NASA - National Aeronautics and Space Administration

Rate coefficients k for the OH + Cl2O reaction are measured as a function of temperature (230–370 K) and pressure by using pulsed laser photolysis to produce OH radicals and laser-induced fluorescence to monitor their loss under pseudo-firstorder conditions in OH. The reaction rate coefficient is found to be independent of pressure, within the precision of our measurements at 30–100 Torr (He) and 100 Torr (N2). The rate coefficients obtained at 100 Torr (He) showed a negative temperature dependence with a weak non-Arrhenius behavior. A room-temperature rate coefficient of k1(297 K) = (7.5  1.1)  1012 cm3 molecule1 s1 is obtained, where the quoted uncertainties are 2 s and include estimated systematic errors. Theoretical methods are used to examine OH···OCl2 and OH···ClOCl adduct formation and the potential-energy surfaces leading to the HOCl + ClO (1 a) and Cl + HOOCl (1 d) products in reaction (1) at the hybrid density functional UMPW1K/6-311 + + G(2df,p) level of theory. The OH···OCl2 and OH···ClOCl adducts are found to have binding energies of about 0.2 kcal mol1. The reaction is calculated to proceed through weak pre-reactive complexes. Transition-state energies for channels (1 a) and (1 d) are calculated to be about 1.4 and about 3.3 kcal mol1 above the energy of the reactants. The results from the present study are compared with previously reported rate coefficients, and the interpretation of the possible non-Arrhenius behavior is discussed.