Hypervalent pyrrolidinium radicals by neutralization-reionization mass spectrometry: Metastability and radical leaving group abilities

Neutralization-reionization mass spectrometry was used to generate hypervalent radicals pyrrolidinium (1H ^·), N-methylpyrrolidinium (2H ^·), N-ethylpyrrolidinium (3H ^·), N-phenylpyrrolidinium (4H ^·), N,N-dimethylpyrrolidinium (5^·), N-methyl-N-ethylpyrrolidinium (6 ^·), and their deuterium-labeled derivatives and to study their dissociations in the gas phase. Isotopomers of pyrrolidinium and N-phenylpyrrolidinium showed small fractions of stable radicals of microsecond lifetimes that were detected following collisional reionization. The leaving group abilities in radical dissociations were established as H^· » C₂H₅ ^· ≈ C₆H₅ ^·> CH₃ ^·. The hydrogen atom was the best leaving group in secondary and tertiary pyrrolidinium radicals 1H ^·–4H^·, whereas losses of ethyl, phenyl, and ring openings by N-C bond cleavages were less facile. Methyl was the worst leaving group among those studied. Ring cleavages dominated the dissociations of quaternary pyrrolidinium radicals 5^· and 6^·, whereas losses of alkyl substituents were less efficient. The electronic properties of hypervalent ammonium radicals are discussed to rationalize the experimental leaving group abilities of hydrogen atom, alkyl, and phenyl radicals.