Observed anion-to-neutral abundance ratios (referred to the left y axis) in cold interstellar clouds where molecular anions have been detected to date. Values are given in Table 4. Referred to the right y axis and following the same color code we also plot as dotted horizontal lines the calculated rate coefficients at 300 K for the reaction of radiative electron attachment to the neutral counterpart. Adopted values are 1.1 × 10⁻⁸ cm³ s⁻¹ for C₄H, 6.2 × 10⁻⁸ cm³ s⁻¹ for C₆H and C₈H (Herbst & Osamura 2008; they are shown slightly displaced for visualization purposes), 2.0 × 10⁻¹⁰ cm³ s⁻¹ for C₃N (Petrie & Herbst 1997; Harada & Herbst 2008), and 1.25 × 10⁻⁷ cm³ s⁻¹ for C₅N (Walsh et al. 2009). The scale of the right y axis is chosen to make the rate coefficient of electron attachment to C₆H to coincide with the mean of C₆H⁻/C₆H ratios and to cover the same range in logarithmic scale than the left y axis, which allows to visualize any potential proportionality between anion-to-neutral ratio and radiative electron attachment rate.