Metastable FTICR

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

More recently, Audier and McMahon have shown that the unimolecular dissociation spectrum of transient ions can be directly obtained from a simple manipulation of a series of FTICR spectra. The data arising from this approach very closely resemble those obtained from metastable dissociations in conventional sector spectrometers (MIKES), and it has been consequently dubbed metastable ion cyclotron resonance (MICR) spectrometry. Very briefly, the method functions as follows ... 

CH3 after rearrangement to the enamine leads to m/z 56 fragment ions and cycloreversion leads to ionized vinylamine 34 and ethene. In fact, these decomposition processes have final states with very close standard enthalpies and they are found to compete in the metastable time frame. However, only the formation of 34 is observed in the experimental conditions of FTICR (long life time and deactivation with Ar) which analyses least-energized parent ions43. This shows the existence of an energy barrier in the formation of the m/z 56 ions attributable to the difficult 1,3 H-transfer required by the rearrangement to enamine. 

Lin, C.-Y., Chen, Q., Chen, H. and Preiser, B.S. (1997) Observing unimolecular dissociation of metastable ions in FTICR a novel appbcation of tbe continuous ejection technique. J. Phys. Chem. A, 101, 6023-6029. 

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