^a The C₃H₆⁺ + C₂H₂ reaction is a potential source of C₅H₇⁺, but it must be a minor source since C₃H₆⁺ is not very abundant. We use the same rates as for the iso-electronic reaction C₃H₄⁺ + C₂H₄.

^b There are two isomers for C₃H₇⁺: CH₃CHCH₃⁺ (the most stable) and CH₃CH₂CH₂⁺ (+52 kJ/mol) separated by TS a localized +85 kJ/mol above CH₃CHCH₃⁺. Both isomers should exist in dense molecular cloud. We conducted preliminary calculations showing that the isomer CH₃CH₂CH₂⁺ reacts with C₂H₂ to produce C₅H₇⁺ + H₂. Further study is needed, but this reaction is likely an important pathway for the production of C₅H₆, as both C₃H₇⁺ and C₂H₂ are fairly abundant in the model.

^c l–C₅H₅ (several mesomeric forms: H₂C=CH–CH–CC–H ↔ H₂C=CH–CH=C=C–H ↔ H₂C–CH=CH–CC–H) is produced by C + CH₂CHCHCH₂ and will react without a barrier on the singlet surface (doublet-doublet reaction) to yield H₂C=CH–CH₂–CC–H (likely the main product as H₂C=CH–CH–CC–H is the most stable configuration), H₂C=CH–CH=C=CH₂ and CH₃–CH=CH–CC–H. H₂C=CH–CH₂–CC–H can lead to c–C₅H₆ through a TS localized -84 kJ/mol below the H + l–C₅H₅ entrance level and then to C₅H₅ + H (without exit TS), or to CH₂CCH₂ + C₂H₂ through a TS localized -65 kJ/mol below the H + l–C₅H₅ entrance level Bacskay & Mackie (2001). The TS involved have similar energies and structures and then H₂C=CH–CH₂–CC–H should lead to H + c–C₅H₅ and CH₂CCH₂ + C₂H₂ in a similar amount. H₂C=CH–CH=C=CH₂ can lead to c–C₅H₆ through a TS localized -144 kJ/mol below the H + l–C₅H₅ entrance level and then to C₅H₅ + H (without exit TS) or to CH₂CCH₂ + C₂H₂ through a TS localized -77 kJ/mol below the H + l–C₅H₅ entrance level Bacskay & Mackie (2001). So H₂C=CH–CH=C=CH₂ should lead mainly to H + c–C₅H₅. The preferential production of c–C₅H₅ is also observed during the pyrolysis of c–C₅H₆ Bacskay & Mackie (2001); Roy et al. (1998). Further studies on this reaction will be necessary to obtain more accurate rates for this important reaction for c–C₅H₆ modeling in interstellar media.

^d The reaction CH₂ + C₄H₅ → C₅H₆ + H is most certainly rapid at low temperatures as radical-radical reaction. The first step of this reaction is the formation of the linear C₅H₇ adduct. There are several isomers of C₄H₅ produced in the network through mostly through the C + C₃H₆ reaction Chastaing et al. (1999); Loison & Bergeat (2004); Chin et al. (2013); Capron et al. (2015) and for the most stable one, CH₃CCCH₂, multiple mesomeric forms exist, leading to the possible production of a large number of C₅H₇ isomers. The production of c–C₅H₆ may be possible but would require numerous isomerization of the initial C₅H₇ adducts and is likely unfavored. Moreover, even though the reactants are produced by efficient reactions, they react with atoms (H, C, O, and N) without a barrier and have relatively low concentrations in dense clouds, limiting the significance of this reaction. Nevertheless, we performed a run assuming a rate of 1 × 10⁻¹⁰ cm³ s⁻¹ for the CH₂ + C₄H₅ → C₅H₆ + H reaction as a test and thus showing that this reaction is negligible under dense cloud conditions.

^e The first step is CH₂CHCH₂CCH and CH₂CHCHCCH₂ formation without barrier (radical-radical association). The TS for CH₂CHCH₂CCH → CH₂CHCHCCH₂ is located -63 kJ/mol below C₂H₃ + C₃H₃. The TS for CH₂CHCHCCH₂ → CH₂CHCHCHCH is located -115 kJ/mol below the entrance level and it seems that there is no, or very low, barrier for CH₂CHCHCHCH cyclisation toward C₅H₆.