Absolute rate coefficients of the reactions of C2H with NO and H-2 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 C2H2 (or C2HCF3), and the real-time pseudo-first-order decay of thermalized C2H was monitored by the luminescence of CH(A(2) Delta) produced by their reaction with O-2, present in large excess. The temperature dependence of the rate coefficient k(C2H+NO), reported for the first time, can be expressed by k(T) = (1.0 +/- 0.2) x 10(-10) exp[-(287 +/- 65)/T] cm(3) molecule(-1) s(-1); the k(T = 295 K) value of (3.9 +/- 0.4) x 10(-11) is independent of pressure (2-10 Torr) and agrees with literature data at 20 Torr. It is argued that the fast C2H + 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(C2H+H-2) result at 295 K, of (6.9 +/- 0.7) x 10(-13), supports the higher of the literature values rather than the slightly lower data (of about 5 x 10(-13)). The ab initio transition state theory k(T) expression of Harding et al.,(18)with the barrier height and the closely correlated omega(6) double dagger TS bending frequency adjusted to fit the average of all k(C2H+H-2) measurements at 295 K, thus resulting in k(T) = 1.31 x 10(-18)T(2.39) exp(-174/T) cm(3) molecule(-1) s(-1), is found to provide a dose 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(C2H2+H)(T) = 1.50 x 10(-13)T(1.32) exp(-15400/T), implying values at flame temperatures almost 20 times higher than recent recommendations.