A Simple Method for Estimating Activation Energies Using the Fragmentation Yield: Collision-Induced Dissociation of Iron(II)-Phenanthroline Complexes in an Electrospray Ionization Mass Spectrometer

The gas-phase stabilities of Fe(Φ)32+ complexes, where Φ represents the 1,10-phenanthroline, 5-chloro-1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline ligands were investigated by collision-induced dissociation (CID) in the capillary-first skimmer region upon changing the voltage difference between the capillary and the skimmer. The loss of only one ligand from the Fe(Φ)32+ complexes was observed with each of the phenanthroline ligands studied. An increase in the voltage difference between the capillary and the skimmer resulted in a higher fragmentation yield as calculated from the intensity of the precursor and the fragment ion. The fragmentation yield versus capillary-skimmer voltage difference plots were evaluated by means of the Arrhenius and the Rice-Ramsperger-Kassel (RRK) model by fitting the model parameters to the experimental data. Both models yielded practically the same results. In addition, if the internal energy gained through the capillary-skimmer region is estimated correctly, the approximate value of the critical energy (activation energy) for fragmentation can be extracted from the fragmentation yield versus capillary-skimmer voltage difference plots. It was found that the gas-phase stabilities of the Fe(Φ)32+ complexes are nearly identical except for the more stable Fe(II)-4,7-diphenyl-1,10-phenanthroline complex. The critical energy for fragmentation was estimated to be ∼1.2 and 0.9 eV for the Fe(II)- 4,7-diphenyl-1,10-phenanthroline, and the other complexes, respectively.