TEMPERATURE-DEPENDENCE AND MECHANISM OF THE REACTION BETWEEN O(3P) AND CHLORINE DIOXIDE

Second-order rate constants for the decay of O(3P) in excess chlorine dioxide, k(II), 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(II) is pressure dependent with a value, k(b), that is nonzero at zero pressure and (2) both the third-order rate constant (dk(II)/d[M])[M] = 0 = k(o), 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 <--> ClO3* (1, 2); ClO3* + M --> ClO3 + M (3); ClO3* --> ClO + O2* (4), with E(o2) > E(o4). The measured k(o) values were used in conjunction with Troe's expression for unimolecular decomposition rates in the low-pressure limit to derive a critical energy for ClO3 of 10 700 cm-1, which leads to DELTA-H(f)(ClO3) = 51.9 +/- 5 kcal/mol. This is approximately 4 kcal/mol smaller than the value derived in our previous room temperature study of this reaction.