A Theoretical Investigation of the Triplet Carbon Atom C(3P) + Vinyl Radical C2H3(A′) Reaction and Thermochemistry of C3Hn (n ) 1-4) Species

The mechanism for the C(3P) + C2H3 reaction has been studied via ab initio calculations to investigate possible formations of C3H2 and C3H isomers in an extraterrestrial environment, combustion processes, and CVD. The C(3P) + C2H3 reaction, which produces C3H3 radical intermediates on the ground-state potential energy surface (PES), is studied employing the B3LYP/6-311G(d,p) and RCCSD(T)/6-311+G(3df,2p) levels of theory. Initially formed C3H3 intermediates have enough energy to undergo unimolecular rearrangements. Further, H or H2 eliminations then lead to C3H2 or C3H fragments. The most energetically favorable channel is found to be the formation of ground-state singlet cyclopropenylidene (c-C3H2, A1) by splitting H from cycloprop-2enyl (c-C3H3, 2A′). The other reaction mechanisms leading to H2CCC(A1) + H, HCCCH(3B) + H, and H2 + HCCC(2Π) exhibit barriers only 1-5 kcal/mol higher than those to produce H + c-C3H2. Detailed RRKM calculations will be needed to predict the product branching ratios under various reaction conditions. The C(3P) + C2H3 reaction channel, yielding intermediate C3H3 radicals on the first excited doublet state PES, is also studied by utilizing the CASSCF(11,11)/6-311+G(d,p) and MRCI+D(7,8)/ANO(2+) levels of theory. Three local minima and six transition states are located on the excited-state C3H3 PES. Various H and H2 loss channels are studied as well. The C-H fission of H2CCCH(1A′′) leading to HCCCH(11A′′) + H is the most energetically favorable channel. Finally, thermochemical parameters for the C3Hn (n ) 1-4) species are determined by employing the G3 theory and the CCSD(T)/6-311+G(3df,2p) method. The differences between the calculated results and available literature data do not normally exceed 1-2 kcal/mol. On the basis of the present calculations and previous theoretical and experimental data, ∆Hf298(H2CCCH) ) 84.5 ( 1 kcal/mol, ∆Hf298(c-C3H3) ) 114.5 ( 2 kcal/mol, ∆Hf298(H3CCC) ) 124 ( 2 kcal/mol, ∆Hf298(c-C3H2) ) 118.0 ( 1 kcal/mol, ∆Hf298(H2CCC) ) 133 ( 1 kcal/mol, ∆Hf298[HCCCH(B)] ) 132.5 ( 1 kcal/mol, ∆Hf298[HCCCH(A1)] ) 144 ( 1 kcal/mol, ∆Hf298[HCCC(Π)] ) 173 ( 2 kcal/mol, and ∆Hf298(c-C3H) ) 170 ( 2 kcal/ mol are recommended.