Intramolecular Charge Transfer in the Gas Phase: Fragmentation of Protonated Sulfonamides in Mass Spectrometry

The fragmentation of protonated molecules (MH(+)) in mass spectrometry usually results in even-electron product ions, but the MH(+) ions of sulfonamides are different as they often produce dominant radical cations of the constituent amines. For a series of benzenesulfonamides of anilines that bear various substituents, we found that the sulfonamides are preferentially protonated at the nitrogen, which is different from the carboxylic amides. Upon N-protonation, the S-N bond dissociates spontaneously to produce an intermediate [sulfonyl cation/aniline] complex. Within the ion-neutral complex, charge transfer between the two partners occurs in the gas phase to give rise to the ionized anilines. A substantial energy barrier was found to govern the reaction, which is consistent with the outer-sphere electron transfer mechanism. This energy barrier prevents the charge transfer when a strong electron-withdrawing substituent is attached to the aniline moiety. In contrast, when the aniline bears an electron-donating group, charge transfer is still more favorable than the dissociation of the intermediate ion-neutral complex, in spite of the existence of the energy barrier, and therefore dominates. A correlation was observed between the intensities of the ionized anilines and the ionization energies of these anilines.