THE HOMOGENEOUS PYROLYSIS OF ACETYLENE .2. THE HIGH-TEMPERATURE RADICAL CHAIN MECHANISM

A comprehensive kineticithermochemical model for the high-temperature radical chain decomposition of acetylene and the polyacetylenes is presented. This mechanism is tested against new shock tube data: time-of-flight mass spectra in C4H2/H2 mixtures over 1600-2100K and laser schlieren experiments in C2H2covering 2700–3500 K. Some earlier time-of-flight measurements on C2H2are also modeled. The paper includes a very extensive consideration of species thermochemistry, and the model is in full accord with the current consensus, having Δ∫H0(C2H) = 135.5kcal/mol. Entropies are computed from recent determinations of molecular parameters for C2H and from new high-level ab initio calculations for C4H reported here The time-of-flight measurements support Δ∫H0(C4 H2) = 111 keal/mol or 28.24 keal/mol for the [Ct-(Ct)] group. Rate constants for the key C-H fission reactions of the acetylenes are taken from RRKM calculations calibrated against the direct measurements of dissociation in C2H2and C4H2. Although there are still some problems with the thermochemistry (in particular, the model probably underestimates the participation of small carbon clusters), it does accurately simulate the early kinetics of the pyrolysis quite generally. The model also resolves some long-standing problems: for example, the current high value for Δ∫ H0 of C2H is now fully compensated by an increase in its entropy, and the chosen rate of C4H2 dissociation is completely in line with the direct measurements of this rate. © 1992, Taylor & Francis Group, LLC. All rights reserved.