We experimentally determined complete fallo † curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two di †erent techniques, laser Ñash photolysis and fast Ñow reactor were used both coupled to a detection of radicals by laser induced Ñuorescence. Experiments were performed at total C 2 H 5 Op ressures between 0.001 and 60 bar of helium and in the temperature range of 391È471 K. Under these conditions the b-CÈC scission (1a)is the dominating decompositionFrom a complete analysis of the experimental fallo † curves the low and the high pressure limiting rate constants of 3.3 ] 10~8 exp([58.5 kJ mol~1/RT ) cm3 s~1 and exp([70.3 kJ k 1a,0 \ [He] k 1a,= \ 1.1 ] 1013 mol~1/RT ) s~1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ^30%. Preexponential factor and activation energy are signiÐcantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the b-CÈH scission (1b) of exp([84 kJ CH 3 CH 2 O~] M ] CH 3 CHO ] H~] M k 1b,= \ 1.3 ] 1013 mol~1/RT ) s~1. Atmospheric implications are discussed.

The temperature and pressure dependence of the rate constant for the unimolecular decomposition of i-propoxy radicals has been determined using the laser photolysis/laser induced Ñuorescence technique. Important features of the potential energy surface have been calculated by ab initio methods. Experiments have been performed at total pressures between 0.01 and 60 bar of helium and in the temperature range 330È408 K. The low and the high pressure limiting rate constants have been extracted from a complete fallo † analysis :exp([43.8 kJ mol~1/RT ) cm3 s~1 and exp([63.7 kJ mol~1/RT )We estimate an uncertainty for these rate constants of ^30%. Both rate constants have been discussed in terms of statistical unimolecular rate theory. Very good agreement between the calculated and the experimental rate constants has been found.

The reaction of 1-methylvinoxy radicals, CH3COCH2, with molecular oxygen has been investigated by experimental and theoretical methods as a function of temperature (291-520 K) and pressure (0.042-10 bar He). Experiments have been performed by laser photolysis coupled to a detection of 1-methylvinoxy radicals by laser-induced fluorescence LIF. The potential energy surface calculations were performed using ab inito molecular orbital theory at the G3MP2B3 and CBSQB3 level of theory based on the density function theory optimized geometries. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reaction under investigation. At 295 K, no pressure dependence of the rate constant for the association reaction has been observed: k(1,298K) = (1.18 +/- 0.04) x 10(-12) cm3 s(-1). Biexponential decays have been observed in the temperature range 459-520 K and have been interpreted as an equilibrium reaction. The temperature-dependent equilibrium constants have been extracted from these decays and a standard reaction enthalpy of deltaH(r,298K) = -105.0 +/- 2.0 kJ mol(-1) and entropy of deltaS(r,298K) = -143.0 +/- 4.0 J mol(-1) K(-1) were derived, in excellent agreement with the theoretical results. Consistent heats of formation for the vinoxy and the 1-methylvinoxy radical as well as their O2 adducts are recommended based on our complementary experimental and theoretical study deltaH(f,298K) = 13.0 +/- 2.0, -32. 9+/- 2.0, -85.9 +/- 4.0, and -142.1 +/- 4.0 kJ mol(-1) for CH2CHO, CH3COCH2 radicals, and their adducts, respectively.