Predicting unimolecular chemical reactions: Chemical flooding

We present a method to predict products, transition states, and reaction paths of unimolecular chemical reactions such as dissociation or rearrangement reactions of small to medium sized molecules. The method thus provides the necessary input for established procedures to compute barrier heights and reaction rates, which conventionally have to be assumed heuristically. The method is an extension of the force field based conformational flooding procedure, but here aims at an accelerated barrier crossing of chemical reactions rather than conformational motions. Accordingly, it is now coupled to density functional molecular dynamics, such that the chemical reaction under study takes place at the picoseconds time scale set by todays computer technology. Barrier crossings are accelerated by means of an additional energy term ~flooding potential! that locally destabilizes the educt conformation without affecting possible transition states or product states. The method is applied to two test systems, bicyclopropylidene and methylenecyclopropane, for which the reaction paths are predicted correctly. New details of reaction pathways are found, such as a transient concerted, but asynchronous rotation of the two methylene groups for the bicyclopropylidene→methylenespiropentane reaction. Our method can be applied to simulations in the gas phase as well as in solution and can be combined with force field simulations, e.g., in hybrid density functional/force field ~QM/MM! computations. © 2002 American Institute of Physics. @DOI: 10.1063/1.1427722#