The decompositions of N-(substituted benzalamino)phthalimide radical cations embody ion-neutral complexes and Stevenson's rule.

A previously unreported series of N-(substituted benzalamino)phthalimides was investigated by using the combined techniques of high resolution electron ionization mass spectrometry, metastable decomposition, and collisional activation mass spectrometry. The predominate fragmentation pathway is a McLafferty-type rearrangement. There also occurs, to a lesser extent, a transfer of hydrogen that originates from a substituent remote from the phthalimide moiety and terminates on the phthalimide, The process is interpreted as proceeding via an ion-neutral complex. The effects of substituents on both of the aforementioned fragmentation pathways provide a striking example that gives quantitative evidence for Stevenson's rule. The substituent effects are responsible for a trend in ion abundance that shows a sharp reversal at approximately the ionization energy of the iminium isomer of the phthalimide molecular ion.