Characterization and differentiation of heterocyclic isomers. Part 3. Study of high internal energy ions produced by electron ionization mass spectrometry on 3-methyl-1,2- and 2-methyl-1,3-thiazolopyridines

The eight members of two classes of heterocyclic isomers, namely 3-methyl-1,2-, 1a-d and 2-methyl-1,3-thiazolopyridines 2a-d have been characterized by mass spectrometry under electron ionization. High internal energy ions formed in the source have been studied by low and high resolution mass spectrometry. The data show remarkable differences among the various components of each class depending on the position of the nitrogen in the pyridine ring. Furthermore, by comparing the mass spectra of members of series 1 with those of their corresponding isomers belonging to series 2 , it is still possible to obtain evidence for different behaviors in the fragmentation pathways. It seems to exclude the occurrence of inter-conversion phenomena from one isomer into another, as well as conversion to a common intermediate before fragmentation. This also suggests that each member of series 1 and 2 retains its own structure after electron ionization. The data obtained on a double focusing instrument equipped with electrostatic and magnetic analyzers have been compared with those obtained on a mass spectrometer with an ion trap as the analyzer.