Kinetics of methylidyne (CH X ²Π) radical reactions with ammonia and methylamines

Laser photolysis/laser-induced fluorescence (LIF) was employed to measure the rate constants for CH (X ²Π) radical reactions with NH₃, CH₃NH₂, (CH₃)₂NH, and (CH₃)₃N. The ground state CH radicals were generated by the multiphoton dissociation of CHBr₃ at 266 nm. The relative CH concentration was probed by LIF at 429.8 nm. The experiments were performed on flowing gas mixtures thus allowing pressure and temperature studies to be performed. No variation of the measured rate constants with pressure was observed over the range 30 to 100 Torr total pressure (mostly argon). The following Arrhenius equations, covering the temperature range 297 to 677 K, are derived: CH + NH₃, k₂ = (8.6±0.6 )×10^-11 exp [(230±30) /T] cm³ s^-1; CH + CH₃NH₂, k₂ = (3.1±0.2)×10^-10 exp [(170±30)/T] cm₃s^-1; CH + (CH₃)₂NH, k₂ = (2.4±0.2)×10^-10 exp [(290±30)/T] cm³ s^-1; CH + (CH₃)₃N, k₂ = (2.6±0.2)×10^-10 exp [ (230±30)/T] cm₃ s^-1. The observed temperature and pressure dependences are in accord with an insertion-elimination mechanism. On a per bond basis, CH reactivity toward NH bonds is only slightly larger than its reactivity toward CH bonds. The observed large rate constants for the methylamine reactions could be attributed to the change in ionization potential (lowering) and polarizability (increasing) as compared with alkanes.