Reactions of chlorine oxide radicals. Part 4.—Rate constants for the reactions Cl + OClO, O + OClO, H + OClO, NO + OClO and O + ClO

Abstract

Two independent methods, atomic resonance fluoresence and mass spectrometry with collision-free sampling, have been used to measure separately the rates of consumption of both reactants involved in various rapid bimolecular reactions involving ground state atoms and OClO or ClO radicals: Cl ²P+ OClO [graphic omitted] ClO + ClO (1), O ³P_J+ OClO [graphic omitted] ClO + O₂(4), O ³P_J+ ClO [graphic omitted] Cl + O₂(5) The magnitudes of the rate constants at 298 K obtained from both methods were in good agreement, and gave the following overall mean values: k₁=(5.9 ± 0.9)× 10^–11 cm³ molecule^–1 s^–1, k₄=(5⁺¹_–2)× 10^–13 cm³ molecule^–1 s^–1, k₅=(5.3 ± 0.8)× 10^–11 cm³ molecule^–1 s^–1, Additionally, k₁ was found to be invariant between 298 and 588 K, and mass spectrometric data gave the following further results: NO X²Π+ OClO [graphic omitted] NO₂+ ClO (2), k₂=(3.4 ± 0.5)× 10^{–1 3} cm³ molecule^–1 s^–1 at 298 K; H ²S+ OClO [graphic omitted] OH + ClO (6), k₆=(5.7 ± 1.2)× 10^{–1 1} cm³ molecule^–1 s^–1 at 298 K. The high values for k₁ and k₅, and the low value for k₄, indicate that the interpretations of previous work on O + OClO systems, particularly the flash photolysis of OClO, require major revision. The principal steps following the flash photolysis of OClO are reactions (1) and (5), and not reaction (4) as assumed previously. The high apparent rates of the O + OClO and NO + OClO reactions previously found in discharge-flow systems, where [OClO]₀≲[O]₀ or [NO]₀ are explained on the basis of a rapid autocatalytic scheme consisting of reactions (5) and (1), triggered by the slow reactions (4) or (3).