FRAGMENTATION REACTIONS OF THE SINGLY AND DOUBLY CHARGED MOLECULAR-IONS OF DIMETHYLANILINES

The electron ionization mass spectra of seven isomeric dimethylanilines are reported. The major fragmentation reactions of the singly charged molecular ions of the ring-substituted isomers involve loss of a methyl group and loss of a hydrogen atom, while the principal fragmentation reaction of the N,N-dimethylaniline molecular ion involves loss of a hydrogen atom. The electron ionization mass spectra permit distinction among the isomeric molecules except for the 2,5- and 2,6-isomers. Unimolecular metastable ion, low energy and high energy collision induced dissociation, charge stripping and neutralization-reionization mass spectrometric methods were applied to the molecular ions in order to explore the utility of these methods for isomer differentiation. While all these methods provided clear identification of the N,N-isomer none provided complete differentiation of all the ring-substituted isomers. Partial charge transfer (2E) mass spectra of the doubly ions in the mass spectra are reported. In contrast to many aromatic compounds, which show abundant C(n)H2(2+) ions, the major doubly charged ions contain nitrogen. Limited differentiation of isomers could be achieved; in particular, the 2,5-isomer could be distinguished from the 2,6-isomer. Metastable molecular dications show two unimolecular charge separation reactions to form C7H8N+ + CH3+ and C7H10+ + HCN+. The kinetic energy releases associated with these reactions lead to a calculated intercharge separation of 4.1-angstrom for the ring substituted isomers and 3.4-angstrom for the N,N-dimethylaniline di-cation, both indicating cyclic structures for the transition states leading to charge separation.