Kinetics of reactions between neutral free radicals. Rate constants for the reaction of CH radicals with N atoms between 216 and 584 K

Abstract

Rate constants have been determined for the reaction between CH radicals and N atoms at temperatures between 216 and 584 K. Discharge-flow methods were used to generate known steady-state concentrations of N atoms in a cell which could either be heated within an oven or cryogenically cooled. CH radicals were produced, in concentrations much smaller than those of the atomic radicals, by pulsed laser photolysis (PLP) of a small concentration of CHBr₃ introduced into the gas flow, and their first-order kinetic decays were observed using the time-resolved laser-induced fluorescence (LIF) technique. The rate constants show only a slight dependence on temperature in the range covered in the present experiments and they are fitted by the functional form: (k₁(T)/cm³ molecule^–1 s^–1=(1.6₆± 0.1₂)× 10^–10(T/298)^{(–0.09 ± 0.2)} where the errors are single standard deviations. If corrections are made to allow for the effects of electronic degeneracies and near degeneracies (Smith and Stewart, J. Chem. Soc. Faraday Trans., 1994, 90, 3221), rate constants, k^′₁(T), can be derived for just that fraction of collisions which occur on the potential-energy surfaces assumed to lead to reaction. The values of k^′₁(T)/cm³ molecule^–1 s^–1 fit the expression (3.4 ± 0.2₅)× 10^–10(T/298)^{(0.04 ± 0.2)}, suggesting that the rate of reaction is determined by ‘capture’ on the long-range potentials which correlate adiabatically with reagents and products. It is proposed that the value k₁(T)/cm³ molecule^–1 s^–1= 2.0 × 10^–10 is used in chemical models of dense interstellar clouds.