Calculated fractional abundances of C₆H₂ (including all possible isomers) and HC₄S as a function of time. Horizontal dotted lines correspond to observed values in TMC-1; the dotted black lines account for the sum of the abundances of the two C₆H₂ isomers detected in TMC-1: H₂C₆ and HCCCHCCC. The most stable isomer, HC₆H, is probably the most abundant one, although it cannot be probed since it is non-polar.