Unimolecular Reactions of Diethyl Malonate Cation in Gas-phase

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Unimolecular reactions of diethyl malonate cation (CH₃CH₂OC(=O)CH₂C(=O)OCH₂CH₃^+· ; 1^+·, M_w : 160) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemistry. In the metastable time window, the molecular ions 1^+· lead not only to the formation of the major fragment ions m/z 133 ([M-C₂H₃]⁺) through a McLafferty + 1 rearrangement, but also to two minor fragment ions m/z 132 ([M-C₂H₄]^+·) through a McLafferty rearrangement (or [M-CO]^+·) and m/z 88 ([M-C₃H₄O₂]^+·). These dissociations are rationalized by the idea of ion-neutral complex. The m/z 88 ions are generated via at least two different routes. The m/z 133 ([M-C₂H₃]⁺) ions decompose into the ions at m/z 115, 106, and 105 by the losses of H₂O, C₂H₃, and C₂H₄, respectively. A large part of the first fragment ions would be in the keto form and it decomposes into the m/z 71 and 43 ions by the losses of CO₂ and C₃H₄O₂ (or C₂O₃), and a small part would be in the enol form and decomposes into the m/z 87 ions by the loss of C₂H₄, not CO. The m/z 43 ions are isobaric, C₂H₃O⁺ and C₃H₇⁺.