Observing Unimolecular Dissociation of Metastable Ions in FT-ICR:  A Novel Application of the Continuous Ejection Technique

The application of the continuous ejection technique to observe unimolecular dissociation of metastable organometallic ions in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer is demonstrated. A radio-frequency ejection voltage is continuously applied at a frequency corresponding to the exact mass of a metastable adduct ion, [MC 6 H 6 + ]* (M = Cr and Mn), formed in the reactions of Cr + and Mn + with C 6 H 6 . By monitoring the initial metal ion intensity as a function of ion ejection time, metastable ion lifetimes and unimolecular dissociation rate constants are obtained. The bond dissociation energies D 0 (Cr + −C 6 H 6 ) = 1.75 ± 0.1 eV and D 0 (Mn + −C 6 H 6 ) = 1.50 ± 0.1 eV are derived from RRKM modeling and are in excellent agreement with other literature values.