Calculated fractional abundances of the three C₄H₃N isomers as a function of time. Solid and dashed lines correspond to two models in which we use branching ratios for the CN + CH₃CCH reaction from Abeysekera et al. (2015) and from Balucani et al. (2000), respectively (see text). The abundances observed in TMC-1 for the three C₄H₃N isomers (from Table 1, adopting an H₂ column density of 10²² cm⁻²; Cernicharo & Guelin 1987) are shown as horizontal dotted lines.