Abundance ratios [MH⁺]/[M] between the protonated and non-protonated form of some molecules as a function of the proton affinity of M. Calculated values at steady state (reached after some 10⁵ yr; see Fig. 3), adopting two different cosmic-ray ionization rates are compared with values derived from observations of TMC-1 (Agúndez & Wakelam 2013). In the case of NH, the observed value is derived from observations of the monodeuterated species in Barnard 1 (Cernicharo et al. 2013). The values for HCN/HNC include both isomers and are plotted at the mean value of the proton affinities of HCN and HNC, 170.4 kcal mol^-1 and 184.6 kcal mol^-1, respectively.