Detection of n-C₃H₇CN propyl cyanide isomer towards IRc21 (see map Fig. 1). In this and the following figures, the most intense lines are displayed (i.e. lines typically stronger than ~5σ. Weaker lines are hardly or not at all detectable and have no impact on the results, nor do lines blended with much stronger lines of other species). The data are in black and the fit is in red. The summation of the fit of all the species we have identified is displayed in blue.