Kinetics of the Thermal Reaction of H Atoms with Propyne

Abstract

The reaction of hydrogen atoms with propyne (CH[triple bond]CCH(3)) was investigated in a heated single pulse-shock tube at temperatures of 874-1196 K and pressures of 1.6-7.6 bar. Stable products from various reaction channels (terminal and nonterminal H addition, and by inference H abstraction) were identified and quantified by gas chromatography and mass spectrometry. The rate constant for the channel involving the displacement of methyl radical from propyne (nonterminal H addition) was determined relative to the methyl displacement from 1,3,5-trimethylbenzene (135-TMB), with k (H + 135-TMB --> m-xylene + CH(3)) = 6.70 x 10(13) exp(-3255/T[K]) cm(3)/mol x s, k(H+propyne-->CH[triple bond]CH+CH3))=6.26 x 10(13) exp(-2267/T[K]) cm3/mole x s. Our results show that the acetylene to allene yield is approximately 2 at 900 K, and decreases with increasing temperature. The rate expression is: k(H+propyne-->CH2=C=CH2+H))=2.07 x 10(14) exp(-3759/T[K]) cm3/mole x s. This is a lower limit for terminal addition. Kinetic information for abstraction of the propargylic hydrogen by H was determined via mass balance. The rate expression is approximately k(H+CH3C[triple bond]CH-->CH[triple bond]C-CH2+H2))=1.20 x 10(14) exp(-4940/T[K])cm3 /mole x s and is only 10% of the rate constant for acetylene formation. All channels from H atom attack on propyne at combustion temperatures have now been determined. Comparisons are made with results of recent ab initio calculations and conclusions are drawn on the quantitative accuracy of such estimates.