KINETIC AND MECHANISTIC STUDY OF X + CLOCL-]PRODUCTS (X = BR, CL, F, O, OH, N) OVER THE TEMPERATURE-RANGE 240-373-K

The rate constants for the reactions of X + ClOCl --> products for X = Br, Cl, F, O, OH, and N have been measured over the temperature range 230-400 K. The rate constants are (in units of cm3 molecule-1 s-1) as follows: (2.1 +/- 0.2) x 10(-11) exp[(-425 +/- 30)/T] for Br + ClOCl; (6.0 +/- 0.6) x 10(-11) exp[(127 +/- 30)/T] for Cl + ClOCl; (1.5 +/- 0.5) x 10(-10) exp[(-47 +/- 94)/T] for F + ClOCl; (1.3 +/- 0.8) x 10(-11) exp[(-510 +/- 30)/T] for O + ClOCl; (1.7 +/- 0.8) x 10(-12) exp[(420 +/- 170)/T] for OH + ClOCl; and k298 < 6 x 10(-15) for N + ClOCl. The rate constants for X = Br, Cl, F, and N are found to correlate with the electron affinity of attacking radical, suggesting that the mechanism for these reactions involves the partial transfer of an electron from ClOCl to X, and the activation energy for reaction is determined by the ability of the transition state to accommodate the shift in electron density. This trend is similar to that found for a number of non-hydrogen abstraction reactions (X + ClNO, O3, Cl2), where the reactivity scales with the quantity IP(molecule) - EA(radical), where IP refers to the ionization potential and EA the electron affinity. The reactions of O and OH with ClOCl are significantly faster than predicted by the trend, suggesting that the electron-transfer mechanism is not the only driving force in these reactions, which may involve long-range attractive forces leading to stable intermediates.