Keywords: Electrospray mass spectrometry / Radical molecular ions / Even electron rule / Reaction mechanism studies by ESI-MS/MSThe 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

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The structure of proline in [proline + K]+ has been investigated in the gas phase using high level DFT and MP2 calculations and infrared photo dissociation spectroscopy with a free electron laser (FELIX). The respective FELIX spectrum of [proline + K]+ matches convincingly the calculated spectra of two structurally closely related and nearly iso-energetic zwitterionic salt bridge (SB) structures. An additional unresolved band at approximately 1725 cm(-1) matching with the characteristic CO stretching mode of charge solvation (CS) structures points toward the presence of a minor population of these conformers of proline in [proline + K]+. However, theory predicts a significant energy gap of 18.9 kJ mol(-1) (B3LYP/6-311++G(2d,2p)) or 15.6 kJ mol(-1) (MP2) between the lowest CS conformer of proline and the clearly favored SB structure.

The gas-phase structures of a series of potassiated tertiary amino acids have been systematically investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, ion mobility spectrometry (IMS), and computational modeling. The examined analytes comprise a set of five linear N,N-dimethyl amino acids derived from N,N-dimethyl glycine and three cyclic N-methyl amino acids including N-methyl proline. The number of methylene groups in either the alkyl chain of the linear members or in the ring of the cyclic members of the series is gradually varied. The spectra of the cyclic potassiated molecular ions are similar and well resolved, whereas the clear signals in the respective spectra of the linear analytes increasingly overlap with longer alkyl chains. Measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311++G(2d,2p) level of theory to identify the structures present in the experimental studies. On the basis of these experiments and calculations, all potassiated molecular ions of this series adopt salt bridge structures in the gas phase, involving bidentate coordination of the potassium cation to the carboxylate moiety. The assigned salt bridge structures are predicted to be the global minima on the potential energy surfaces. IMS cross-section measurements of the potassiated systems show a monotonic increase with growing system size, suggesting that the precursor ions adopt the same type of structure and comparisons between experimental and theoretical cross sections are consistent with salt bridge structures and the IRMPD results.

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