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

• Published in: Journal of organic chemistry Vol. 75; no. 12; pp. 4244 - 4250
• Main Authors: Hu, Nan, Tu, Ya-Ping, Jiang, Kezhi, Pan, Yuanjiang
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 18.06.2010
• Subjects: Chemical reactivity; Chemistry; Exact sciences and technology; Gases; Kinetics and mechanisms; Mass Spectrometry; Molecular Structure; Noncondensed benzenic compounds; Organic chemistry; Phase Transition; Preparations and properties; Protons; Quantum Theory; Reactivity and mechanisms; Sulfonamides - chemistry; Nitro compound; Correlation; Sulfone; Electron transfer; Energy barrier; Intramolecular charge transfer; Fragmentation; Amine; Carboxylic acid; Reaction mechanism; Potential energy surfaces; Charge transfer; Radical cation; Sulfonamide; Aniline derivatives; Ionization; Outer sphere; Density functional method; Carboxamide; Benzenic compound; Protonation; Cations; Gas phase; Mass spectrometry; Electron acceptor; Electron donor
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo100761k

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.