Predictive Theory for the Addition and Insertion Kinetics of ~1CH_2 Reacting with Unsaturated Hydrocarbons

摘要

The reaction of singlet methylene, ~1CH_2, with unsaturated hydrocarbons is 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 (C_2H_2), ethylene (C_2H_4), propyne (CH_3CCH), allene (CH_2CCH_2), 1,3-butadiene (CH_2CHCHCH_2), and benzene (C_6H_6). 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. We treat the addition kinetics with direct CASPT2 based variable-reaction-coordinate transition-state-theory (VRC-TST). One dimensional corrections to the CASPT2 interaction energies are obtained from geometry relaxation calculations and CCSD(T)/CBS evaluations. The insertion kinetics are treated with traditional TST methods employing CCSD(T)/CBS energies obtained along CASPT2/cc-pVTZ distinguished reaction coordinate 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. Our 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 maximum observed discrepancies of 20-30%.