Variation of the ¹²C/¹³C isotope ratios derived from and C₂ (a³Π_u) (by means of reactions (2) and (4), respectively) as a function of the gas kinetic temperature (T_kin). Solid (blue and red) lines represent the theoretical values calculated from Eq. (18) assuming chemical equilibrium conditions, while the corresponding (pink and back) dashed lines show their behavior as obtained from a reduced kinetic model with fixed integration time of 1.6 × 10⁵ yr (see text). Also shown by the gray points (with error bars) are the corresponding values obtained from observational surveys conducted by Hamano et al. (2019) and Bakker & Lambert (1998) towards Cyg OB2 No. 12 and HD 56126, respectively, as well as those reported by Colzi et al. (2020) for 10 K using a gas-grain chemical model in three different simulation timescales. The horizontal dashed line (also in the inset) highlights the elemental [¹²C/¹³C] Solar System abundance ratio.