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

Reaction of pentadienyl radicals (C5H7) with O-2 has been studied by combin ation of pulsed laser photolysis and photoionization mass spectrometry. The se radicals could be generated either by the photolysis of 1.3-pentadiene o r by the two-step reaction of carbon tetrachloride photolysis followed by t he H-atom abstraction reaction of Cl atom with 1,4-pentadiene. The equilibr ium between pentadienyl radicals, O-2 and pentadienylperoxy radicals could be observed over the range 268-308 K. An analysis of the temporal signal of pentadienyl radicals was used to evaluate the equilibrium constant. Third- law analysis was used to evaluate the enthalpy change for the reaction C5H2 + O-2 reversible arrow C5H-O-2. The observed C-O bond energy in the C5H7-O -2 adduct was found to be 56.0 +/- 2.2 kj.mol(-1) which is lower than the v alues of peroxy radicals formed with allyl and cyclohexenyl radicals which have an allylic resonance structure. (C) 2000 Academie des sciences/Edition s scientifiques et medicales Elsevier SAS.