Mass spectrometry of stable free radicals—II: pyrrolidine nitroxides

3‐Substituted‐2,2,5,5‐tetramethylpyrrolidine nitroxides are stable free radicals used extensively in the synthesis of ‘spin labels’. The high resolution mass spectra of these nitroxides substituted with CH 2 OH, OH, NH 2 and o have been recorded on magnetic tape and the elemental compositions of the ions calculated by computer. Ionisation by electron bombardment(70eV), gives rise to an even‐electron molecular ion species. [M+1] + . ions are observed in the spectra of all compounds examined, except in the case of the 3‐carbonyl compound, 2,2,5,5‐tetramethylpyrrolid‐3‐one‐1‐oxyl. Loss of a methyl radical from these ions leads to the appearance of ions at [M ‐14] + . The predominant fragmentation for those compounds in which the substituents can supply electrons to the ring, is the sequential elimination of isobutene, nitric oxide and a hydrogen radical. In the case of the 3‐hydroxy compound, these ions account for 23 percent of the total ion current. 2,2,5,5‐Tetramethylpyrrolid‐3‐one‐1‐oxyl, which bears an electron‐withdrawing substituent gives rise to a fragmentation pattern somewhat different from those of the other compounds. The main features are the absence of a peak at [M + 1] + ˙ and the general phenomenon of fewer peaks but with higher intensities.