Exploring the potential energy surface of ion-molecule pairs by experiment and by theory: acetaldehyde and methanol

The energy barrier for the keto-enol isomerization of the isolated acetaldehyde ion to vinyl alcohol lies above its lowest dissociation limit and so the spontaneous isomerization is never observed. However, it has been shown that the distonic methanol ion can ionize the acetaldehyde molecule and greatly lower the energy barrier of the keto-enol isomerization. The present study involved two systems in which the acetaldehyde was associated with the distonic methanol cation, CH2OH2+, and with its conventional isomer, CH3OH+, in stable and metastable adducts. The isomerization barrier for acetaldehyde was lowered by association with CH2OH2+; in contrast, the CH3OH+ ion did not facilitate the rearrangement of acetaldehyde to its enol ion. Tandem mass spectrometry combined with ab initio calculations was applied to investigate the two systems. Potential energy surface diagrams were obtained by MP2/6-31+g(d) and B3-LYP/6-31+g(d) calculations to further elucidate the reaction mechanisms.