Ion−Molecule Branching Ratios at High Temperature: Vibrational Energy Promotes Formation of New Channels in the Reaction of O2+with CH4

A High-Temperature Flowing Afterglow (HTFA) instrument has been modified to allow measurements of ion−molecule branching ratios at temperatures above 700 K for the first time. The technique is capable of temperatures at least as high as 1400 K, and here we report the branching ratios for the reactions of O2+ with CH4 and CD4 measured at 1400 K (for the reaction of O2+ with CH4, the rate constant was also measured). Comparison to work using drift tube and guided ion beam techniques shows that heating the CH4 vibrations produces new reaction product channels and enhances others. In particular, HCO+, CH3O+, and H2O+ products are seen for the first time, and the H3O+ product is much more intense in comparison with experiments on vibrationally cold CH4. The previously observed products CH3+ and CH4+ are comparable to those found with vibrationally cold CH4. We conclude that vibrational excitation promotes those channels which require extensive bond rearrangement. Vibrational energy is found not only to promote...