The use of mechanical stresses to induce chemical reactions has attracted significant interest in recent years. Computational modeling can play a significant role in developing a comprehensive understanding of the interplay between stresses and chemical reactivity. In this review, we discuss techniques for simulating chemical reactions occurring under mechanochemical conditions. The methods described are broadly divided into techniques that are appropriate for studying molecular mechanochemistry and those suited to modeling bulk mechanochemistry. In both cases, several different approaches are described and compared. Methods for examining molecular mechanochemistry are based on exploring the force-modified potential energy surface on which a molecule subjected to an external force moves. Meanwhile, it is suggested that condensed phase simulation methods typically used to study tribochemical reactions, i.e., those occurring in sliding contacts, can be adapted to study bulk mechanochemistry.

The use of mechanical stress to guide molecular systems along specific reaction pathways has gained considerable interest recently. [1] This interest has been driven by sonication experiments, [2] which showed that the ring opening of benzocyclobutene (BCB) with polymeric substituents on the carbon atoms of the scissile bond can be selectively directed along competing pathways in violation of the Woodward-Hoffmann (WH) rules.[3] Specifically, the thermally allowed conrotatory pathway was followed if the substituents were in a trans configuration with respect to the ring, and the thermally forbidden disrotatory pathway was followed if the substituents were in a cis configuration. These results were attributed to stress applied across the scissile bond during ultrasound pulses, causing the ring to open along the pathway that moves the polymer substituents farthest apart. Circumventing the WH rules is fundamentally interesting, and more generally, the ability to selectively activate competing reactions with mechanical stress may be of synthetic value.The mechanochemical ring openings of cyclobutene (CB) and BCB were further investigated using quantum chemical methods. [4,5] These studies confirmed that applying an external force of magnitude F ext can selectively activate the ring opening of CB along controtary or disrotatory pathways based on the location of the atoms used as pulling points (PPs). This was determined on the basis of reaction barriers calculated by treating the system as moving on a forcemodified potential energy surface (FMPES) [Eq. (1)]:where q represents the atomic positions, V BO is the BornOppenheimer (BO) potential energy, and x(q) is the distance between the PPs. Pathways along which x(q) increases upon moving from reactant to transition state (TS) will experience a decreased barrier relative to that in the absence of F ext . The theoretical studies explained the mechanochemical ring opening of CB along the forbidden pathway in terms of energetics. However, the WH rules are ultimately based on orbital symmetries. The typical route taken to follow a thermally forbidden pathway is to irradiate the system, changing the electronic state and orbital occupations. F ext does not interact directly with the electronic structure (ES) in a manner that can induce electronic excitations. Therefore, it would be of fundamental value to determine how applying F ext promotes WH forbidden reactions. Herein, we use quantum chemical methods to study how the ES evolves during the disrotatory ring opening of CB under mechanochemical conditions.The disrotatory ring opening of CB was investigated by performing molecular dynamics (MD) simulations on the FMPES. CASSCF(4,4)/6-31G(d,p) was used to evaluate V BO and hydrogens on the carbon atoms of the scissile bond were used as PPs, with the PPs in a cis configuration to activate the disrotatory process. The simulations showed that the forbidden process occurs when F ext ! 2800 pN. These results are consistent with previous MD simulations except that the minimum value ...

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Stress test: Quantum chemical methods were used to study how subjecting molecules to mechanical stresses (see picture; Fext is an external force) can overcome the Woodward–Hoffmann (WH) rules. The results show that applied stresses do not alter the electronic structure (as is the case for irradiation) but rather render the WH rules secondary to mechanochemical factors that favor progression to the products.

The synthesis, structure determination and oxidative stability of novel Rh-NHC complexes which feature pyridine-derived ligands have been described. All complexes described herein were synthesized from common dinuclear precursors of general structure [Rh(NHC)(L)Cl](2), where L is a monodentate olefin. We demonstrate that the use of these precursors is critical for the formation of all complexes since related cyclooctadiene containing precursors ([Rh(NHC)(COD)Cl]) were completely unreactive under identical conditions. We further demonstrate that complexes with the general formula [Rh(NHC)(olefin)(Py)Cl] or ([Rh(NHC)(BiPy/Phen)Cl]) are extremely sensitive to oxygen, reacting initially to give an adduct with dioxygen, and then decomposing further. The series of compounds and their oxidation products gave a remarkable range of colours which may be useful in the preparation of colourometric oxygen sensors.