Catalysis models for the enolization of the acetaldehyde radical cation

The catalysis of the enolization of the prototype aldehyde radical cation in the gas phase using methanol or acetaldehyde as the catalyst was studied theoretically. For the computations the CBS-Q model was used, in combination with RHF/DZP optimized geometries. Various reaction path models were tested, including the proton-transport catalysis (PTC) model, the Spectator model and the Quid pro Quo (QpQ) model. For the latter model there are several possibilities, depending on the choice made for the catalyst hydrogen to be exchanged. For some reaction steps no transition states exist. Instead, there are intersections of two potential energy surfaces corresponding to different localizations of the radical site in the ion-molecule complex. For acetaldehyde both the PTC and QpQ (all variants) models are shown to be feasible. However, the barrier heights involved in these models are very different and the corresponding reaction rates are also expected to differ widely. As a consequence different reaction pathways may be applicable for reactions taking place in different time frames. (C) 2002 Elsevier Science B.V. All rights reserved.