Quantum Chemical Simulations of the Mechanical Activation of Pericyclic Reactions

Abstract

Mechanochemistry, the use of mechanical stress to activate chemical reactions, has emerged as a significant area of interest in recent years. Two theoretical approaches have been developed to simulate mechanochemical processes: COnstrained Geometries Simulate External Force (COGEF) and External Force is Explicitly Included (EFEI). In the COGEF method, mechanical stress is simulated by increasing the distance between atoms in a molecule that serve as pulling points (PPs) at a constant rate. In the EFEI methods, a constant external force (Fext) is applied between PPs, allowing the atoms to move to maintain that force. Both methods have been used in the literature to study the ring opening of cyclobutene under mechanochemical conditions. These studies have shown that applying a force between cis PPs in cyclobutene induces ring opening along the conrotatory pathway in COGEF-based simulations and ring opening along the disrotatory pathway in EFEI-based simulations. The latter is consistent with experiments. The work in this thesis identifies the origin of the differences in the outcomes obtained with these two methods, which may be of interest in the context of researchers selecting methods to simulate mechanochemical processes. The results demonstrate that the origin of the difference in behaviour is related to the manner in which these methods alter the potential energy surface (PES) through the application of a mechanical stress. Specifically, the PES obtained with the COGEF method does not contain a minimum energy pathway (MEP) linking cyclobutene to the disrotatory product, whereas the EFEI surface does contain such a path. The differences in PESs suggest that the EFEI method is more suitable to simulate mechanochemical processes. The EFEI approach was then used to examine how the electronic structure evolves to permit a formally forbidden disrotatory reaction to occur. The circumvention of the Woodward-Hoffmann rules was not due to a change in the electronic structure. Instead, the application of an external force shifts the transition state along the reaction coordinate towards the reactants, lowering the barrier for the reactions. The orbital effects that disfavor movement from reactants to products are rendered secondary to mechanochemical factors.