Direct determination of the equilibrium constant and thermodynamic parameters in the reaction of pentadienyl radicals with O 2

Abstract

The reaction of pentadienyl radicals (C5H7) with O2 has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals were generated either by the photolysis of 1,3-pentadiene or by a two step process, photolyzing carbon tetrachloride to form Cl atoms, which then abstracted a hydrogen atom from 1,4-pentadiene. The equilibrium between pentadienyl radicals, O2 and pentadienylperoxy radicals could be observed over the temperature range 268−308 K. An analysis of the time-dependent signal of pentadienyl radicals was used to evaluate the equilibrium constant. From the temperature dependence of the equilibrium constant, the enthalpy change for the reaction C5H7 + O2 → C5H7O2 was found to be 56 ± 5 kJ mol-1. The C−O bond energy in the C5H7−O2 adduct is weaker than those of allyl-type peroxy radicals. Possible values of the heats of formation of C5H7 and C5H7−O2 radicals and the resonance stabilization energy of the C5H7 radical are discussed.