In order to determine the unknown rate constant that will be used in the network, we used a methodology developed in previous articles (Loison et al. 2014a,b, 2015) and summarized in Appendix A. This methodology includes an extensive literature review, various Density Functional Theory (DFT) and ab initio calculations for critical gas phase reactions, namely H + l-C3H2, H + t-C3H2, O + c-C3H2, O + l-C3H2, N + C3, N + c-C3H2, N + l-C3H2, H + C3O, O + C3O, O + c-C3H3+, OH + C3, OH + c-C3H2, H2 + l-C3H and H2 + c-C3H, to determine the presence, or not, of a barrier. When there is no barrier in the entrance valley and exothermic bimolecular exit channels, we chose to use the capture rate constant or sometimes a fraction of the capture rate constant by comparison with similar reactions (the capture rate is the upper limit of the rate constants for barrierless reactions). The dissociative recombination (DR) of c,l-C3H2+ and c,l-C3H3+ is an important source of c,l-C3H and is the main source c,l,t-C3H2 in our network. The first step of c,l-C3H2+ and c,l-C3H3+ DR is the formation of highly excited C3H2** and C3H3**, which leads to bond fragmentation. Angelova et al. (2004) have shown that the DR of c-C3H2+ leads to 87.5 per cent of C3Hx and 12.5 per cent of C2Hy + CHz, and the DR of c-C3H3+ leads to 90.7 per cent of C3Hx and 9.3 per cent of C2Hy + CHz. Moreover, in DR processes, the H ejection is in general favoured more than H2 ejection (Plessis et al. 2010, 2012; Janev & Reiter 2004). Considering the exothermicity for the ejection of two hydrogen atoms (endothermic for c-C3H3+ DR and only slightly exothermic for l-C3H2+, c-C3H2+ and l-C3H3+ DR, see Appendix B), this process will have a low branching ratio. Then, the dissociation of C3H2** and C3H3** will produce mainly C3H + H and C3H2 + H, both C3H and C3H2 species being also excited considering the exothermicity of the DR and the fact that the hydrogen atom will carry only a limited part of the available energy through kinetic energy. Part of the excited C3H* and C3H2* molecules will lead to dissociation when they are populated above the dissociation limit, but most of them will relax through radiative emission of an infrared photon. As noted by Herbst et al. (2000), the typical time-scale for isomeric conversion is much shorter than that for relaxation by one infrared photon. Thus, because radiative relaxation occurs slowly, isomeric conversion leads to equilibrated isomeric (c-C3H $\leftrightarrows $ l-C3H, c-C3H2$\leftrightarrows $ l-C3H2) abundances at each internal energy. The final balance is determined at or near the effective barrier to isomerization, which corresponds to the energy of the transition state. The ratio between the isomeric forms are then approximated by the ratio of the rovibrational densities of states of the isomers at the barrier to isomerization calculated using the MESMER program (Glowacki et al. 2012). Fig. 2 shows the isomerization pathway calculated at the DFT level. The calculated geometries of the stationary point can be found in Appendix A. The t-C3H2 has a triplet ground state and its production in the excited singlet ground state is neglected here. The production of t-C3H2 from the DR of c,l-C3H3+ is supposed to come from c,l-C3H3+ + e− → c,l-C3H3 → t-C3H2 + H, which is supposed to be a minor channel in comparison to c,l-C3H3+ + e− → c,l-C3H3 → c,l-C3H2 + H (t-C3H2 has a ground triplet state, in contrast to c,l-C3H2, which has a singlet ground state).
Citation: Janev & Reiter 2004
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