Abstract
The electrospray ionization (ESI) mass spectrum in positive-ion mode of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) in aqueous solution exhibits peaks at m/z 156, 157 and 158 that are assigned to TEMPO+, TEMPOH+ and TEMPOH2+. MS/MS analysis reveals that the even-electron TEMPO+ and TEMPOH2+ ions fragment via the expected CN bond cleavage but the TEMPOH+ odd-electron ion undergoes facile loss of a CH3 radical. Density-functional theory (DFT) calculations confirm that TEMPO protonation dramatically lowers the CCH3 bond dissociation energy to ∼22 kcal/mol, affording a low-energy fragmentation pathway for TEMPOH+. Thus, the experimental and computational data concur that TEMPO protonation activates the CCH3 bond and promotes the formation of reactive CH3 radicals in the gas phase.
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