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

Abstract

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: k₀=[He]×1.5×10⁻⁸ exp(−38.5 kJ mol⁻¹/RT) cm³ s⁻¹, k_∞=1.0×10¹⁴ 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 β 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 β 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_∞=1.0×10¹⁴ exp(−78.2 kJ mol⁻¹/RT) and 1.0×10¹⁴ exp(−63.1 kJ mol⁻¹/RT) s⁻¹, respectively. For the reverse reactions, the addition of CH₃ radicals to CH₂O, CH₃CHO, and (CH₃)₂CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol⁻¹, respectively.