Further study of cyclopropane structural isomerization behind reflected shock waves

The homogeneous thermal isomerization of cyclopropane to propene was studied in the presence of large excesses (99.6%–99.8%) of argon or helium diluent. Reaction temperatures ranged from 1038°?1208°K, and total gas pressures were varied from 533 to 5097 torr. The comparative-rate single-pulse shock-tube method was used, with the well characterized decomposition of cyclohexene serving as the internal standard reaction. Comparison of the measured rate constants for cyclopropane isomerization with k_∞ values extrapolated from “preferred?” lower-temperature rate constants suggests that collision efficiencies for helium and argon relative to cyclopropane, under the present conditions, are β_c ≈ 0.04 and 0.05, respectively. Although the uncertainties are rather large, these results do not support the suggestion that rapidly declining β_c values are largely responsible for the anomalously low rate constants for this reaction at T≥1130°K previously reported by other workers.