A study of the isomerization and dissociation of formal [acetone–methanol]+· ion–molecule complexes

Abstract

The energy barrier for the keto–enol isomerization of the isolated acetone ion to its distonic (enol) isomer lies above its lowest dissociation limit and so the spontaneous isomerization can never be observed. Keto–enol isomerizations can be catalyzed within appropriate ion–molecule complexes. The present study involved two systems, [(CH3)2C=O···H+···O(H)CH2·] (ion 1) and [(CH3)2C=O···H+····OCH3] (ion 2), in both stable and metastable adducts. When acetone is bound to ·CH2OH though a proton bridge, shown as ion 1, an enol acetone ion is produced. This reaction results from a proton attaching to the acetone, which then gives an H· atom back to the radical site by a 1,6-H transfer, involving a transition state of low energy requirement. In contrast, when the acetone is protonated and bound to the radical CH3O· (ion 2), the above rearrangement does not take place. The metastable complex ion 2 loses a methyl radical, producing a new [C3H7O2]+ isomer of structure [CH3C+(O)···(H)OCH3]. Tandem mass spectrometry combined with ab initio calculations were used to investigate the two systems. Potential energy surface diagrams were obtained by calculations at the MP2/6-31+G(d) level of theory to aid further elucidation of the reaction mechanisms. Key words: ion–molecule complexes, keto–enol mechanisms, ion rearrangements and structures.