Quantitative ESR Measurements of Gas-Phase H and OH Concentrations in the H–NO2 Reaction

Abstract

An application of the intensity relations derived previously for determining absolute concentrations of H atoms and OH radicals in the gas phase by ESR spectroscopy is described. Magnetic‐dipole transitions in O₂ and electric‐dipole transitions in NO are used as calibration standards for determining H and OH, respectively. The construction of a cylindrical TE₀₁₁ cavity suitable for observing both types of transitions in a fast flow system is described. Results of measurements of H and OH concentrations during the ``titration'' reaction H+NO₂→NO+OH are presented. The internal consistency of the theoretical OH intensity relations is demonstrated by experimental measurements on $\mathrm{J=}\frac{3}{2}$ and $\mathrm{J=}\frac{5}{2}$ OH lines. At reaction times of a few milliseconds, the H titration curve is shown to have an initial linear portion which extrapolates to an NO₂/H ratio of unity. It then becomes curved as expected by the generation of an additional H from $$\mathrm{OH}+\mathrm{OH}\to {\displaystyle {lim}_{k_2}}{\mathrm{H}}_2\mathrm{O}+\mathrm{O}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{and}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{OH}+\mathrm{O}\to {\mathrm{O}}_2+\mathrm{H}.$$ The OH titration curves yield k₂=1.2±0.3×10¹² cm³ mole⁻¹·sec⁻¹ at 300°K in good agreement with the only other reported value. Finally, some qualitative observations on a striking generation of free electrons associated with the main H–NO₂ reaction are noted.