Temperature dependence and mechanism of the reaction between atomic O(^3P) and chlorine dioxide

Abstract

Second-order rate constants for the decay of O(^3P) in excess chlorine dioxide, k_(11), were measured by flash photolysis-atomic resonance fluorescence as a function of total pressure (20-600 Torr argon) and temperature (248-312 K). It was found that (1) k_(11) is pressure dependent with a value, k_b, that is nonzero at zero pressure and (2) both the third-order rate constant (dk_(11)/d[M])_(lMl=O) = k_0, and k_b have negative temperature dependences. These results are consistent with an association reaction leading to an intermediate having two decomposition channels: O + OClO ⇌ ClO_3* (1, 2); ClO3_* + M → + ClO_3 + M (3); ClO_3* → ClO + O_2* (4), with E_(o2) > E_(o4). The measured k_0 values were used in conjunction with Troe's expression for unimolecular decomposition rates in the low-pressure limit to derive a critical energy for ClO_3 of 10 700 cm^(-1), which leads to ΔH_f(ClO_3) = 51.9 ± 5 kcal/mol. This is ~4 kcal/mol smaller than the value derived in our previous room temperature study of this reaction.

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