A molecular force probe.

Force probes allow reaction rates to be measured as a function of the restoring force in a molecule that has been stretched or compressed. Unlike strain energy, approaches based on restoring force allow quantitative molecular understanding of phenomena as diverse as translation of microscopic objects by reacting molecules, crack propagation and mechanosensing. Conceptually, localized reactions offer the best opportunity to gain fundamental insights into how rates vary with restoring forces, but such reactions are particularly difficult to study systematically using microscopic force probes. Here, we show how a molecular force probe, stiff stilbene, simplifies force spectroscopy of localized reactions. We illustrate the capabilities of our approach by validating the central postulate of chemomechanical kinetics--force lowers the activation barrier proportionally to the difference in a single internuclear distance between the ground and transition states projected on the force vector--on a paradigmatic unimolecular reaction: concerted dissociation of the C-C bond.