Theoretical studies of the thermal and chemically activated decomposition of CF3CY2O (Y = F, H) radicals

Abstract

The C–C bond scission of three fluorinated ethoxy radicals CF₃CF₂O, CF₃CFHO and CF₃CH₂O has been investigated using current theoretical methods. Quantum chemical calculations were performed to derive structures and vibrational frequencies at the B3LYP/cc-pVTZ(+1) level. Higher level ab initio data such as critical energy barriers are provided by application of a modified G3(MP2)-theory. The results have been used as input to statistical kinetic calculations. Complete fall-off curves for the thermal decomposition of all three radicals were calculated in a weak collision treatment by use of RRKM theory and subsequent solution of the master equation. As a part of our calculations rate coefficients of k_dec = 1.1 × 10⁴ s⁻¹, 5.3 × 10⁶ s⁻¹ and 7.4 × 10⁻⁵ s⁻¹ for the thermal decomposition of CF₃CFHO, CF₃CF₂O and CF₃CH₂O radicals by C–C fission, respectively, at atmospheric pressure and a temperature of 300 K were obtained. From a comparison with previous experimental studies on the ratio k_dec/k_O2 for the CF₃CFHO radical we recommend a rate coefficient of k_O2 = 4.2 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ for the reaction of CF₃CFHO with O₂ at 300 K. In addition, a simple approach is developed to calculate the time-dependent decomposition rate coefficient of vibrationally excited CF₃CFHO radicals as they might originate through chemical activation from the exothermic reaction of CF₃CFHOO with NO. It is concluded, that the inclusion of vibrational chemical excitation of CF₃CFHO radicals results in prompt decomposition yields likely to be larger than 35%.