Ab initio and direct dynamics studies of the reaction of singlet methylene with acetylene and the lifetime of the cyclopropene complex

The energetics of the ¹CH ₂ + C ₂H ₂ → H + C ₃H ₃ reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates - cyclopropene (c-C ₃H ₄, allene (CH ₂CCH ₂), and propyne (CH ₃CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C ₃H ₄), 109.69 (CH ₂CCH ₂), and 110.78 kcal/mol (CH ₃CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C ₃H ₄ isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the ¹CH ₂ + C ₂H ₂ reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 ± 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of ¹CH ₂ into a C-H bond of C ₂H ₂. However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.