Rate Constants for D + C2H2 → C2HD + H at High Temperature: Implications to the High Pressure Rate Constant for H + C2H2 → C2H3

Abstract

The reflected shock tube technique with D atom atomic resonance absorption spectrometry (ARAS) detection has been used to study the bimolecular reaction, D + C2H2 → C2HD + H. D atoms were produced from the thermal decomposition of C2D5I above ∼1150 K. The initially formed C2D5 radicals rapidly decompose to give D + C2D4. Rate constant values were obtained from both reactant and product hydrogen atom measurements, and these were found to be identical within experimental error. The title reaction proceeds through a vibrationally excited vinyl radical, and the equivalence of results based on reactant and product measurements suggests that radical stabilization is negligible over the temperature and pressure ranges of the experiments. For 1100 ≤ T ≤ 1630 K, the results can be described by the linear-least-squares Arrhenius expression: k = (2.77 ± 0.45) × 10-10 exp(−3051 ± 210 K/T) in units of cm3 molecule-1 s-1, with the one standard deviation of the values from the equation being ±10.7%. Application of RRKM theory with negligible stabilization shows that k = kD∞〈kfε/(kfε+ kbε)〉 where the kiε's refer to RRKM evaluated specific rate constants for forward and backward dissociations, and kD∞ is the high-pressure limiting rate constant for D addition to acetylene. Hence, the present measurements coupled with earlier measurements and modern ab initio potential energy determinations allow for specification of the high-pressure limiting rate constants. The same model can then be used for the protonated reaction, H + C2H2, where a considerable ambiguity has existed for about 30 years.