VIBRATIONAL-ENERGY TRANSFER IN SHOCK-HEATED NORBORNENE

Recently, Kiefer et al. [J. H. Kiefer, S. S. Kumaran, and S. Sundaram, J. Chem. Phys. 99, 3531 (1993)] studied shock-heated norbornene (NB). in krypton bath gas using;the laser-schlieren technique and observed vibrational relaxation, unimolecular dissociation (to 1,3-cyclopentadiene and ethylene), and dissociation incubation times. Other workers have obtained an extensive body of high-pressure limit unimolecular reaction rate data at lower temperatures using conventional static and how reactors. In the present work, we have developed a vibrational energy transfer-unimolecular reaction:model based on steady-state RRKM calculations and time-dependent master equation calculations to satisfactorily describe all of the NE data (incubation times, vibrational relaxation times, and unimolecular rate coefficients): The results cover the temperature range from similar to 300 to 1500 K and the excitation energy range from similar to 1000 to 18 000 cm-l. Three different models (based on the exponential step-size distribution) for the average downward energy transferred per collision, [Delta E](down) were investigated. The experimental data are too limited to enable the identification of a preferred model and it was not possible to determine whether the average [Delta E](down) is temperature dependent. However, all three [Delta E](down) models depend linearly on vibrational energy and it is concluded that standard unimolecular reaction rate codes must be revised to include energy-dependent microcanonical energy transfer parameters. The choice;of energy transfer model affects the deduced reaction critical energy by more than 2 kcal mol(-1), however, which shows the importance of energy transfer in determining thermochemistry from unimolecular reaction fall-off data. It is shown that a single set of Arrhenius parameters gives a good fit of all the low temperature data and the shock-tube data extrapolated to the high pressure limit, obviating the need to invoke a change in reaction mechanism from concerted to diradical for high temperature conditions. Some possible future experiments are suggested. (C) 1995 American Institute of Physics.