Semi-Classical Transition State Theory (SCTST) Rate Coefficients for the Reaction of H and H₂O₂: A High-Level...

The bimolecular reaction of hydrogen atom (H) and hydrogen peroxide (H₂O₂), which plays an important role in combustion of H₂-fuel, has been characterized using a combination of a high-accuracy coupled-cluster-based composite method for constructing the potential energy surface and W. H. Miller's semiclassical transition state theory (SCTST) for determining the reaction rate coefficients from first principles. Two distinct reaction mechanisms have been characterized, pathway R1 for a direct H-abstraction mechanism yielding H₂ + HO₂ and pathway R2 for a nucleophilic substitution (S_N2) mechanism leading to OH + H₂O. Pathway R1 is found to be most important at temperatures below 200 K due to quantum tunneling effects while pathway R2 having a lower barrier dominates at higher temperatures and in combustion conditions (ca. 90%). For the temperature range of 200 - 2000 K, the calculated rate coefficients of the two channels can be represented by the expressions k₁ (T) = 8.5 × 10^-26 × T^4.316 × exp (195/T) cm³ s^-1 and k₂ (T) = 2.0 × 10^-18 × T^2.40 × exp (-1221/T) cm³ s^-1, respectively. The reliability of these theoretical rate coefficients is supported by the close agreement of the calculated total rate coefficient k₁(T) + k₂(T), within 30%, with the available, directly measured experimental k_tot data, lending confidence to our kinetic predictions for use at combustion temperatures where experimental data are lacking.