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

1998-10-21T00:00:00Z (GMT) by Vadim D. Knyazev Irene R. Slagle
The kinetics of the reaction CH<sub>3</sub>CHCHCH<sub>2</sub> + O<sub>2</sub> ⇄ CH<sub>3</sub>CHCHCH<sub>2</sub>O<sub>2</sub> has been studied using laser photolysis/photoionization mass spectrometry. Room-temperature decay constants of the CH<sub>3</sub>CHCHCH<sub>2</sub> radical were determined in time-resolved experiments as a function of bath gas density ([He] = (3−24) × 10<sup>16</sup> molecule cm<sup>-3</sup>. 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 CH<sub>3</sub>CHCHCH<sub>2</sub> with molecular oxygen could be observed and upper limits to the rate constants were determined (1 × 10<sup>-16</sup> cm<sup>3</sup> molecule<sup>-1</sup> s<sup>-1</sup> at 600 K and 2 × 10<sup>-16</sup> cm<sup>3</sup> molecule<sup>-1</sup> s<sup>-1</sup> at 700 K). Structures, vibrational frequencies, and energies of several conformations of CH<sub>3</sub>CHCHCH<sub>2</sub>, CH<sub>3</sub>CHCHCH<sub>2</sub>O<sub>2</sub>, and CH<sub>3</sub>CH(OO)CHCH<sub>2</sub> 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−O<sub>2</sub> bond energy for terminal and nonterminal addition:  Δ<i>H</i><sup>o</sup><sub>298</sub> = 82.6 ± 5.3 kJ mol<sup>-1</sup>. Earlier experimental results on the kinetics of relaxation to equilibrium in the reaction of allyl radical with O<sub>2</sub> are reanalyzed using an improved kinetic mechanism which accounts for heterogeneous wall decay of the CH<sub>2</sub>CHCH<sub>2</sub>O<sub>2</sub> adduct. The corrected value of the CH<sub>2</sub>CHCH<sub>2</sub>−O<sub>2</sub> bond energy (77.0 kJ mol<sup>-1</sup>) is determined from the reinterpreted data.