Low‐pressure thermal decomposition of ONBr and ONCl in shock waves

Rate constants for the low‐pressure unimolecular decomposition of ONBr and ONCl in an argon bath have been determined at temperatures in the vicinity of 1000°K. Both molecules exhibit the usual depression of the observed activation energy below the bond dissociation energy. The Arrhenius expressions obtained are (units of cc mole −1 sec −1 ):\documentclass{article}\pagestyle{empty}\begin{document}$$ ONBr - Ar:\log k = (15.06 \pm 0.27) - (25.06 \pm 1.13)kcalmole^{ - 1} /\theta $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ ONCl - Ar:\log k = (15.95 \pm 0.52) - (43.79 \pm 2.04)kcalmole^{ - 1} /\theta $$\end{document}Treatment of the data by the classical RRK theory yields s ≅ 2.7 ± 1 for ONCl and 3.0 ± 0.6 for ONBr. Coupling the shock tube results for ONCl with lower‐temperature data from Ashmore and Burnett [3], one obtains s ≅ 2.5 ± 0.5 and λ ≈ 1. If it is assumed that s is also 2.5 for ONBr, then one finds the surprising (but tentative) result that λ ONClAr/λONBrAr ≈ 3 to 4.