarius B2(N) in the course of a molecular line survey in the 3 mm wavelength domain with additional observations at 2 and 1 mm carried out with the IRAM 30 m radio telescope.
Aminoacetonitrile was detected earlier in the course of this molecular line survey. At a rotational temperature of about 150 K, very many transitions of n-propyl cyanide have non-negligible intensities. However, many of them are blended severely with emissions or absorptions of other molecules. Nevertheless, a considerable number of transitions of the lower energy anti-conformer are not or only slightly blended. Its column density is about a factor of 100 lower in Sgr B2(N) than those of methyl and ethyl cyanide.

n-Propyl cyanide is the third alkyl cyanide to be detected in space after methyl cyanide, CH₃CN, and ethyl cyanide, C₂H₅CN.

Transitions of the lower energy g-n-propyl cyanide were detected unambiguously in the course of a line survey of the same region carried out with ALMA which lead to the detection of i-propyl cyanide as described in the paragraphs further below.
See also below for observations of both isomers in other sources.

H. S. P. Müller, A. Walters, N. Wehres, A. Belloche, O. H. Wilkins, D. Liu, R. Vicente, R. T. Garrod, K. M. Menten, F. Lewen, and S. Schlemmer
reported on
Laboratory Spectroscopic Study and Astronomical Detection of Vibrationally Excited //n//-Propyl Cyanide
Astron. Astrophys. 595, (2016) Art. No. A87.
The four lowest vibrational states of each conformer, gauche and anti, were detected in the ALMA line survery described below. The intensities of all vibrational states are compatible with the earlier determined rotational temperature of 150 K.

A. Belloche, R. T. Garrod, H. S. P. Müller, and K. M. Menten
carried out a molecular line survey of Sagittarius B2(N) at 3 mm (between 84 and 111 GHz) with the Atacama Large Millimeter Array (ALMA) and reported on the
Detection of a Branched Alkyl Molecule in the Interstellar Medium: //iso//-Propyl Cyanide
Science 345, (2014) 1584–1587.
See also the MPIfR press release (in English; German version also available) or the Universität zu Köln press release (in German).

The molecule is the first one found in the interstellar medium with a branched carbon backbone. It is a first step towards confirming earlier assumptions that branched molecules are abundant in space. This is a particularly important finding for the search for amino acids and other biogenic molecules in space. The molecule is also the fourth alkyl cyanide found in space.

About 50 unblended or slightly blended transitions were identified for i-propyl cyanide in the norther hot core of Sgr B2(N) and about 120 for both conformers of n-propyl cyanide. A rotational temperature of about 150 K was deduced. The iso- to normal-propyl cyanide ratio was determined to be 0.40 ± 0.06.

L. Pagani, C. Favre, P. F. Goldsmith, E. A. Bergin, R. Snell, and G. Melnick
reported on
The complexity of Orion: an ALMA view – I. Data and first results
Astron. Astrophys. 604, (2017) Art. No. A32.
Various spectral windows between 215.1 and 252.1 GHz covered a fairly large portion of the Orion KL region with ALMA in its Cycle 2. Several clear or nearly clear lines of both isomers were detected toward the sources IRc7, IRc21, MF2, and MF10.

Contributor(s): H. S. P. Müller; 04, 2009; 07, 2013; 10, 2014; 08, 2017