Reaction of hydrogen atoms with propyne at high temperatures: An experimental and theoretical study

The kinetics of the reaction of hydrogen atoms with propyne (pC ₂H₄) was experimentally studied in a shock tube at temperatures ranging from 1200 to 1400 K and pressures between 1.3 and 4.0 bar with Ar as the bath gas. The hydrogen atoms (initial mole fraction 0.5-2.0 ppm) were produced by pyrolysis of C₂H₅I and monitored by atomic resonance absorption spectrometry under pseudo-first-order conditions with respect to propyne (initial mole fraction 5-20 ppm). From the hydrogen atom time profiles, overall rate coefficients k_ov ≡= -([pC ₃H₄][H]^-1 × d[H]/dt for the reaction H + pC₃H₄ → products (≠ H) were deduced; the following temperature dependence was obtained: k_ov = 1.2 × 10 ^-10 exp(-2270 K/T) cm^{3 -1} with an estimated uncertainty of ±20%. A pressure dependence was not observed. The results are analyzed in terms of statistical rate theory with molecular and transition state data from quantum chemical calculations. Geometries were optimized using density functional theory at the B3LYP/6-31G(d) level, and single-point energies were computed at the QCISD(T)/cc-pVTZ level of theory. It is confirmed that the reaction proceeds via an addition-elimination mechanism to yield C ₂H₂ + CH₃ and via a parallel direct abstraction to give C₃H₃ + H₂. Furthermore, it is shown that a hydrogen atom catalyzed isomerization channel to allene (aC ₃H₄), H + pC₃H₄ → aC ₃H₄ + H, is also important. Kinetic parameters to describe the channel branching of these reactions are deduced.