Studies of Hydroxyl Radical Kinetics by Quantitative ESR

Reactions of the OH radical with OH, H₂, and CO have been studied at room temperature in a fast flow system at pressures of the order of 1 mm. The very fast reaction between H atoms and NO₂ was used as the source of OH. Electron spin resonance techniques were employed to examine the decay of OH concentration along the flow tube, using NO gas as the calibration standard. Rate constants at 300°K were found to be, in cubic centimeters per mole·second units $$\begin{array}{cc}\mathrm{OH}+\mathrm{OH}=\mathrm{O}+{\mathrm{H}}_2\mathrm{O},& k_2{\text{=1.55±0.12×10}}^{12},\\ \mathrm{OH}+{\mathrm{H}}_2={\mathrm{H}}_2\mathrm{O}+\mathrm{H},& k_4{\text{=3.9±0.2×10}}^9,\\ \mathrm{OH}+\mathrm{CO}={\mathrm{CO}}_2+\mathrm{H},& k_5{\text{=1.15±0.05×10}}^{11}.\end{array}$$ Combination of the last two of these with published results at higher temperatures leads to $$\begin{array}{c}{k}_4{\text{=3.9±0.2×10}}^9\exp \text{{[(5200±300)/R][(1/300)−(1/T)]},}\\ k_5{\text{=1.15±0.05×10}}^{11}\exp \text{{[(600±300)/R][(1/300)−(1/T)]},}\end{array}$$ or in simple Arrhenius form $$\begin{array}{c}{k}_4{\text{=2.3×10}}^{13}\exp \text{(−5200/RT),}\\ k_5{\text{=3.1×10}}^{11}\exp \text{(−600/RT).}\end{array}$$