Absolute Rate Coefficients of the Reactions of C2H with NO and H2 between 295 and 440 K

The rate coefficients of the title reactions were measured at 295 < T < 440 K and at He pressures in the 2−20 Torr range by a laser photodissociation/chemiluminescence technique (LPD/CL), which was validated earlier. Ethynyl radicals were generated by 193 nm excimer laser photolysis of C 2 H 2 (or C 2 HCF 3 ), and the real-time pseudo-first-order decay of thermalized C 2 H was monitored by the luminescence of CH(A 2 Δ) produced by their reaction with O 2, present in large excess. The temperature dependence of the rate coefficient k (C 2 H+NO), reported for the first time, can be expressed by k ( T ) = (1.0 ± 0.2) × 10 -10 exp[−(287 ± 65)/ T ] cm 3 molecule -1 s -1; the k ( T =295 K) value of (3.9 ± 0.4) × 10 -11 is independent of pressure (2−10 Torr) and agrees with literature data at 20 Torr. It is argued that the fast C 2 H + NO reaction is an association−elimination process, probably forming HCN + CO or CN + HCO (or H + CO), and is of major importance to NO-reburning chemistry in fuel-rich hydrocarbon flames. The k (C 2 H+H 2 ) result at 295 K, of (6.9 ± 0.7) × 10 -13, supports the higher of the literature values rather than the slightly lower data (of about 5 × 10 -13 ). The ab initio transition state theory k(T) expression of Harding et al., 18 with the barrier height and the closely correlated ω 6 ‡ TS bending frequency adjusted to fit the average of all k (C 2 H+H 2 ) measurements at 295 K, thus resulting in k(T) = 1.31 × 10 -18 T 2.39 exp(−174/ T ) cm 3 molecule -1 s -1, is found to provide a close representation (standard deviation 21%) of all three available sets of k(T) measurements at higher temperatures, including the present data in the 295−440 K range. Combination with the equilibrium constant leads to k (C 2 H 2 +H)( T ) = 1.50 × 10 -13 T 1.32 exp(−15400/ T ), implying values at flame temperatures almost 20 times higher than recent recommendations.