PUB - Publikationen an der Universität Bielefeld

The electron impact-induced fragmentation of 2,2-dimethyl- and 2-ethyl-1,3-indandione, 1 and 2, and their isomers, 3-isopropylidene- and 3-propylidenephthalide, 3 and 4, respectively, was studied in detail by mass-analysed ion kinetic energy (MIKE) and collision-induced dissociation (CID-MIKE) spectrometry, including H-2 and C-13-labelled analogues of 1 and 2. In all regimes of internal energy, the molecular ions 1+. - 4+. interconvert by up to seven consecutive, reversible isomerization steps prior to the main fragmentation processes, viz. loss of CH3. and C2H4 . 1,3-Indandione and 3-methylenephthalide ions with identical alkylidene moieties (i.e. 1+. half arrow right over half arrow left 3+. and 2+. half arrow right over half arrow left 4+.) equilibrate rapidly and completely prior to fragmentation, whereas these pairs of isomers interconvert only slowly via a five-step rearrangement of the indandione ions 1+. half arrow right half arrow left 2+.. Distinct from the behaviour of simpler ionized carbonyl species, a 1,2-C shift of a (formally) neutral carbonyl group is found to occur along with that of a protonated one. Also distinct from simpler cases, methyl loss does not take place from the ionized enol intermediates formed within the interconversion 1+. half arrow right over half arrow left 2+. of the diketone ions but rather from the n-propylidenephthalide ions 4+.. This follows from CID-MIKE spectrometry of the [M - CH3]+ ions of 1-4 and two reference C10H7O2+ (m/z 159) ions of authentic structures (protonated 2-methylene-1,3-indandione and protonated 1,4-naphthoquinone). The characteristic CID fragmentation of the C10H7O2+ ions is rationalized. Finally, the multistep isomerization of ionized 1,3-indandiones apparently also extends to higher homologues [eg. 5+. from 2-ethyl-2-methyl-1,3-indandione (5) and 6+. from 2,2-diethyl-1,3-indandione (6)]: the ionized phthaloyl group of 1,3-indandione radical cations 1+., 2+., 5+. and 6+., originally attached with its two acyl functionalities to the same carbon of the aliphatic chain, performs, in fact, a 'multi-step migration'.