Radical Cations in Electrospray Mass Spectrometry: Formation of Open‐Shell Species, Examination of the Fragmentation Behaviour in ESI‐MS n and Reaction Mechanism Studies by Detection of Transient Radical Cations

The ion formation mechanism in electrospray MS is reviewed, with special focus on the electrochemical red/ox reactions responsible for the formation of radical molecular ions. Prerequisites influencing the likelihood of formation and observation of a particular compound as an open‐shell molecular species in ESI‐MS (i.e., the structure and the oxidation potential of the analyte, the solvent and additives) are evaluated. For illustration of the ESI phenomena governing radical cation formation, an ESI‐MS study of tetra(aryl)benzidine compounds is presented. The facile formation of abundant radical molecular cations in ESI‐MS demonstrates imposingly that the basicity of the analyte's nitrogen atoms is strongly overcompensated by the ability to stabilize unpaired electrons. ESI‐MS n spectra of the tetra(aryl)benzidine molecular ions exhibit a characteristic feature in the loss of radicals. This process is the major fragmentation pathway of open‐shell molecular precursor ions in their MS 2 spectra, and also that of even‐electron ions in sequential MS n spectra. The collision‐induced dissociation (CID) behaviour suggests a general assumption: easily oxidizable compounds (e.g., hydrocarbon polyenes, polycyclic aromatic hydrocarbons, porphyrins etc.) generating predominantly molecular radical cations in ESI‐MS contradict the even electron rule in ESI‐MS n experiments. The strong ability to stabilize unpaired electrons is preserved in product ions and makes the formation of open‐shell species energetically less demanding. A selection of solution‐phase reaction mechanistic studies in which open‐shell intermediates were detected and structurally characterized by ESI‐MS and ESI‐MS/MS, respectively, is presented. The merits of ESI‐MS and ESI‐MS n for mechanistic studies of chemical reaction are critically discussed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)