Predictive Theory for the Addition and Insertion Kinetics of 1CH2 Reacting with Unsaturated Hydrocarbons

The reactions of singlet methylene, (CH2)-C-1, with unsaturated hydrocarbons are of considerable significance to the formation and growth of polycyclic aromatic hydrocarbons (PAHs). In this work, we employ high level ab initio transition state theory to predict the high pressure rate coefficient for singlet methylene reacting with acetylene (C2H2), ethylene (C2H4), propyne (CH3CCH), propene (CH3CHCH2), allene (CH2CCH2), 1,3-butadiene (CH2CHCHCH2), 2-butyne (CH3CCCH3), and benzene (C6H6). Both addition and insertion channels are found to contribute significantly to the kinetics, with the insertion kinetics of increasing importance for larger hydrocarbons due to the increasing number of CH bonds and increasingly attractive interactions. We treat the addition kinetics with direct CASPT2 based variable-reaction-coordinate transition state theory. One-dimensional corrections to the CASPT2 interaction energies are obtained from geometry relaxation calculations and CCSD(T)/CBS evaluations. The insertion kinetics is treated with traditional variational TST methods employing CCSD(T)/CBS energies obtained along the CASPT2/cc-pVTZ distinguished coordinate reaction paths. The overall rate constant and branching fractions are obtained from a multiple transition state model that accounts for the physical distinction between tight inner and loose outer transition states. The predicted rate constants, which cover the range from 200 to 2000 K, are found to be in excellent agreement with the available experimental data, with a maximum observed discrepancy of about 40%.