Upper panel: rotational diagram of the first 36 rotational transitions of HC₃N generated from the SLIM output synthetic spectra. Physical parameters of T_ex = 22 K, Δ = 10 km s⁻¹ and extended source size have been used. The inset figure in the top left corner matches the range of energies displayed in Fig. 2 of Goldsmith & Langer (1999). Lower panel: synthetic spectra of three transitions at representative upper-level energies for the four column densities used to generate the rotational diagram. The effect of opacity is clearly seen in both line saturation and broadening.