Thermal Ion–Molecule Reaction Rate Constants at Pressures up to 10 torr with a Pulsed Mass Spectrometer. Reactions in Methane, Krypton, and Oxygen

A mass spectrometer for high‐pressure ion–molecule reaction studies was constructed using a pulsed 4‐keV electron beam and pulsed “gate‐open” mass‐spectrometer detection. The ions drift out of the field‐free ion source by diffusion and mass flow. Pressures up to 10 torr can be used and the ion reaction times can be as long as several hundred microseconds. The system is specially suited for study of reactions with very small rate constants, long reaction sequences, and reactions dependent on third‐body deactivation. Also when reverse reactions are operative the establishment of the equilibrium can be directly followed. The thermal rate constants for the reactions of Kr⁺ with CH₄, and CH₄⁺ and CH₃⁺ with CH₄, were measured. The reaction of O₂⁺ with O₂ to form O₄⁺ is found to be third order, proceeding with a rate constant ${\text{K\hspace{0.17em}=\hspace{0.17em}2.8\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−30}}{\mathrm{cm}}^6{\mathrm{molecules}}^{\text{−2}}·{\sec }^{\text{−1}}$ at 298°K. The activation energy for the reaction is negative and equal to 1–2 kcal/mole. The equilibrium constants for the reaction O₂⁺ + 20₂⇄O₄⁺+O₂ were determined over the temperature range 295°–350°K. A Vant Hoff plot of the equilibrium constant gives $\mathrm{D}{\mathrm{(O}}_2^{+}{\mathrm{–O}}_2\text{≃10}\mathrm{kcal/mole}$ . The equilibrium constants and bond dissociation energy are in very good agreement with earlier results by Yang and Conway. Interesting comparisons with the N₄⁺ formation in nitrogen can be made.