title: Intramolecular charge transfer in the gas phase: fragmentation of protonated sulfonamides in mass spectrometry.
abstract: 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.
authors.affiliation: Department of Chemistry, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, China.
authors.forename: Nan
authors.initials: N
authors.lastname: Hu
authors.forename: Ya-Ping
authors.initials: YP
authors.lastname: Tu
authors.forename: Kezhi
authors.initials: K
authors.lastname: Jiang
authors.forename: Yuanjiang
authors.initials: Y
authors.lastname: Pan
mesh_terms: D005740:Gases; D013058:Mass Spectrometry; D015394:Molecular Structure; D044367:Phase Transition; D011522:Protons; D011789:Quantum Theory; D013449:Sulfonamides / Q000737:chemistry*
chemical_list: D005740:Gases; D011522:Protons; D013449:Sulfonamides