Unimolecular Dissociation of Cyclohexene at Extremely High Temperatures: Behavior of the Energy-Transfer Collision Efficiency

Article abstract of DOI:10.1021/j100295a076

The dissociation of cyclohexene has been observed in shock waves with the laser schlieren and pulsed laser flash absorption techniques over 1200-2000 K and 110-550 Torr, using 2percent and 4percent cyclohexene in krypton and 0.2percent cyclohexene in argon.The inverse Diels-Alder molecular elimination to 1,3-butadiene and ethylene is clearly the dominant dissociation channel under all conditions.Rate constants derived for this reaction have a high precision, with rms deviations of only a few percent.Unimolecular falloff, although slight at the lowest temperatures, is clearly discernible for all temperatures.A simple RRKM calculation, using a nonspecific vibration model transition state, is fit the measured rate constants, giving log k_<*> = 15.57-65.7 (kcal/mol)/2.303RT with a barrier of 61.9 kcal/mol.These RRKM calculations were performed with the energy-transfer collision efficiency derived from both fixed (ΔE)_down and (ΔE)_all.A good fit is achieved with either below 1500 K, but above this a fixed (ΔE)_down gives rates which are much too small, and this deficiency cannot be compensated by other changes, whereas a fixed (ΔE)_all provides quite satisfactory agreement with the measurements for all conditions.This situation is similar to that indicated by other large molecule dissociations at extreme temperatures.