Competition between Orbitals and Stress in Mechanochemistry

Abstract

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 ...

Model studies of force-dependent kinetics of multi-barrier reactions

“…Here Q is a vector of 3N-6 non-redundant internal coordinates of the molecule with Q(1) being the constrained molecular coordinate (note that the system of equations does not generally have a unique solution, that is, even a single conformer would have several geometries that would satisfy the system of equations). This strategy was used successfully [20][21][22]24 to calculate electronic energies of activation of cyclobutene ring-opening and analogous reactions in single ground and transition-state conformers of small molecules. This approach is particularly useful for MD simulations, but its current implementations remain too expensive for optimizations of thermodynamic states comprising dozens or hundreds of individual conformers.…”

“…Such force-dependent reactivity is thought to account for the behaviour of polymeric materials, melts and solutions under load, may be exploited to yield new stress-responsive, actuating and energy transducing materials [1][2][3][4][5][6][7][8][9][10] and may even allow characterization of transition states. 11 Considerable theoretical, 3,[12][13][14][15][16][17][18][19] computational 13,[20][21][22][23][24][25][26] and experimental [27][28][29][30][31][32][33][34] effort has been devoted to refine and validate a model of forcedependent kinetics for elementary (single-barrier) reactions. Yet many reactions proceed through one or more intermediates, that is, multiple activation barriers separate the reactant(s) from the product(s).…”

Perspective: Mechanochemistry of biological and synthetic molecules

“…102). Those include explicit incorporation of the force term into a PES 21,[106][107][108] (as in Eq. (8)), using constraints to simulate stretching, 1,[109][110][111] numerical integration of Eqs.…”

Theoretical Approaches for Understanding the Interplay Between Stress and Chemical Reactivity

“…A large number of theoretical studies of chemical processes occurring under mechanochemical conditions have been reported to complement experimental efforts, help explain experimental outcomes, and provide new insights into the interplay between applied forces or stresses and chemical reactivity [29][30][31][32][33][34][35][36][37][38]. These studies have focused primarily on simulating molecular systems exposed to external forces by treating the system as though it moves on a force-modified potential energy surface that incorporates the work performed on a chemical system as it undergoes structural changes in the presence of an external force.…”