THERMOCHEMISTRY AND KINETICS OF THE REACTION OF 1-METHYLALLYL RADICALS WITH MOLECULAR-OXYGEN

The kinetics of the reaction CH3CHCHCH2 + O-2 reversible arrow CH3CHCH CH2O2 has been studied using laser photolysis/ photoionization mass sp ectrometry. Room-temperature decay constants of the CH3CHCHCH2 radical were determined in time-resolved experiments as a function of bath ga s density ([He] (3-24) x 10(16) molecule cm(-3). The rate constants ar e in the falloff region under the conditions of the experiments. Relax ation to equilibrium in the addition step of the reaction was monitore d within the temperature range 345-390 K. Equilibrium constants were d etermined as a function of temperature and used to obtain the enthalpy of reaction 1. At high temperatures (600-700 K), no reaction of CH3CH CHCH2 with molecular oxygen could be observed and upper limits to the rate constants were determined (1 x 10(-16) cm(3) molecule(-1) s(-1) a t 600 K and 2 x 10(-16) cm(3) molecule(-1) s(-1) at 700 K). Structures , vibrational frequencies, and energies of several conformations of CH 3CHCHCH2, CH3CHCHCH2O2, and CH3CH(OO)CHCH2 were calculated using ab in itio UHF and MP2 methods. The results were used to calculate the entro py changes of the addition reaction. These entropy changes combined wi th the experimentally determined equilibrium constants resulted in the average R-O-2 bond energy for terminal and nonterminal addition: Delt a H-298(0) = 82.6 +/- 5.3 kJ mol(-1). Earlier experimental results on the kinetics of relaxation to equilibrium in the reaction of allyl rad ical with O-2 are reanalyzed using an improved kinetic mechanism which accounts for heterogeneous wall decay of the CH2CHCH2O2 adduct. The c orrected value of the CH2CHCH2-O-2 bond energy (77.0 kJ mol(-1)) is de termined from the reinterpreted data.