title: Recombination of allyl radicals in the high temperature fall-off regime.
abstract: 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) × 10(55)T( -12.995) exp(-8426/T), k(1a,57Torr) = (1.7 ± 0.5) × 10(60)T( -14.49) exp(-9344/T), and k(1a,30Torr) = (7.5 ± 2.3) × 10(66)T( -15.935) exp(-10192/T) cm(3) mol(-1)s(-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.
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Patrick T
authors.initials: PT
authors.lastname: Lynch
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Annesley
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Aul
authors.forename: Xueliang
authors.initials: X
authors.lastname: Yang
authors.forename: Robert S
authors.initials: RS
authors.lastname: Tranter
mesh_terms: D000498:Allyl Compounds / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D013696:Temperature
chemical_list: D000498:Allyl Compounds; D005609:Free Radicals