PROPERTIES OF TRANSITION SPECIES IN THE REACTIONS OF HYDROXYL WITH AMMONIA AND WITH ITSELF

Quantum transition state theory was used to model two hydrogen atom ab straction reactions of hydroxyl radicals. Moments of inertia for the t ransition species were calculated from geometries obtained by ab initi o or bond energy bond order methods. The transition state equation was fit by nonlinear least squares to experimental rate constants to dete rmine parameters of the transition species. For the reaction of hydrox yl with ammonia, the geometric mean, Wb, of four transitional. vibrati onal frequencies was found to be 660 +/- 20 cm(-1), the activation bar rier height, V-b, including zero point energy, to be 8.5 +/- 0.5 kJ, m ol(-1), and the barrier thickness, at half its height, to be 65 +/- 30 pm. For the disproportionation reaction of hydroxyl radicals, the cor responding values of the first two parameters, based on the most recen t experimental data, were 890 +/- 20 cm(-1) and 2.7 +/- 0.8 kJ mol(-1) . Ab initio calculations on the latter reaction gave omega(b) = 746 cm (-1) at the HF/6-31G(d,p) level and V-b = 13.6 kJ mol(-1) at the MP4/6 -311G(d,p) level. The curvature of the Arrhenius plots can be explaine d by the contributions of the low-frequency vibrations and, for the di sproportionation reaction, of the low-lying electronic states.