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.
The surface tribological chemistry of acetic acid on copper is studied using an ultrahigh vacuum tribometer, supplemented by first-principles density functional theory calculations of the surface structure and reaction pathways. Acetic acid forms η 2 -acetate species on bridge sites at room temperature as identified by reflection-absorption infrared spectroscopy. Rubbing the surface with a tungsten carbide ball reduces the amount of carbon and oxygen in the rubbed region at the same rates to leave some carbon and oxygen on the surface. This is different from the thermal decomposition pathway, where heating to ~ 580 K removes all oxygen, but leave a small amount of carbon on the surface. It is postulated that this arises because sliding along a direction aligned within the plane of the adsorbed acetate species can induce a high-energy-barrier pathway in which the η 2 -acetate tilts to form an η 1 -acetate that can react to form a bent CO 2 δ− species that decomposes to evolve carbon monoxide and deposit atomic oxygen on the surface. Repeated acetic acid dosing and rubbing reduces the total amount of acetic acid that can adsorb on the surface by ~ 50% after ~ 4 cycles, resulting is a stable, low-friction film. At this point, the adsorbed acetic acid is completely tribochemically removed. This suggests that adsorbed acetic acid can form a selfhealing film in which any wear of the low-friction film will then allow it to be replenished by shear-induced decomposition of adsorbed acetate species.
The rate of mechanochemical decomposition of C8-carboxylates on copper was found to be independent of the nature of the terminal group despite differences in the strength of binding to the moving counterface.
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