Reactions of chlorine oxide radicals. Part 1.—Reaction kinetics of the ClO radical

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Abstract
The reactions of atomic chlorine and atomic oxygen with chlorine dioxide have been investigated in a flow system, at total pressures between 0.5 and 3 mm Hg, and at 298°K. A major product of both reactions is the ClO molecule, which was detected by observations on bands of its ultra-violet absorption spectrum. The reaction of oxygen atoms in a nitrogen carrier with chlorine dioxide was very rapid (k1 > 2 × 1013 cm3 mole–1 sec–1 at 298°K): O + ClO2= O2+ ClO (1). However, one ClO2 molecule was found to remove 2.0 oxygen atoms, and it is concluded that reaction (2), O + ClO = O2+ Cl (2), competes for oxygen atoms with reaction (1). The ratio k1/k2 was approximately 4 at 298°K. When ClO radicals free from ClO2 were mixed with streams either of oxygen atoms or of nitric oxide, rapid reactions occured in both cases: O + ClO = O2+ Cl NO + ClO = NO2+ Cl The same concentrations of oxygen atoms and nitric oxide react with an equal concentration of ClO. Either oxygen atoms or nitric oxide may therefore be used to “titrate” ClO radicals in order to measure their absolute concentrations and hence to find the value of the extinction coefficient ε for the ClO radical. At 298°K and at a wavelength of 2577 Å, ε was (1.36±0.04)× 106 cm2 mole–1 using total pressures between 1.25 and 2.11 mm Hg. Kinetic experiments on the decay of ClO radicals showed that the reaction was second-order in [ClO], and no detectable contribution from any first-order removal of ClO was observed. In terms of the extinction coefficient ε(cm2 mole–1) of ClO in the absorption continuum at 2577 Å, the second order rate constant k3/ε=(1.26±0.05)× 104 cm sec–1 at 298°K. No dependence of the rate constant k3/ε upon total pressure of argon in the range 1.3 to 3.0 mm Hg was detected, and similar second-order kinetics and values of k3/ε were found when molecular oxygen was present in the carrier gas. Combining this value of k3/ε with the value of ε found in this work, k3=(1.7±0.1)× 1010 cm3 mole–1 sec–1 at 298°K.

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