Atom—Molecule Kinetics at High Temperature Using ESR Detection. Technique and Results for O +H2, O +CH4, and O +C2H6

Abstract

A technique is described for the measurement of atom—molecule rate constants over the temperature range 300°—1000°K. It combines a heated fast‐flow reactor with ESR atom detection, the ESR cavity being located at a fixed position outside the heated reactor. The system should be useful with a wide variety of reactions, and yields rate constants having ±10% precision independent of temperature. This is also the absolute accuracy when the reaction stoichiometry is known or can be determined by auxiliary mass‐spectrometric analysis. Results on some O‐atom reactions are (in cubic centimeters per mole·second units): $$\mathrm{O}+{\mathrm{H}}_2\to \mathrm{H}+\mathrm{OH}{\mathrm{,\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}k}}_1{\text{=4.0±0.4×10}}^{13}\hspace{0.17em}\exp \text{(−10 200/RT),}$$ $$\mathrm{O}+{\mathrm{CH}}_4\to {\mathrm{CH}}_3+\mathrm{OH}{\mathrm{,\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}k}}_3{\text{=1.7±0.2×10}}^{13}\hspace{0.17em}\exp \text{(−8700/RT),}$$ $$\mathrm{O}+{\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_2{\mathrm{H}}_5+\mathrm{OH}{\mathrm{,\hspace{0.17em}\hspace{0.17em}k}}_{10}{\text{=1.8±0.2×10}}^{13}\hspace{0.17em}\exp \text{(−6100/RT).}$$ In the O+CH₄ case, the stoichiometry determined both in this work and by others indicates that the mechanism subsequent to the initial step may be $$\mathrm{O}+\mathrm{OH}\to {\mathrm{O}}_2+\mathrm{H},$$ $$\mathrm{O}+{\mathrm{CH}}_3\to \mathrm{CH}+{\mathrm{H}}_2\mathrm{O},$$ $$\mathrm{O}+\mathrm{CH}\to \mathrm{CO}+\mathrm{H},$$ although [8] and [9] are not definitely established, and other mechanisms are discussed. Little can be said about the O+C₂H₆ mechanism at present. It is shown that the pre‐exponential factors and activation energies for these three hydrogen abstraction reactions are in reasonable relation to each other.