Direct Rate Measurements for OH + OH → H2O + O in the Range 1200–1800 K

Rate constants for the disproportionation reaction (1) OH + OH → H2O + O in the range T = 1200 ‐ 1800 K have been measured by a direct technique using shock heating of HNO3/Ar mixtures as a source of a strongly non‐equilibrated OH concentration and time resolved UV resonance absorption as its monitor. The result. k1 = 3.4 1013 exp(−21 kJ mol−1/RT) cm3/mol·s does not correlate with the accepted absolute data at room temperature and implies that the Arrhenius plot for this reaction is strongly curved. It is shown that presently available data in the range 300–2000 K can be represented best by a k = ATm expression. Whereas an empirical interpolation based on the weighted absolute values of all rate constants yields k1 = 1.5 109(T/K)1.14 cm3/mol·s, the result of a transition state theory calculation implies a slightly stronger temperature dependence (k1TST = 6.6. 108 (T/K)1.23 cm3/mol·s) with apparent Arrhenius activation energies of zero around room temperature and ∼20 kJ mol−1 in the 1000–2000 K range