Reaction of hydroxyl with nitrosyl chloride: kinetics and mechanisms (original) (raw)

The kinetics and products of the gas-phase reaction of OH with ClNO have been studied in a fast flow discharge system using the reaction of hydrogen atoms with NOz as the source of OH. The decay of OH in excess ClNO at a total pressure of 1.05 0.05 Torr in He was followed by resonance fluorescence. at 309.5 nm at temperatures from 262 to 373 K in 2.4-cm-diameter flow tubes coated with boric acid, phosphoric acid, or halocarbon wax. Kinetic studies were also carried out in a 4.8-cm-diameter boric acid coated flow tube. While the rate of decay of OH increased both above and below room temperature in both boric acid coated flow tubes, more typical Arrhenius behavior was observed in the phosphoric acid and halocarbon wax coated tubes. The stable products observed by mass spectrometry from 276 to 353 K and from 0.2 to 0.8 Torr total pressure were HOCl, HONO, and CIz, which supports the existence of two reaction channels suggested in earlier work by Poulet and co-workers (1981): OH + ClNO-HOCl + NO (la), OH + ClNO-HONO + C1 (lb). Molecular CIz results from the reaction of Cl atoms with the excess ClNO present. The ratio of the products HOC1/ClZ did not change significantly with temperature or total pressure. All of these data taken together indicate that the temperature dependence observed in the boric acid coated flow tubes is due to an unusual surface reaction rather than intermediate complex formation in reaction 1 b. Taking only the data for which the surface reaction is believed to be negligible, the Arrhenius expression for the rate constant kl (= k l , + klb) for the reaction of OH with ClNO is kl = (1.5T; X exp[-(458 f 2 7) / a cm3 molecule-' s-l, with kl = (3.2 0.5) X cm3 molecule-' s-' at 298 K, where all errors are two standard deviations. From these kinetic data, a typical tropospheric lifetime of ClNO with respect to reaction with OH is calculated to be-36 days, too slow to compete with ClNO photolysis.