Unimolecular gas-phase reactions of methyl and ethyl trifluoroacetoacetates upon electron ionization

摘要

The unimolecular metastable decompositions of methyl and ethyl trifluoroacetoacetates, CF_3COCH_2COOCH_3 (MW: 170 (1)) and CF_3COCH_2COOCH_2CH_3 (MW: 184 (2)) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. In the metastable time window, the molecular ions 1~(·+) decompose to give exclusively to ions at m/z 101 [M - CF_3]~+. However, the metastably decomposing ions 2~(·+) lead not only to the formation of the major fragment ion m/z 115 [M - CF_3]~+, but also to three minor fragment ions m/z 164 [M - HF]~+, m/z 156 [M - C_2H_4]~+ and m/z 87. A large part of the metastably decompositions 1~(·+) and 2~(·+) has the enol form. The loss of CO_2 from the ions m/z 101 and m/z 115 occurs through migration of the methyl and ethyl groups, respectively. The source-generated m/z 69 ions from 1~(·+) and 2~(·+) are most abundant and consist of both CF_3~+ and OCCHCO~+. The latter ion, a protonated carbon suboxide, is generated by at least three and four different fragment fragmentation routes from 1~(·+) and 2~(·+), respectively. The m/z 43 ion, C_2H_3O~+, from 2~(·+) is formed by at least two different routes.