A theoretical study on the potential energy surface of the ³C₂ + NO reaction

The reaction of ³C₂ radical with NO molecule is studied at the B3LYP/6-311G(d) level of theory. Both doublet and quartet potential energy surfaces (PES) are considered. On the doublet potential energy surface, we obtain four major pathways in which the pathways ³C ₂ + NO ⇌ 1 CCNO → 2 (NC)CO → 3 CNCO → P ₁ CN + CO and ³C₂ + NO ⇌ 1 → 4 CC(NO) → 2 → 3 → P₁ CN + CO have lower energetics than the other two pathways that form the ³O atom, namely ³C₂ + NO ⇌ 1 CCNO → P₂ ³O + ²CCN and ³C₂+ NO ⇌ 1 CCNO → 5 ON(CC) → P₃ ³O + ²CCN _ring. However, the latter two pathways may compete with the former two pathways. The calculated energetics of the four pathways may account for the experimental results that CN is the primary product at low temperatures and ³O is the main product at higher temperatures. The barrierless entrance to the first adduct isomer 1 and the tight transition states to the products P₁, can provide a reasonable explanation for the experimentally observed negative temperature dependence of rate constants at low temperatures. The pathways on the quartet potential energy surface are less competitive than those on the doublet potential energy surface. Thus, they are negligible.