VIBRATIONAL-RELAXATION, DISSOCIATION, AND DISSOCIATION INCUBATION TIMES IN NORBORNENE

Shock waves in norbornene (bicyclo [2,2,I] 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 t(i)/tau decreases from approximately 5 to 2 in 900-1300 K. Vibrational relaxation is rapid, log10 Ptau (mus 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, [DELTAE]down = 280 cm-1), finds the reaction closer to the low pressure limit k0 than to k(infinity) for 1400 K and 100 Torr. Earlier work at low temperatures ( < 750 K) has E(a) too low to extrapolate to the present results. Thus a significant increase in E(a)infinity is indicated over 700-1000 K, suggesting a change in mechanism.