Fragmentation reactions of the singly and doubly charged molecular ions of dimethylanisoles

Abstract

The major primary fragmentation reaction of the singly charged molecular ions of the dimethylanisoles involves elimination of a methyl radical. Deuterium labelling shows that, in contrast to the methylanisoles, this fragmentation reaction involves not only the methoxy methyl but also the methyl groups attached to the aromatic ring. The electron impact mass spectra permit limited distinction among the isomeric molecules. The charge exchange (2E) mass spectra of the doubly charged ions produced from the dimethylanisoles permit limited identification of individual isomers. These spectra also show that loss of CH 3 from the molecular dication is a minor process and involves only the methoxy methyl. The major doubly charged fragment ions observed are C n H 2+ 2 ( n = 5–9), C n H 2+ 6 ( n = 7, 8) and C 7H 2+ 7. The molecular dications of the methoxy- d 3 dimethylanisoles show two unimolecular charge separation reactions to form C 8H 9O + + CD + 3 and C 7H 5D 2O + + C 2H 4D +. The kinetic energy releases associated with these charge separation reactions lead to calculated intercharge distances of 5.3 and 4.7 Å, respectively, suggesting acyclic structures for the transition states leading to charge separation. However, the fragmentation reactions of the singly charged molecular ions produced by charge exchange of the doubly charged molecular ions suggest a cyclic structure for the stable molecular dications.