Gas-phase structures of solution-phase zwitterions: Charge solvation or salt bridge?

Sodium and lithium adduct ions of a synthetic guanidiniocarbonylpyrrole-derivative are examined in the gas phase. A wavelength tunable free electron laser (FEL) was used for photo-dissociation spectroscopy experiments in the infrared (1400–1800 cm−1). The photo-dissociation spectra are compared to calculated IR spectra of structures identified by theory. All photo-dissociation spectra acquired are strikingly similar, indicating that all ions adopt analogous gas-phase structures. Additionally, the respective sodium adduct ions were examined with ion mobility mass spectrometry (IMS). Although computational efforts succeeded in finding a charge solvated conformer that matched both the photo-dissociation spectra and the IMS data, the predicted global minimum was a salt-bridge structure.