Predicting unimolecular chemical reactions: Chemical flooding

Abstract

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. (C) 2002 American Institute of Physics

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

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Flooding in GROMACS: Accelerated barrier crossings in molecular dynamics

“…Therefore, if only the educt state is known, these methods cannot be applied, and the prediction of product states becomes a real challenge. This scenario is addressed by the flooding technique, 5,8 which aims at predicting both, the unknown product state as well as the transition pathway.…”

Anharmonic Downward Distortion Following for Automated Exploration of Quantum Chemical Potential Energy Surfaces

“…The artificial force induced reaction (AFIR) method has been developed for finding TSs for associative pathways among two or more components efficiently. 6567 Some approaches, such as the sphere optimization technique, 68 the iso-potential contour following method, 69 and the chemical flooding method, 70 find only a single path starting from a local minimum, where this single path often corresponds to the lowest barrier path. Moreover, there is a rigorous, deterministic approach called ¡BB global optimization method which has been applied to triatomic molecules and the conformational search in simple peptides.…”