Shear-Induced Mechanochemistry: Pushing Molecules Around

The molecular mechanisms by which mechanical energy accelerates a chemical reaction at sliding solidsolid interfaces are not well understood because of the experimental difficulties in monitoring chemical processes and their rates, and in controlling parameters such as interfacial temperature. These issues are addressed by measuring the shear-induced rate of methane formation from the decomposition of adsorbed methyl thiolate species on copper in ultrahigh vacuum (UHV), where the frictional heating is negligible. The effect of a constant force F on the energy profile for thiolate decomposition from density functional theory calculations is modeled by superimposing a linear potential, V(x) = -Fx. This enables the change in activation barrier to be calculated as a function of force. The mechanically induced reaction rate is measured by sliding a ball over a methyl thiolate-covered copper surface from the methane yield measured by a mass spectrometer placed in the UHV chamber. Molecular dynamics simulations reveal that a wide distribution of forces are exerted on the thiolates and comparing the measured methyl thiolate decomposition rate with the rate calculated by assuming a wide force distribution reproduces the experimental data. This reveals that only a small proportion of the adsorbed thiolates experience sufficiently high forces to reduce the activation barrier to reproduce the experimentally measured rate constant.