Quantum Chemical Investigations of Unimolecular and Bimolecular Reaction Barriers under Mechanochemical Conditions

Abstract

Mechanochemistry, i.e. the application of external forces at the molecular level, has emerged as a significant area of research in recent years. This emergence can be attributed to the development of experimental techniques such as atomic force microscopy, molecular force probes, optical and magnetic tweezers, and sonication. The application of such techniques has rendered it possible to activate chemical reactions along particular pathways. Theoretical models have been reported in the literature to account for experimental outcomes and to understand the interplay between mechanical stress and chemical reactivity. The work in this thesis uses quantum chemical calculations to examine reaction barriers, ΔE‡’s, under mechanochemical conditions for a set of unimolecular and bimolecular reactions.First, a code is written to predict changes in ΔE‡’s under mechanochemical conditions. The code is used to select two atoms that a force, F, is applied along, which are termed pulling points. The predicted ΔE‡’s are consistent with those calculated using quantum chemical methods at low values of F, in which the reactants or transition state (TS) structures are not significantly altered. The structural changes are prevented by applying constraints of varying size and chemical composition. The nature of the constraint plays a significant role in activating or deactivating a particular reaction. This result may be of value in the area of mechanoselectivity, in which the application of F can either activate or deactivate a reaction along a particular pathway. Substituents of varying size are added to bimolecular reactions to investigate the underlying energetics associated with ΔE‡’s under mechanochemical conditions. Increasing the sterics of the substituents increases the mechanical work contribution to the reduction in the force-dependent ΔE‡’s.The differences in the abilities to mechanically eliminate ΔE‡’s in unimolecular and bimolecular reactions are identified in this thesis. In unimolecular reactions, ΔE‡ can be rendered to zero if the reactant or TS is shifted along the zero-F reaction coordinate, S0. The ΔE‡’s for bimolecular reactions can only be rendered to zero if there is sufficient coupling between F and other nuclear degrees of freedom in the reactants that shift the reactants along S0 toward the TS.