Computational Study of the Kinetics of Hydrogen Abstraction from Fluoromethanes by the Hydroxyl Radical

One of the fastest steps in the initial decomposition of HFCs under combustion conditions is hydrogen atom abstraction by hydroxyl radicals. We have utilized ab initio quantum mechanics and transition-state theory (TST) to calculate the temperature dependence of rate constants for the reactions of OH with CH4, CH3F, CH2F2, and CHF3. Rate constants calculated using HF/6-31G(d) frequencies and MP2(full)/6-31G(d) structures to evaluate reactant and transition-state partition functions and the Hartree-Fock imaginary frequency, ωi, to compute Eckart tunneling factors, Γ, yielded rate constants that were substantially greater than experiment. Adjustment of the energy barrier to effect agreement between experimental and calculated rate constants at 298 K gave Arrhenius plots that exhibited markedly greater curvature than measured rate constants. When the imaginary frequency and barrier height were calculated by fitting high-level (G2) energies along the reaction path with a semiempirical Eckart function, it was found that the calculated imaginary frequency is a factor of 2.5 lower than the HF/6-31G(d) value, indicating that the energy barrier is considerably broader than predicted by the latter frequency. When the new imaginary frequencies and barrier heights were used to calculate rate constants, it was found that kTST < kexpt but that lowering the barrier height (by an average of 4.7 kJ/mol for the four reactions) yields calculated rate constants that are in excellent agreement with experiment at all temperatures.