Unimolecular gas-phase reactions of diethyl phthalate, isophthalate, and terephthalate upon electron ionization

Unimolecular gas-phase reactions of diethyl phthalate (1), isophthalate (2), and terephthalate (3), upon electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labelling. The metastable molecular ions (1)⁺ decompose to give exclusively the ions m/z 176 ([M-CH₃CH₂OH]⁺) and not the ions by the loss of CH₃CH₂O as proposed earlier in the literature. The metastable molecular ions (2)⁺ and (3)⁺ fragment differently from (1)⁺ and lead not only to the formation of the major fragment ions m/z 194 ([M-CH₂CH₂]⁺) via a McLafferty rearrangement but also to minor fragment ions m/z 193 ([M-CH₂CH₃]⁺). Yet, molecular ions decomposing in the ion source all show as primary fragmentation channel the loss of CH₃CH₂O to give the ions at m/z 177, which further dissociate to give the ions at m/z 149 through the loss of C₂H₄ or CO, indicating the resulting ions are ⁺COC₆H₄COOH and ⁺C₆H₄COOCH₂CH₃. The ⁺COC₆H₄COOH ions decompose into the m/z 121, 93, and 65 ions by the consecutive losses of three carbon monoxide molecules, respectively. Prior to the second CO loss, a migration of the OH group to the benzene ring occurs. During the metastable fragmentation of the ⁺C₆H₄COOCH₂CH₃ ions no ethoxy migration occurs, in contrast to the methoxy migration taking place in the metastable decomposition of the lower homologue ⁺C₆H₄COOCH₃ ions.