The possibility to modify chemical reactions rates by applying external mechanical stimuli such as force or pressure opens new opportunities for designing novel materials with switchable properties in response to mechanical stimuli, e.g. for self-healing and shock absorbing applications. An efficient rational design of such materials would be greatly facilitated by quantitative modeling of mechanical sensitivity, which also provides a clear microscopic physical explanation of the phenomena.  In this work we develop an atomistic model of a mechanophore, a chemical species responsible for the mechanical sensitivity. We start by analyzing the chemical reaction rate in terms of transition state theory, which relates the rate constant to the activation free energy. Thus the mechanical sensitivity is quantified as difference in the activation free energy barriers for a reaction in the presence and in the absence of a mechanical stimulus. The free energy barrier is evaluated by computing the free energy surfaces from ab initio molecular dynamics simulations, in which the electronic structure of the mechanophore is described with ab initio quantum mechanics. We demonstrate the model by application to pressure-induced isomerization of bis-anthracene into its photoisomer. The mechanical impact of pressure is captured by immersing the mechanophore in a fluid of soft spheres and by adjusting density of the fluid.Furthermore, we demonstrate that the mechanism of altering the barrier is destabilization (increase in the free energy) of either reactants or products by the external work. Finally, we identify the molecular deformations, which are responsible for converting the external work into free energy, and demonstrate that the mechanical sensitivity of the activation barrier is related to the existence of selective molecular deformations either in the reactants or in the products. These findings provide guidance to the rational design of mechanophores.

Theoretical insight on mechanical sensitivity of chemical reactions rates from ab initio molecular dynamics free energy modeling

248th National Meeting (2014)

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