Identification of the Sequence CH(2Π) + C2H2 → C3H2 + H (and C3H + H2) Followed by C3H2 + O → C2H + HCO (or H + CO) as C2H Source in C2H2/O/H Atomic Flames

In this work, the novel reaction mechanism initiated by the fast CH( 2 Π) + C 2 H 2 → C 3 H 2 + H reaction (r13a) and followed by C 3 H 2 + O → C 2 H + HCO (or H + CO) (r25a) was established as the dominant C 2 H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C 2 H 2 /O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reaction-time data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH( 2 Π) was evidenced by CH 4 -addition experiments on room-temperature C 2 H 2 /O/H systems, where CH( 2 Π) and CH 2 ( 1 A 1 ) are the sole intermediates that react rapidly with CH 4 . The observed strong reduction of [C 3 H 2 ], [C 2 H], and [C 4 H 2 ] upon CH 4 addition could be correlated quantitatively with the known removal of CH( 2 Π) by CH 4 . The reaction channels r13a and r25a as sources of C 3 H 2 and C 2 H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C 2 H 2 /O/H mixtures at 600 K. By a similar method, the observed C 3 H radicals could be attributed to a minor channel of the CH( 2 Π) + C 2 H 2 reaction (r13) parallel to that producing C 3 H 2 . At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived: C 3 H 2 plus H, and C 3 H plus H 2 . Concomitantly with the identification of reaction r25a as dominant C 2 H source, the rate constant of the reaction C 2 H + O (r19) was determined relative to the well-known kinetic coefficients of C 2 H + C 2 H 2 and C 2 H + O 2: k 19 = (9 ± 4) × 10 -11 cm 3 molecule -1 s -1 at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C 3 H x radicals ( x = 1−3) that arise in the fast reactions of CH(X 2 Π) and CH 2 (a 1 A 1 ) with C 2 H 2 .