Role of the Recombination Channel in the Reaction between the HO and HO₂ Radicals

The kinetics of the gas phase recombination reaction HO + HO₂ + He → HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k_∞ = 3.55 × 10^-12 (T/300)^0.20 cm³ molecule^-1 s^-1 and k₀ = [He] 1.55 × 10^-31 (T/300)^-3.2 cm³ molecule^-1 s^-1. The rate coefficients calculated over the 6 × 10^-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO₂ → H₂O + O₂, indicating that the recombination pathway is irrelevant.