Thermochemistry and Kinetics of the Reaction of 1-Methylallyl Radicals with Molecular Oxygen

The kinetics of the reaction CH₃CHCHCH₂ + O₂ ⇄ CH₃CHCHCH₂O₂ has been studied using laser photolysis/photoionization mass spectrometry. Room-temperature decay constants of the CH₃CHCHCH₂ radical were determined in time-resolved experiments as a function of bath gas density ([He] = (3−24) × 10¹⁶ molecule cm^-3. The rate constants are in the falloff region under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the temperature range 345−390 K. Equilibrium constants were determined as a function of temperature and used to obtain the enthalpy of reaction 1. At high temperatures (600−700 K), no reaction of CH₃CHCHCH₂ with molecular oxygen could be observed and upper limits to the rate constants were determined (1 × 10^-16 cm³ molecule^-1 s^-1 at 600 K and 2 × 10^-16 cm³ molecule^-1 s^-1 at 700 K). Structures, vibrational frequencies, and energies of several conformations of CH₃CHCHCH₂, CH₃CHCHCH₂O₂, and CH₃CH(OO)CHCH₂ were calculated using ab initio UHF and MP2 methods. The results were used to calculate the entropy changes of the addition reaction. These entropy changes combined with the experimentally determined equilibrium constants resulted in the average R−O₂ bond energy for terminal and nonterminal addition:  ΔH^o₂₉₈ = 82.6 ± 5.3 kJ mol^-1. Earlier experimental results on the kinetics of relaxation to equilibrium in the reaction of allyl radical with O₂ are reanalyzed using an improved kinetic mechanism which accounts for heterogeneous wall decay of the CH₂CHCH₂O₂ adduct. The corrected value of the CH₂CHCH₂−O₂ bond energy (77.0 kJ mol^-1) is determined from the reinterpreted data.