High-Pressure Photoionization Mass Spectrometry. Reactions of Alkane and Cycloalkane Molecular Ions with Water Vapor at Thermal Kinetic Energies

The reactions of alkane molecular ions (RH₂⁺) with water vapor were found to proceed via a bimolecular mechanism in both ethane and propane. Parent ions from cyclohexane, cyclopentane, i‐butane, n‐butane, i‐pentane, n‐pentane, and n‐hexane were observed to react exclusively via a termolecular mechanism involving two water molecules: RH₂⁺+2H₂O→H⁺(H₂O)₂+RH. Thermal rate constants of 1.2 and 1.4 × 10⁻⁹ cm³/molecule·sec, respectively, were derived for the bimolecular reactions of C₂H₆⁺ and C₃H₈⁺ with H₂O. The termolecular rate constants found in other RH₂⁺–H₂O combinations were quite high, falling in the range 10⁻²⁵–10⁻²⁷ cm⁶/molecule²·sec. The nature of the collision complex is discussed, and new limits are estimated for ${\mathrm{\delta H}}_f({\mathrm{H}}_3{\mathrm{O}}^{+})$ .