CH(A 2 Δ) Formation in Hydrocarbon Combustion: The Temperature Dependence of the Rate Constant of the Reaction C 2 H + O 2 f CH(A 2 Δ) + CO 2 Rehab M. I. Elsamra, Stijn Vranckx, and Shaun A. Carl* UniVersity of LeuVen, Department of Chemistry, Celestijnenlaan 200F, LeuVen, Belgium ReceiVed: July 6, 2005; In Final Form: August 26, 2005 The temperature dependence of the rate constant of the chemiluminescence reaction C 2 H + O 2 f CH(A) + CO 2 , k 1e , has been experimentally determined over the temperature range 316-837 K using pulsed laser photolysis techniques. The rate constant was found to have a pronounced positive temperature dependence given by k 1e (T) ) AT 4.4 exp(1150 ( 150/T), where A ) 1 × 10 -27 cm 3 s -1 . The preexponential factor for k 1e , A, which is known only to within an order of magnitude, is based on a revised expression for the rate constant for the C 2 H + O( 3 P) f CH(A) + CO reaction, k 2b , of (1.0 ( 0.5) × 10 -11 exp(-230 K/T) cm 3 s -1 [Devriendt, K.; Van Look, H.; Ceursters, B.; Peeters, J. Chem. Phys. Lett. 1996, 261, 450] and a k 2b /k 1e determination of this work of 1200 ( 500 at 295 K. Using the temperature dependence of the rate constant k 1e (T)/k 1e (300 K), which is much more accurately and precisely determined than is A, we predict an increase in k 1e of a factor 60 ( 16 between 300 and 1500 K. The ratio of rate constants k 2b /k 1e is predicted to change from 1200 ( 500 at 295 K to 40 ( 25 at 1500 K. These results suggest that the reaction C 2 H + O 2 f CH(A) + CO 2 contributes significantly to CH(AfX) chemiluminescence in hot flames and especially under fuel-lean conditions where it probably dominates the reaction C 2 H + O( 3 P) f CH(A) + CO. Introduction In the absence of heated soot particles, having a continuum emission perceived as orange-yellow, hydrocarbon flames would be invisible were it not for the handful of electronically excited species with electronic transitions in the 150-300 kJ mol -1 range and superthermal populations. Most of these species have long been identified spectroscopically, CH(A 2 Δ,B 2 Σ - ), C 2 - (d 3 Π g ), CN(B 2 Σ + ), HCO(A ˜ 2 A′′,B ˜ 2 A′), CO(a′ 3 Σ,d 3 ∆,e 3 Σ), and CO 2 (A ˜ 1 B 2 ), 1-7 and from their non-Boltzmann concentrations one must conclude their source to be either chemical reaction or rovibronic-to-rovibronic (E, υ, j f E′, υ′, j′) energy transfer. 8 However, finding the reactions responsible for their production and determining the associated rate constants have proven to be difficult challenges. Quantitative interpretation of flame chemiluminescence data, in terms of the concentration of one of the precursor species or the precursor concentration product, requires some knowledge of the magnitude of the rate constant. Unfortunately though, despite several attempts at their determination, accurate absolute rate constants have not been forthcoming for the gas-phase chemiluminescence reactions. This is due simply to the com- bined uncertainties associated with the determination of absolute concentrations of both (mainly radical) precursors and the electronically excited product. Thus, absolute rate constants determined so far for these reactions are probably known only within about an order of magnitude. Nevertheless, attempts to interpret flame chemiluminescence go back many years, with much recent work focusing on qualitatively linking OH(AfX) and CH(AfX) emission intensities 9 to flame equivalence ratios 10,11-13 heat release rates, 14,15 and the final stages of the C 2 -hydrocarbon and CH x reaction chains. 16,17 In this paper we focus on the chemical reactions responsible for the production of CH(A 2 ∆) and, by extension, CH(B 2 Σ - ). The former gives rise to the relatively intense blue emission at ca. 430 nm, and the latter to a less intense emission, just beyond the visible range, at ca. 390 nm. The transition at 430 nm can be readily observed in natural gas flames by the unaided eye. Only a few reactions stand out as being plausible sources of electronically exited CH in flame environments. Of these, only studies of R1, R2, and R4 have so far appeared in the literature: for the other reactions, the overall rate constants and yield of electronically excited CH is unknown. However, it has become clear over recent years that most, if not all, CH(A) formed in hydrocarbon flames is associated with the simulta- neous presence of C 2 H 2 and O (and therefore usually O 2 ). Reaction R2b was proposed many years ago by Glass et al. 18 as being the main source of CH(A) in ethyne and methane flames. The other main contenders for CH(A) formation have been the C 2 + OH reaction (R4c) proposed by Gaydon 19 and by Porter et al. 20 and reaction R1e proposed by Matsuda et al. 21 and by Renlund et al. 22 For R1 and R2, the combination of the high C-H bond strength of C 2 H and a very stable coproduct leads to sufficient * Corresponding author. E-mail: shaun.carl@chem.kuleuven.be. ∆rH298K/kJ mol -1 C2H + O2 f CH(B, A) + CO2 -56, -85 (R1c,e) C2H + O f CH(B, A) + CO -22, -51 (R2a,b) C + H + M f CH(B, A) + M -24, -53 (R3a,b) C2 + OH f CH(C, B, A) + CO -1.5, -74, -103 (R4a,b,c) C2 + HO2 f OH(A) + C2O -36 (R5a) f CH(C, B, A) + CO2 -262, -335, -364 (R5b,c,d) C2 + NH2 f CH(B, A) + HCN +19, -10 (R6a,b) C2 + CH3O f CH(A) + H2CCO -21 (R7) CH + HONC f CH(B, A) + HNCO -15, -44 (R8a,b) CH + HCNO f CH(A) + HNCO -3 (R9a) HOCC + O f CH(B, A) + CO2 -29, -58 (R10a,b) 10287 J. Phys. Chem. A 2005, 109, 10287-10293 10.1021/jp053684b CCC: $30.25 © 2005 American Chemical Society Published on Web 10/20/2005