Kinetic Isotope Effect in the CH[2Π] + O2 Reaction

The reaction rates for CH and CD with O 2 have been measured for temperatures between 293 and 720 K. The rate constant has a slight negative temperature dependence which can be well fit over the temperature range by (4.7 ± 0.2) × 10 -11 ( T /293) -0.22±0.1 cm 3 molecule -1 s -1 . The deuterium isotope effect has been measured to be k H / k D = 1.16 ± 0.06 at 293 K and is approximately constant to within experimental error between 293 and 720 K. The isotope effect and the temperature dependences can be described by a barrierless addition with no collisional stabilization of the adduct before dissociation to products. The effects of the known species HCO 2 and HOCO are not evident in the CH + O 2 reaction rate constant at these temperatures and pressures. Simple transition state theory can model the kinetic isotope effect if the transition state for addition involves a weakening of the C−H bond from the free CH molecule.