Theoretical studies of the thermal and chemically activated decomposition of CF3CY2O (Y = F, H) radicals†
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 kdec = 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 kdec/kO2 for the CF3CFHO radical we recommend a rate coefficient of kO2 = 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%.