Electron ionization mass spectrometric analysis of 5-nitro octaethylprophyrin: evidence for scission of the porphyrin macrocycle

Abstract

Analysis of the electron ionization mass spectrum (EIMS) of 5-nitro octaethylporphyrin reveals that the molecular ion m/z 579 undergoes novel fragmentations. The typical fragmentation pathway of β-cleavage of peripheral substituents is suppressed. Furthermore, the presence of the fragment ion m/z 375 provides strong evidence for a fragmentation pathway that involves cleavage of the porphyrin macrocycle. Such a fragmentation pathway has not been reported previously for porphyrins bearing β-pyrrolic substituents. Subsequent EIMS and electron ionization tandem mass spectrometric analyses, both of the 5-nitro octaethylporphyrin and of its analogs isotopically labeled at the meso (bridge) position (13C) or at the pyrrolic nitrogens (15N) or at the nitro group (15N), confirmed the ring scission process. The elemental composition of the significant fragment ions was confirmed by high-resolution EIMS. The anomalous behavior may be attributed to macrocyclic distortion and/or the electron-withdrawing effect of the nitro group. © 1997 John Wiley & Sons, Ltd.