Anisotropy of Shear-Induced Mechanochemical Reaction Rates of Surface Adsorbates; Implications for Theoretical Models

Abstract

Mechanochemical reactions occur by an applied force modifying and ideally accelerating the rate of reaction of a mechanically active species, a mechanophore. Thermal reactions are described by the steepest descent pathway (SDP) of the potential energy surface (PES) from the transition state to the reactant state, which are stationary points on the SDP. The activation energy is calculated from the energy difference between these two points. The PES is modified by an imposed force to yield a reaction pathway given by the force-displaced stationary points (FDSPs), which depend on the magnitude and direction of the force and shape of the PES. However, the SDP has zero force in a direction perpendicular to it so that the PES can be visualized as a “harmonic valley” that forms a potential energy trough about the SDP. If the walls of the potential trough are sufficiently steep, a mechanochemical reaction should be constrained to occur along the SDP, and the influence of an applied force should depend only on the component of the force along it. If this is the case, it should be possible to use just the shape of the PES around the initial and transition states to calculate the effect of an imposed force or stress on mechanochemical reaction rates. The postulate is tested for the mechanically induced decomposition of an adsorbed methyl thiolate species on a Cu(100) single-crystal surface by measuring the azimuthal angular dependence of the mechanochemical methyl thiolate decomposition rate by varying the sliding direction of a sharp atomic force microscope tip over the surface in ultrahigh vacuum. The concept is also illustrated using a model 4-fold potential using a Remoissenet–Peyrard function to mimic the potential of a Cu(100) surface. This yields an angular dependence that agrees well with the prediction from the above postulate. This simplification will facilitate the analysis of mechanochemical rates of both surface and bulk reactions.