Kinetics and mechanisms of CH radical reactions with fluoromethanes and carbon tetrachloride

Abstract

Laser photolysis/laser-induced fluorescence is employed to determine absolute rate coefficients for the reaction of the methylidyne radical, CH, with CH 3F, CH 2F 2, CHF 3, and CF 4 over the temperature range 295–672 K. Multiphoton dissociation of CHBr 3 at 266 nm forms CH radicals. Relative CH concentrations are monitored by LIF at 430 nm. The following Arrhenius parameters are obtained: k = (2.8±0.2)×10 −10 exp[(280±50 cal/mol)/ RT] cm 3 s −1 for CH+CCl 4, k = (2.0±0.2)×10 −11 exp[(460±70 cal/mol)/ RT] cm 3 s −1 for CH+CH 3F, k = (4.8±0.4)×10 −12 exp[−(330±50 cal/mol)/ RT] cm 3 s −1 for CH+CH 2, F 2, k = (1.4±0.2)×10 −13 exp[(40±110 cal/mol)/ RT] cm 3 s −1 for CH+CHF 3, and k < 7×10 −14 cm 3 s −1 for CH + CF 4. The experimental evidence supports a mechanism of CH radical insertion into CH or CCl bonds followed by fragmentation of the excited adducts as the dominant process in these reactions. The large variance in measured rate constants results mainly from steric hindrance by the unreactive CF moieties.