Kinetics and mechanism of X + CINO → XCl + NO (X = Cl, F, Br, OH, O, N) from 220 to 450 K. Correlation of reactivity and activation energy with electron affinity of X

The rate constants for a series of radical reactions with ClNO, X + ClNO → products where X = Cl, F, Br, OH, O, N, have been measured as a function of temperature in discharge flow systems at pressures between 1 and 2 Torr of helium buffer gas. Radicals were detected by resonance fluorescence (X = Cl, Br, OH, O), laser magnetic resonance (X = OH), and chemical conversion/resonance fluorescence (X = F, N). The rate constants, with units of cm³ molecule^-1 s^-1 and to 95% confidence level, are for Cl + ClNO → Cl₂ + NO, [(6.6±1.2) × 10^-11]e^(128±46)/T; for F + ClNO → FCl + NO, [(1.4 ±0.4) × 10^-10]e^(-28±84)/T. for Br + ClNO → BrCl + NO, [(1.5±0.2) × 10^-11]e^(-52±43)/T; for OH + ClNO → ClOH + NO, [(9.0±4.5) × 10^-12]e^{(-1130±170)/T}; for OH + ClNO → HONO + Cl, [(9.2±6.5) × 10^-14]e^(240±130)/T; for O + ClNO → ClO + NO, [(8.3±0.9) × 10^-12]e^{(-1520 ± 36)/T}; and for N + ClNO → NCl + NO, [(9.2 ± 2.2) × 10^-12]e^{(-2250±90)/T}. Both the reaction activation energies and the logarithms of the room temperature rate constants are found to correlate strongly with the electron affinity of the radical in such a way that high electron affinity leads to enhanced reactivity. The reactivity trend is rationalized by a frontier orbital interaction dominated by the ease with which electron transfer from the ClNO molecule to the X radical can occur to stabilize a polar transition state, a mechanism shown to be widely prevalent in radical-molecule systems. The propensity for this type of interaction is determined by the energy required for electron transfer which, in this case, is given by IP^ClNO - EA^X, where IP refers to ionization potential and EA to electron affinity. In addition, the A factors for the X/ClNO reactions are found to increase as the electron affinity of the radical increases, indicating that the tightness of the transition state is directly related to the height of the activation barrier.