Product Branching Ratios of the C(3P) + C2H3(2A‘) and CH(2Π) + C2H2(1Σg+) Reactions and Photodissociation of H2CC⋮CH(2B1) at 193 and 242 nm: an ab Initio/RRKM Study

Abstract

The C(3P) + C2H3(2A‘) and CH(2Π) + C2H2 reactions have been studied using ab initio/RRKM calculations to investigate possible formations of C3H2 and C3H isomers in extraterrestrial environments such as circumstellar envelopes of carbon stars and cold molecular clouds, combustion processes, and CVD. Microcanonical rate constants and product branching ratios have been calculated. Product branching ratios for the C(3P) + C2H3(2A‘) are obtained as 78.3−81.8% for the HCCCH(3B) +H products, 6.1−7.3% for c-C3H2(1A1) + H, 4.4−8.1% for H2CCC(1A1) + H, 5.5% for HCCC(2A‘) + H2, and 1.0−2.0% for the CH(2Π) + C2H2 products, depending on the initial concentrations of intermediates c-H2CCCH and H2CC(H)C, both of which can be produced at the initial reaction step without entrance barrier. Thus, the C(3P) + C2H3(2A‘) reaction can be expected to mainly produce C3H2 isomers in extraterrestrial environments. Product branching ratios for the CH(2Π) + C2H2 reaction slightly vary with the initial concentrations of two initial complexes c-C3H3 and CHCHCH, which can be formed from the reactants without barriers and are calculated as 84.5−87.0% for HCCCH(3B) + H, 10.2−12.8% for c-C3H2(1A1) + H, 1.8−1.9% for HCCC(2A‘) + H2, and 0.9% for H2CCC(1A1) + H. The photodissociation of propargyl radical has been also investigated at photon energies of 193 and 242 nm, assuming internal conversion into the ground electronic state before dissociation. Product branching ratios calculated at 193 nm are 86.5% for the HCCCH(3B) + H channel, 3.6% for c-C3H2(1A1) + H, 5.5% for HCCC(2A‘) + H2, 3.5% for H2CCC(1A1) + H, and 0.9% for CH(2Π) + C2H2. These results are in line with experimental measurements (ref 24), which gave 96% and 4% branching ratios for the C3H2 + H and C3H + H2 channels, respectively. Product branching ratios obtained at 242 nm are 90.2% for the HCCCH(3B) + H channel, 5.1% for c-C3H2(1A1) + H, 3.0% for HCCC(2A‘) + H2, 1.6% for H2CCC(1A1) + H, and 0.1% for CH(2Π) + C2H2. Thus, HCCCH(3B) and H are predicted to be the major products, while c-C3H2 and H are expected to play only a minor role.