Reaction of CH radicals with methane isotopomers

Abstract

The kinetics of the reaction of methylidene [CH(D)] radicals with methane and [ ² H ₄ ]methane have been studied over the temperature range 290–772 K, using pulsed laser flash photolysis of bromoform to generate CH with observation of the resulting temporal CH profile by laser-induced fluorescence. The reactions are all fast [e.g. k _298,CH+CH4 =(9.79±0.37)× 10 ^-11 cm ³ molecule ^-1 s ^-1 ] and exhibit a negative temperature dependence which weakens above 500 K. The reactions can be represented over the experimental temperature range by the following parametrizations, where the error limits on the parameters correspond to ±1 standard deviation (SD): Deuteriation of the methylidene radical has little effect on the kinetics of the reaction, however, deuteriation of methane reduces the rate coefficient by ca. 30% over the entire temperature range. The results of reaction (2) are compared with previous determinations. The isotopic dependence of the reaction is incompatible with a Gorin model of a loose transition state. The suitability of reaction (2) as a calibration reaction, uniquely linking CH and H, is discussed.