Population diagram of n-propyl cyanide toward Sgr B2(N2). Only the lines that are clearly detected and do not suffer too much from contamination by other species are displayed. The observed data points are shown in various colors (except red) as indicated in the upper right corner of panel a) while the synthetic populations are shown in red. No correction is applied in panel a). In panel b), the optical depth correction has been applied to both the observed and synthetic populations and the contamination by all other species included in the full model has been removed from the observed data points. The purple line is a linear fit to the observed populations (in linear-logarithmic space). The derived rotation temperature is 142 ± 4 K.