Recombination of Allyl Radicals in the High Temperature Fall-Off Regime

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-06, Vol.117 (23), p.4750-4761
• Main Authors: Lynch, Patrick T, Annesley, Christopher J, Aul, Christopher J, Yang, Xueliang, Tranter, Robert S
• Format: Article
• Language: English
• Subjects: Allyl Compounds - chemistry; Channels; Chemistry; Densitometry; Disproportionation; Exact sciences and technology; Free Radicals - chemistry; Iodides; Kinetics and mechanisms; Lasers; Molecular Structure; Organic chemistry; Precursors; Radicals; Reactivity and mechanisms; Simulation; Temperature
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp402484v

The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900–1700 K, 10 ± 1, 29 ± 3, 57 ± 3, and 120 ± 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I → C3H5 + I and C3H5 + C3H5 → C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k 1a,124Torr = (2.6 ± 0.8) × 1055 T  –12.995 exp(−8426/T), k 1a,57Torr = (1.7 ± 0.5) × 1060 T  –14.49 exp(−9344/T), and k 1a,30Torr = (7.5 ± 2.3) × 1066 T  –15.935 exp(−10192/T) cm3 mol–1s–1. The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C6H10 ⇌ C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol–1 K–1, which is significantly smaller than that group additivity predicts, but larger than other reference literature values.