Mechanism and rate constant of the reaction of atomic hydrogen with propyne

The potential energy surface for the reaction of atomic hydrogen with propy ne has been studied at the G3//UB3LYP/6-31G(d) level of theory. Three react ion entrances were revealed, namely, terminal addition, nonterminal additio n, and direct H-abstraction, leading to CH3CCH2, CH3CHCH, and H-2+C3H3, res pectively. The respective activation barriers are 1.7, 3.9, and 8.4 kcal/mo l. The CH3-extrusion from CH3CHCH forms C2H2 via a barrier of about 32 kcal /mol. Several H-shift paths along the CCC skeleton were also examined for t hree C3H5 isomers. Multichannel RRKM and TST calculations have been carried out for the total and individual rate constants over a wide range of tempe ratures and pressures. The total rate constants possess both positive tempe rature dependence and typical "S" shaped fall-off behavior. At atmospheric pressure, the collisional stabilization of the initial adducts dominates th e H+CH3CCH reaction at temperatures lower than 500 K, and at T > 1000 K, CH 3 and C2H2 are the major products. Moreover, the direct H-abstraction chann el also contributes significantly to the overall reaction. The theoretical results are compared with those of previous studies. (C) 2000 American Inst itute of Physics. [S0021-9606(00)30318-X].