From left to right, top to bottom panels: emission lines of H¹⁵NC, HN¹³C, HC¹⁵N, and hyperfine transitions of H¹³CN observed with IRAM 30 m towards IRDCs C, F, G, and H. Red lines indicate the best Gaussian fit to the lines. The hyperfine components of H¹³CN were initially fitted but given the bad results of the fit; in a second step only the main F = 2 → 1 component of H¹³CN was fitted using a single-Gaussian component profile (see red line in bottom panels and Sect. 3.1 for details). See also Figs. A.1−A.4 for all spectra taken.