Aspects of mass spectra of organic compounds. Part IX. Evidence against charge localization in the fragmentation of methionine and selenomethionine

Abstract

The ionization and fragmentation of selenomethionine (Ia) has previously been interpreted according to Scheme 1 even though limited measurements of appearance potentials did not support such an interpretation. The results have been accepted as strong evidence for the role of charge localization in directing fragmentation. We have studied the fragmentation of selenomethionine at low electron impact energies, by defocussing, high-resolution mass measurements, and measurements of ionization and appearance potentials. Interpretation of these results (Scheme 2), differs in almost every respect from that given for the earlier results (Scheme 1). Since the latter interpretation of the fragmentation of selenomethionine has been claimed as an outstanding success for charge-localization ‘theory’ in preference to product stability arguments, an investigation of the fragmentation forms an ideal means of comparing them.