Ion-Exchange-Mediated Pre-Association Gates Interfacial PCET

Interfacial proton-coupled electron transfer (I-PCET) is typically viewed as a single elementary reaction despite the accepted recognition that analogous solution-phase reactivity requires proton donor/acceptor pre association. Herein, we examine the role of pre-association in I-PCET to a molecularly well-defined graphite-conjugated carboxylic acid (GC-COOH) surface site. We quantify electrolyte proton activity and I PCET kinetics in acidic, acetate-buffered, and alkaline electrolytes as a function of NaClO4 concentration ranging from 1 mole kg⁻¹ to 17 mole kg⁻¹. Measured I-PCET rates at GC COOH are attenuated by a factor of 4.3 in acidic and 4.6 in acetate-buffered media relative to the rates expected based on the solution pH across this ionic strength range. In contrast, in alkaline electrolytes the apparent rate of I-PCET is far less diminished upon increasing NaClO₄ concentration. To account for these findings, we propose a multiple-step model for I-PCET in acidic and acetate-buffered media that invokes quasi-equilibrated exchange of interfacial Na⁺ for H₃O⁺ to form a hydrogen-bonded pre association complex prior to rate-limiting concerted proton-electron transfer. In this model, a decrease in the ratio of interfacial vs bulk Na⁺ activity with added electrolyte inhibits H₃O⁺ pre-association, a phenomenon that is also captured in our molecular dynamics simulations. These studies emphasize the non-innocence of the supporting electrolyte species and exposes the key role pre association equilibria play in I-PCET mechanisms. The work suggests that control over pre-association equilibria could be used as an effective handle for tailoring interfacial ion transfer reaction kinetics.