Theory and computational studies of mechanochemical phenomena
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Mechanochemistry, or the modulation of chemical reactivity through the application of mechanical forces, has shown to facilitate a number of otherwise prohibitive chemical transformations. Computational approaches employing electronic structure calculations have explained a number of mechanochemically activated processes such as thermally inaccessible isomerizations and cycloreversions, symmetry-forbidden electrocyclic ring openings or activation of latent catalysts and, more recently, have been successfully used to design novel mechanosensitive systems. A significant limitation of such approaches, however, is their high computational cost, as finding force dependent transition states requires multiple saddle searches and consequently, multiple energy evaluations. To circumvent this problem, an approximation has been proposed, extending the well know "Bell formula", which estimates the force-dependent reaction barrier based on zero-force transition state properties. We demonstrate the numerical efficiency of this approximation termed as extended Bell theory (EBT) by comparing to existing theories and experiments. We also apply this method to suggest the unexplored, yet potentially useful possibility of suppressing chemical reactions through mechanical perturbation. Furthermore, in sharp contrast to simple, one-dimensional theories, our analysis reveals that the anti-Hammond effect is dominant in the mechanical activation of polyatomic molecules. Finally, we propose a numerical scheme to address the drawback of the EBT approximation, which is the failure to account for force-induced instabilities. Our approach provides a computationally efficient recipe to track the instabilities and follow the evolution of the reactant or transition states at any explicit force. We provide a classification of the different instability scenarios, and provide an illustrative example for each case.