Observed and calculated protonated-to-neutral abundance ratios, MH⁺/M, in cold dense clouds (mostly in TMC-1) as a function of the proton affinity of the neutral M. The figure is based on Agúndez et al. (2015), with updates for H₂COH⁺ (Bacmann et al. 2016), H₂NCO⁺ (Marcelino et al. 2018), HC₅NH⁺ (Marcelino et al. 2020), HC₃O⁺ (Cernicharo et al. 2020a), HC₃S⁺ (Cernicharo et al. 2021a), CH₃CO⁺ (Cernicharo et al. 2021b), HCCS⁺ (Cabezas et al. 2022a), HC₇NH⁺ (Cabezas et al. 2022b), and HCCNCH⁺ (this study). We also include calculated values for C₃H⁺/C₃ and C₅H⁺/C₅ because these two ions were recently detected toward TMC-1 (Cernicharo et al. 2022).