Radical Cations?

The electron impact-induced fragmentation of 2,2-dimethyland 2-ethyI-l,findandione, 1 and 2, and their isomers, 3-isopropylideneand 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 and I3Clabelled 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 CH,' and C,H,. 1,3-Indandione and 3-methylenephthalide ions with identical alkylidene moieties (i.e. 1 +' e 3" and 2+' e 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+'=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 +' + 2" of the diketone ions but rather from the n-propylidenephthalide ions 4+'. This follows from CID-MIKE spectrometry of the IM CH,] + ions of 1-4 and two reference C,oH70,+ (m/z 159) ions of authentic structures (protonated 2-methylene-l,3-indaodione and protonated 1,4naphthoquinone). The characteristic CID fragmentation of the C, oH70, + ions is rationalized. Finally, the multistep isomerization of ionized 1,J-indandiones apparently also extends to higher homologues leg. 5" from 2-etbyl-2-methyl-1,3-indandione (5) and 6" from 2,2-diethyl-1,3-indandione (6 ) ] : the ionized phthaloyl group of 1,findandione radical cations l+', 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'.