abundances of parent molecules derived from IR data [@Faggi2019; @Roth2020] allows us to investigate the origin of the radicals observed in 21P atmosphere. Using high-resolution infrared spectra obtained in 1998, [@Weaver1999] reported upper limits for different species relative to H$_2$O such as C$_2$H$_6$ (2-3%), HCN(0.3-0.4%) and C$_2$H$_2$(0.5-0.8%) assuming that all species are parent molecules. C$_2$H$_2$ has been found depleted with respect to HCN by a factor five compared to other comets like Hyakutake and Hale–Bopp, a result that has been confirmed at this apparition by [@Faggi2019].

We derived a Q(C$_2$)=1.26$\times$10$^{25}$ molec/s which is consistent with the upper limit of Q(C$_2$H$_2$)&lt;4.52$\times$10$^{25}$ molec/s reported by [@Faggi2019] and &lt;1.80$\times$10$^{25}$ molec/s reported by [@Roth2018] at 1.18 au from the Sun. This agreement indicates that C$_2$ could be a daughter species of C$_2$H$_2$. C$_2$ also has the possibility to come from C$_2$H$_6$ and HC$_2$N [@Helbert2005; @Weiler2012; @Holscher2015PhD] or released from organic-rich grains [@Combi1997], but a detailed chemical model of the coma would be needed to go in more details. We also found a good match between Q(CN)=4.40$\times$10$^{25}$ and Q(HCN)=4.30$\times$10$^{25}$ molec/s [@Faggi2019] at 1.01 au, showing that HCN could be the main parent molecule of CN in 21P. This result is known for several comets using 