Pressure and temperature dependence of the rate constants for the association reactions of CH radicals with CO and N2 between 202 and 584 K

Abstract

The pulsed laser photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the reactions of CH radicals with CO and with N₂ at temperatures between 202 and 584 K. The observed rate constants depend on pressure throughout the ranges of pressure and temperature covered in the present experiments, and it is clear that the mechanism of reaction is predominantly association. Using a global fitting procedure, the observed second-order rate constants are fitted to yield the following expressions for the rate constants in the limit of low pressure: CH + CO: k^°_1a(T)[M = Ar]= 4.1 × 10^–30(T/298)^–2.5[Ar] cm³ molecule^–1 s^–1, CH + N₂: k^°_2a(T)[M = Ar]= 1.7 × 10^–31(T/298)^–2.3[Ar] cm³ molecule^–1 s^–1 These limiting low-pressure rate constants are compared with values calculated using the methodology of Troe (J. Chem. Phys., 1977, 66, 4758). The high-pressure rate constants deduced from the kinetic behaviour in the fall-off region are compared with the rate constants for relaxation of CH(v= 1) by CO and N₂ which are reported elsewhere (R. A. Brownsword, L. B. Herbert, I. R. Sims, I. W. M. Smith, A. C. Symonds, D. W. A. Stewart, A. Canosa and B. R. Rowe, Chem. Phys. Lett., 1996, submitted). The agreement is poor, the rate constants given by the global fit being appreciably smaller than those for relaxation. It is concluded that the ‘true’ values are those given by the relaxation measurements, whilst those derived from the fitting procedure should be viewed merely as fitting parameters. The reaction between CH and N₂ to yield HCN + N is too slow to play any significant role at the temperatures of these experiments. However, there is evidence that, at the lowest total pressures, the reaction of CH with CO to yield C₂O + H may occur to a significant extent.