Mechanochemistry
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CHEMISTS TYPICALLY activate chemical reactions
throught the application of heat (thermochemistry), light (photochemistry), or electric potential
(electrochemistry). An alternative approach, called mechanochemistry, involves activating reactions
through the application of mechanical stresses. In this approach specific atoms in a molecule are
subjected to external stresses. These stresses deform the molecular geometry in a manner that alters
the relative energies of reactants and transition states, thus altering reaction barriers. In addition,
the application of an external stress during the course of a reaction performs work on the system, which
contributes to the energy input needed to overcome reaction barriers. In this manner, mechanochemistry
may be a useful tool for activating chemical reactions. Since stress is a directional quantity, the
mechanochemical activation of reactions has the added benefit of selectively guiding chemical systems
through specific changes in molecular structures. This has been demonstrated in numerous experiments,
including sonication studies that showed mechanochemistry can be used to selectively guide the
ring opening of pericyclic molecules in ways the completely ignore the Woodward-Hoffmann rules. The
ability to essentially direct chemical reactions at the atomic level through mechanical manipulation
would be of significant synthetic utility, and thus it would be beneficial to have a better
understanding of the interplay between mechanical stresses and reactivity.
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THE ABILITY, to activate activate chemical
reactions mechanically in a practical manner has been enabled in recent years through developments
in sonication, molecular force probes, and molecular manipulation methods, which have permitted
controlled stresses to be applied to large quantities of molecules. We are using chemical simulation
to complement these experimental studies, gain a better fundamental understanding of how the application
of mechanical stresses induces reactions, and explore reactions that occur under mechanochemical
conditions. These simulations are typically performed with quantum chemical methods, using an approach
that modifies the potential energy surface to account for the effects of an external force applied
between two atoms used as pulling points.
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Mechanochemical reactions: Processes activated by mechanical stresses are modeled by applying
external forces to two atoms in a molecule used as pulling points, and studying the changes in
structure that occur on the resulting force-modified potential energy surface.
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OUR EFFORTS to date in the area of mechanochemistry
have focused on exploring the ways in which applied stresses and electronic structure compete to
control the outcome of mechanochemical reactions, to explore simple models for predicting reaction barriers
under mechanochemical barriers, and exploring the proper way to simulation mechanochemical processes. Ongoing
efforts are aimed at investigating how mechanochemistry can be used in the context of practical chemical
problems, and exploring ways of designing molecules that respond in a controlled and desired manner to
applied mechanical stresses.
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