Theoretical study on the weakly-bound complexes in the reactions of hydroxyl radical with saturated hydrocarbons (methane, ethane, and propane)

Weakly bound reactant and product complexes in the hydrogen abstraction reactions of an 011 radical with CH4, CH6, and C3H8 are investigated with ab initio molecular orbital methods. The calculated binding energy of the CH4 and OH reactant complex at CCSD(T)/aug-cc-pVTZ (aug-cc-pVDZ) levels is 0.54 (0.74) kcal/ mol. The zero-point vibrational energy correction at MP2 level with the corresponding basis sets reduces the binding energy to 0.16 (0.08) kcal/mol, which is substantially smaller than the recent experimental estimation (about 0.60 kcal/mol). A product complex for the CH4 + OH system, which is more stable than the reactant complex, has the binding energy of 0.77 (0.79) kcal/mol at CCSD(T)/aug-ee-pVTZ (aug-cc-pVDZ) levels with MP2 zero-point energy corrections. The reactant and product complexes are also found for the C2H6 + OH and C3H8 + OH reactions at the CCSD(T)/aug-cc-pVDZ level of theory. The binding energies tend to increase with the number of carbon atoms of the hydrocarbon. The reaction rates and their temperature dependence are estimated, and they are more than 1 order of magnitude larger than the experimentally reported values.