Shock wave studies of the pyrolysis of fluorocarbon oxygenates. I. The thermal dissociation of C3F6O and CF3COF

Abstract

The thermal decomposition of hexafluoropropylene oxide, C3F6O, to perfluoroacetyl fluoride, CF3COF, and CF2 has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k(1) = 1.1 x 10(14) exp(-162(+4) kJ mol(-1)/RT) s(-1) agrees well with literature data and modelling results. Using the reaction as a precursor, equimolar mixtures of CF3COF and CF2 were further heated. Combining experimental observations with theoretical modelling (on the CBS-QB3 and G(4)MP(2) ab initio composite levels), CF3COF is shown to dissociate on two channels, either leading to CF2 + COF2 or to CF3 + FCO. By monitoring the CF2 signals, the branching ratio was determined between 1400 and 1900 K. The high pressure rate constants for the two channels were obtained from theoretical modelling as k(5,infinity)(CF3COF -> CF2 + COF2) = 7.1 x 10(14)exp(-320 kJ mol(-1)/RT) s(-1) and k(6),(infinity)(CF3COF -> CF3 + FCO) = 3.9 x 10(15) exp(-355 kJ mol(-1)/RT) s(-1). The experimental results obtained at [Ar] approximate to 5 x 10(-6) mol cm(-3) were consistent with modelling results, showing that the reaction is in the falloff range of the unimolecular dissociation. The mechanism of secondary reactions following CF3COF dissociation has been analysed as well.