In tandem mass spectrometry, the formation of radical anion from fragmentation of deprotonated ions disobeys the “even-electron rule”. However, such unusual fragmentations were often observed and the mechanism was traditionally attributed to be a simple homolysis reaction. In this study, a novel mechanistic insight into radical anion generation in even-electron anion dissociation was gained and supported by experiments and theoretical calculations. A series of deprotonated N,2-diphenylacetamide and its ring-substituted derivatives (R¹C₆H₄CH₂CONHC₆H₄R²) were produced in an electrospray ionization-ion trap mass spectrometer and then were subjected to collision-induced dissociation. In the fragmentation of all the deprotonated amides studied, when R¹ ≠ NO₂ and R²＝NO₂, the 4-nitrophenyl isocyanate radical anion was formed as the dominant product ion, but the corresponding phenyl isocyanate radical anion was not formed for other deprotonated derivatives. Taking compound 3 (R¹＝H, R²＝NO₂) as an example, the mechanism was proposed as a single electron transfer (SET) reaction via an intermediate [benzyl anion/4-nitrophenyl isocyanate] complex which derived from the initial heterolysis of the benzylic CH₂—CO bond of the precursor anion. Benzyl anion could be observed as a minor product due to the separation of the ion/neutral complex. The density functional theory calculations indicated that the formation of 4-nitrophenyl isocyanate radical anion was much more favorable than that of benzyl anion in terms of energy and the activation energy of heterolytic cleavage of the CH₂—CO bond was much lower than that of homolytic cleavage of that bond. Theoretical calculations also revealed that the intra-complex SET reaction was considerably exothermic and so it was feasible. Obviously, SET was the key step in the new mechanism. The gas-phase SET reaction of 3-nitrotoluene with benzyl anion was confirmed to occur in the bimolecular ion/molecule reaction and had been reported by others in the literature, which indirectly supported the SET mechanism in the present study. For fragmentation of different precursor ions, only when the electron affinity of (substituted) phenyl isocyanate was higher than that of the corresponding (substituted) benzyl radical, the SET reaction in the complex was achievable, which well accorded with the thermodynamic properties of electron transfer reaction. This study provides a new perspective on the formation of radical anions in the fragmentation of even-electron negative ions which helps us better understand the mass spectrometric fragmentation reactions.

Key words: electrospray ionization mass spectrometry, radical anion, even-electron ion, ion/neutral complex, single electron transfer, negative ion, N,2-diphenylacetamide