Structures and Dissociation of Iron Porphyrin Complexes by Ion Mobility and Collision-Induced Dissociation Mass Spectrometry

A combination of ion mobility mass spectrometry (IMS-MS), collision-induced dissociation (CID), RRKM modeling, and computational chemistry was used to determine the structure and unimolecular chemistry of dimeric and trimeric sulfonated meso-tetraphenylporphyrins with negative charges from -2 to -5. By comparing experimental collision cross-sections obtained from calibrated IMS drift times with calculated cross sections for the lowest energy calculated structures, it was confirmed that dimer species have a bridged structure where the two monomers are connected through iron-sulfonic interactions. Dimer species with the charge states -4 and -5 dissociate into two monomer units where the charge is distributed between the monomers. Dimers with lower charge states also lose neutral SO2 and SO3 groups. For trimeric species with charge states of -3 and -4, IMS identifies three and two isomers, respectively. It was confirmed that the -4 charged trimer isomers consist of one with three stacked monomers and one in which the third monomer unit is connected to a stacked dimer via two iron-sulfonic bonds (bridged/stacked). Both yielded the same CID breakdown diagram confirming that the two isomers likely interconvert prior to dissociation. The significantly larger density of states of the bridged-stacked structure compared to the stacked structure means the former is likely the reactive configuration.