FAB and chemical ionization in the production of [M+ H]+ and [M− H]− species from 2-aryl-2-methyl-1,3-dithianes

The unimolecular fragmentations of M + H]⁺ and M – H]^? ions from four 2-aryl-2-methyl-1,3-dithianes are described and clarified with the aid of deuterated derivatives. Comparison of the MIKE spectra of M + H]⁺ species obtained under chemical ionization and fast atom bombardment (FAB) conditions reveals differences which are attributed to the different energetics involved in the two ionization processes. It is suggested that FAB is a ‘softer’ ionization technique but, at the same time, it provides, for the possibility of solvation, reaction sites not available in gas-phase protonation. M – H]^? species and anionic fragments thereof were generally not obtained under FAB(?) conditions. M – H]^? ions are readily produced in gas-phase reactions with OH^? via proton abstraction from C(4) or C(5), and from the 2-methyl substituent; and they fragment according to several reaction pathways.