The beta C-C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323-383 K. The low and the high pressure limiting rate constants as well as the broadening factor F-c have been extracted from a complete falloff analysis of the experimental results: k(0) = [He] x 1.5 x 10(-8) exp(-38.5 kJ mol(-1)/RT) cm(3) s(-1), k(infinity) = 1.0 x 10(14) exp(-60.5 kJ mol(-1)/RT) s(-1), and F-c = 0.87 - T/870 K. We anticipate an uncertainty for these rate constants of +/- 30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the beta C-C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for beta C-C bond scission rate constants of all alkoxy radicals of A = 10(14 +/- 0.3) s(-1). Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k(infinity) = 1.0 x 10(14) exp(-78.2 kJ mol(-1)/RT) and 1.0 x 10(14) exp(-63.1 kJ mol(-1)/RT) s(-1), respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)(2)CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol(-1), respectively.