Kinetic (T=201-298 K) and Equilibrium (T=320-420 K) Measurements of the C3H5 + O2 ⇆ C3H5O2 Reaction

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (T = 201-298 K), association kinetics were observed, and the measured time-resolved C3H5 radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {p = 0.4-36 Torr, [He] = (1.7-118.0) x 10(16) cm(-3)}. The bimolecular rate coefficients obtained vary in the range (0.88-11.6) X 10(-13) cm(3) s(-1). In higher-temperature experiments (T = 320-420 K), the C3H5 radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting In K-p values versus 1/T in a modified van't Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of Delta H Alpha degrees(298) = -78.3 +/- 1.1 kJ mol(-1) and Delta S degrees(298) = -129.9 +/- 3.1 J mol(-1) KT-1, with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as Delta H Alpha degrees(298) = -75.6 +/- 2.3 kJ mol(-1).