Vibrational relaxation, dissociation, and dissociation incubation times in norbornene

Abstract

Shock waves in norbornene (bicyclo [2,2,1] hept-2-ene, C7H10)–krypton mixtures have been examined with the laser-schlieren technique over the very wide range of conditions, 542–1480 K, and 34–416 Torr in 0.5%, 2%, and 4% C7H10. The experiments exhibit both vibrational relaxation (542–1480 K) and the retro-Diels–Alder dissociation, norbornene→1,3-cyclopentadiene+ethylene (869–1480 K). Over 869–1304 K, and for pressures below 140 Torr, both relaxation and dissociation are resolved. These experiments provide the first measurements of unimolecular incubation (induction) times for the dissociation of a large polyatomic molecule. The ratio ti/τ decreases from ∼5 to 2 in 900–1300 K. Vibrational relaxation is rapid, log10 Pτ (μs atm)=0.066−6.70/T1/3, with a weak inverse temperature dependence, but is completely consistent with series excitation through the lowest-frequency mode. Dissociation shows very strong unimolecular falloff. A Rice–Ramsperger–Kassel–Marcus (RRKM) model, parametrized to fit the observations (E0=44.2 kcal/mol, 〈ΔE〉down=280 cm−1), finds the reaction closer to the low pressure limit k0 than to k∞ for 1400 K and 100 Torr. Earlier work at low temperatures (<750 K) has Ea too low to extrapolate to the present results. Thus a significant increase in Ea∞ is indicated over 700–1000 K, suggesting a change in mechanism.