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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 ∆) 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 associationelimination 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.

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Pulsed laser photolysis and quantum chemical-statistical rate study of the reaction of the ethynyl radical with water vapor