Dramatic C-C bond activation on protonation of the persistent nitroxyl radical TEMPO•

The electrospray ionization (ESI) mass spectrum in positive-ion mode of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPOS center dot) in aqueous solution exhibits peaks at m/z 156, 157 and 158 that are assigned to TEMPO+, TEMPOH center dot+ and TEMPOH2+. MS/MS analysis reveals that the even-electron TEMPO+ and TEMPOH2+ ions fragment via the expected C-N bond cleavage but the TEMPOH+. odd-electron ion undergoes facile loss of a CH3. radical. Density-functional theory (DFT) calculations confirm that TEMPOS protonation dramatically lowers the C-CH3 bond dissociation energy to 22 kcal/mol, affording a low-energy fragmentation pathway for TEMPOH center dot+. Thus, the experimental and computational data concur that TEMPO center dot protonation activates the C-CH3 bond and promotes the formation of reactive CH3 center dot radicals in the gas phase. (C) 2017 Elsevier B.V. All rights reserved.