The unexpected decarbonylation of ionized 2,3-pentanedione and related diketones

Abstract

The fragmentation of the 2,3-pentanedione radical cation gives rise to an unexpected composite metastable ion peak, m/ z 72, resulting from the isobaric losses of CO and C 2H 4. These two fragmentation channels are energetically competitive (i.e., the transition states have similar energies). The two processes yield [CH 3C(O)CH 2CH 3] + and [CH 3C(OH) C O] +, respectively. The latter new ion, which is produced by a McLafferty rearrangement, has Δ f H° = 604 kJ/mol, obtained from G3 calculations. The four competing processes for metastable [CH 3COCOCH 2CH 3] +, the (intense) losses of CH 3CO , CH 3CH 2CO and the weak losses of C 2H 4 and CO and their transition states were placed on a potential energy surface computed at the G3 level of theory. The homologous ionized diketone 2,3-butanedione also displays the decarbonylation channel and 3,4-hexanedione loses CO and C 2H 4.