Theoretical assessment on reaction kinetics HCO and CH₂OH unimolecular decomposition

The HCO and CH₂OH unimolecular decomposition reaction kinetics have been studied theoretically over a wide temperature (300-2700 K) and pressure range (0.01-1000 atm). The potential energy surfaces (PES) of CH2OH and HCO were examined at the CCSD(T)/CBS//QCISD(T)/aug-cc-pvdz level, and the thermochemistry of species are in very good agreement with the values recommended by Active Thermochemical Tables. The decomposition of HCO shows a very strong non-RRKM behavior and thus was treated using state specificity and collision-averaged unimolecular decomposition theory based on the results from a quantum scattering calculations (Keller et al., J. Chem. Phys. 1996, 105, 4983) in which the key parameters of PES agree with current calculations. The obtained theoretical rates can be presented by a Troe formula using the following parameters: k_inf = 4.93E16 × T_-0.93 × exp(-9927/T); κ₀ = 3.94E21 × T^-2.24 × exp(-9568/T); Fc = 0.852 × exp(- T/51.4) + 0.147 × exp(-T/3570) + exp(-3420/T) for N2 as bath gas; κ₀ = 7.43E21×T-2.36 × exp(- 9755/T); Fc = 0.897 × exp(-T/139) + 0.103 × exp(-T/1.09E4) + exp(-4.55E3/T) for Ar as bath gas and setting Helium collisional efficiency 1.3 times larger than Argon. The results agree well with the most of the currently available HCO decomposition experimental data. The recommended Troe formula for CH2OH decomposition rate is: k_inf = 6.535E11 × T^0.567 × exp(-20744/T); κ₀ = 4.145E31 × T^-4.517 × exp(-21116/T); Fc = 0.578 × exp(-T/2.30) + 0.422 × exp(-T/3174) + exp(-1.47E6/T) for Ar as bath gas. The newly obtained rates are very different from the recommendation by Baulch et al. (J. Phys. Chem. Ref. Data, 2005, 34, 757) based on some empirical estimations, but fall in the right range of scattered experimental measurements in the literature. The combustion simulations using these recommended rates result in much better agreement with experimental measured flame speed.