Quantum tunneling on water. III. An asymmetric Marcus-based electron-transfer model for microdroplet redox chemistry.

Water microdroplets support a broad range of spontaneous redox reactions at rates that often exceed expectations from bulk solution, yet a microscopic framework that is both mechanistic and quantitatively predictive across diverse acceptors remains lacking. In Parts I and II [C. Zhuang, N. Qian, and W. Min, J. Chem. Phys. 164(22), 224709 (2026) and J. Chem. Phys. 164(22), 224710 (2026)] of this series, we described microdroplet redox chemistry as concerted electron tunneling from partially solvated interfacial hydroxide to acceptors within a symmetric Marcus framework. Here, we extend this picture to an asymmetric, solvation-state-dependent framework in which reactant- and product-side reorganization energies are unequal and evolve along the interfacial dehydration coordinate. This generalization captures the partial breakdown of a single linear-response solvent coordinate at heterogeneous water interfaces and yields a corresponding asymmetric rate expression. Reanalysis of previously studied acceptors confirms consistency with the earlier symmetric model, while revealing that asymmetry becomes decisive in strongly imbalanced cases. Proton reduction by interfacial hydroxide provides a striking example: charge recombination generates highly unequal solvation responses and alters the predicted kinetics by orders of magnitude. Comparison with three independent experimental datasets yields rate estimates in encouraging agreement with experiment, supporting proton reduction as a baseline pathway of intrinsic microdroplet reactivity. A paired comparison of pyridine and benzene further defines a practical boundary for interfacial redox observability. Finally, parity analysis of all acceptors examined across this series shows broad quantitative agreement between predicted and experimentally inferred rate constants. Together, these results establish an asymmetric Marcus framework as a more faithful and predictive theory of electron transfer at heterogeneous water interfaces.