Mechanical Activation of Forbidden Photoreactivity in Oxa-di-π-methane Rearrangement

Wandering through quantum-mechanochemistry: from concepts to reactivity and switches

Mechanochemistry has experienced a renaissance in recent years witnessing, at the molecular level, a remarkable interplay between theory and experiment. Molecular mechanochemistry has welcomed a broad spectrum of quantum-chemical methods to evaluate the influence of an external mechanical force on molecular properties. In this contribution, an overview is given on recent work on quantum mechanochemistry in the Brussels Quantum Chemistry group (ALGC). The effect of an external force was scrutinized both in fundamental topics, like reactivity descriptors in Conceptual DFT, and in applied topics, such as designing molecular force probes and tuning the stereoselectivity of certain types of reactions. In the conceptual part, a brief overview of the techniques introducing mechanical forces into a quantum-mechanical description of a molecule is followed by an introduction to conceptual DFT. The evolution of the electronic chemical potential (or electronegativity), chemical hardness and electrophilicity are investigated when a chemical bond in a series of diatomics is put under mechanical stress. Its counterpart, the influence of mechanical stress on bond angles, is analyzed by varying the strain present in alkyne triple bonds by applying a bending force, taking the strain promoted alkyne-azide coupling cycloaddition as an example. The increase of reactivity of the alkyne upon bending is probed by Fukui functions and the local softness. In the applied part, a new molecular force probe is presented based on an intramolecular 6π-electrocyclization in constrained polyenes operating under thermal conditions. A cyclic process is conceived where ring opening and closure are triggered by applying or removing an external pulling force. The efficiency of mechanical activation strongly depends on the magnitude of the applied force and the distance between the pulling points. The idea of pulling point distances as a tool to identify new mechanochemical processes is then tested in [28]hexaphyrins with an intricate equilibrium between Möbius aromatic and Hückel antiaromatic topologies. A mechanical force is shown to trigger the interconversion between the two topologies, using the distance matrix as a guide to select appropriate pulling points. In a final application, the Felkin-Anh model for the addition of nucleophiles to chiral carbonyls under the presence of an external mechanical force is scrutinized. By applying a force for restricting the conformational freedom of the chiral ketone, otherwise inaccessible reaction pathways are promoted on the force-modified potential energy surfaces resulting in a diastereoselectivity different from the force-free reaction.

An Algorithm Predicting the Optimal Mechanical Response of Electronic Energy Difference

The use of mechanical forces at the molecular level has been shown to be an interesting tool for modulating different chemical and physical molecular properties. The so-called covalent mechanochemistry deals with the application of precise mechanical forces that induce specific changes in the structure, stability, reactivity, and other physical properties. The use of this kind of force to modulate photophysical properties and photochemical reactivity has also been studied. Nevertheless, the general problem of mechanical modulation of the energy gap between two electronic states has been addressed only with the development of simple theoretical models. Here, we develop and implement an algorithm providing the Largest energy Gap variation with Minimal mechanical Force (LGMF) that allows the determination of the optimal mechanical forces tuning the electronic energy gap, as well as to identify the maximum mechanical response of a molecular system to the application of any mechanical stimulus. The algorithm has been implemented for diverse molecular systems showing different degrees of flexibility. The phyton code of the algorithm is available in a public repository.