Ab Initio Chemical Kinetics for the HCCO + H Reaction

Ketenyl radical (HCCO) is an important hydrocarbon combustion intermediate. The mechanisms and kinetics for the reaction of HCCO (X²A″) with H(²S) occurring on both singlet and triplet surfaces have been studied by a combination of ab initio calculations and rate constant predictions at the CCSD(T)/6-311++G(3df,2p)//CCSD/6-311++G(d,p) level of theory. The kinetics and product branching ratios have been investigated in the temperature range of 297–3000 K by variational transition state and Rice–Ramsperger–Kassel–Marcus (RRKM) theories for the production of CH₂(a¹A₁) + CO(X¹Σ⁺) and CH₂(X³B₁) + CO(X¹Σ⁺). Our prediction for the primary product CH₂(a¹A₁) + CO(X¹Σ⁺) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed by k₁(T) = 8.62 × 10^–11T^0.16exp(–20/T) cm³ molecule^–1 s^–1. For the production of CH₂(X³B₁) + CO(X¹Σ⁺), the rate constant k₂ can be represented as k₂(T) = 7.63 × 10^–16T^1.56exp(–386/T) cm³ molecule^–1 s^–1. The predicted product branching ratios for the reaction are in close agreement with experimental data as well. We also predicted the heat of formation at 0 K for ²HCCO, ³CCO, and ¹CCO by CCSD(T)/6-311++G(3df,2p), CBS-QB3, and G2M; the values are in good agreement among one another. Specifically, the CCSD(T) values are: Δ_fH°(HCCO, X²A″) = 42.52 ± 0.70; Δ_fH°(CCO, X³Σ_g) = 91.50 ± 0.54; and Δ_fH°(CCO, a¹Δ) = 110.22 ± 0.54 kcal/mol.