Quantum tunneling on water. I. General framework for microdroplet redox chemistry

Microdroplet chemistry has emerged as a fascinating field. In particular, a plethora of unexpected oxidation and reduction reactions are observed at the air-water interface, with wide implications on chemical science, industry, and beyond. However, the explanation of microdroplet redox chemistry is challenging and controversial. Mechanistically, thermodynamic reversal over the bulk reaction, surprisingly fast kinetics (often less than milliseconds), and generality of a broad scope of substrates defy textbook understanding. Herein, we propose and justify a theoretical model of quantum-tunneling-based, concerted, interfacial electron transfer of OH-partiallysolvated+An→OH·partiallysolvated+An-1, rather than the conventional stepwise ionization-capture picture. We then introduce a solvation-dependent free-energy distribution for the partial solvation of OH- at a heterogeneous water interface and develop a Marcus-theory-based reaction rate formalism that integrates the energetic distribution. Four different electron acceptors An (including a transition metal, an organic molecule, oxygen, and ozone) are chosen to cover a wide range of experimental scenarios and chemical diversity. When An in the system is reduced upon receiving an electron from OH- across the thin interface, the accompanied product of OH· further serves as the oxidizing agent, capturing the intrinsic oxidative capacity of microdroplet. We quantitatively analyze both the thermodynamic and the kinetic consequences of this model. For all four cases analyzed, the otherwise unfavorable thermodynamic penalty (ΔG° > 0) for the bulk reaction is reversed at the water interface (ΔG° < 0) after a certain dehydration level. Remarkably, for all these cases, interfacial electron transfer becomes near-barrierless (ΔG°≈-λ, ΔG‡≤3 kcal/mol) for a substantial sub-population of partially solvated OH-. Thus, the water interface can prepare and gate a considerable donor population whose electrons are nearly ready to quantum mechanically tunnel into a broad range of acceptors across the thin interface, which explains the surprisingly fast reaction rate observed in experiments. This model further predicts absolute rate constants that are comparable with kinetic measurement on microdroplets. Finally, a phase diagram is constructed to depict the vast thermodynamic and kinetic (barrierless reaction) zones that are newly opened for a general An by the water interface, thus accounting for reaction generality. Despite its simplicity, this model provides a robust explanation and prediction for the rich redox chemistry at a water interface.