Shock wave and modelling study of the dissociation kinetics of C₂F₅I

The thermal dissociation of C2F5I was studied in shock waves monitoring UV absorption signals from the reactant C2F5I and later formed reaction products such as CF, CF2, and C2F4. Temperatures of 950-1500 K, bath gas concentrations of [Ar] = 3 × 10-5-2 × 10-4 mol cm-3, and reactant concentrations of 100-500 ppm C2F5I in Ar were employed. Absorption-time profiles were recorded at selected wavelengths in the range 200-280 nm. It was found that the dissociation of C2F5I → C2F5 + I was followed by the dissociation C2F5 → CF2 + CF3, before the dimerization reactions 2CF2 → C2F4 and 2CF3 → C2F6 and a reaction CF2 + CF3 → CF + CF4 set in. The combination of iodine atoms with C2F5 and CF3 had also to be considered. The rate constant of the primary dissociation of C2F5I was analyzed in the framework of statistical unimolecular rate theory accompanied by a quantum-chemical characterization of molecular parameters. Rates of secondary reactions were modelled as well. Experimental rate constants for the dissociations of C2F5I and C2F5 agreed well with the modelling results. The comparably slow dimerization 2CF2 → C2F4 could be followed both by monitoring reactant CF2 and product C2F4 absorption signals, while CF3 dimerization was too fast to be detected. A competition between the dimerization reactions of CF2 and CF3, the recombination of CF2 and CF3 forming C2F5, and CF-forming processes like CF2 + CF3 → CF + CF4 finally was discussed.