Deciphering Proton Tunneling in Single-Molecule Chemical Reactions.

Proton tunneling, a fundamental quantum mechanical tunneling effect, profoundly determines catalytic pathways in proton-coupled electron transfer (PCET) reactions, yet it has long been overlooked because of the challenge in deciphering its contribution under ambient conditions. Advancing the on-chip electrochemical mechanically controllable break junction approach, we immobilized a single benzothiadiazole molecule to monitor the PCET reaction that occurred on this molecular catalyst via conductance tracking. Through repeated thousands of molecular junction suspensions, we statistically identified four key PCET intermediates from thousands of PCET reaction cycles by correlating the conductance evolution with Raman spectroscopy, thereby extracting each rate constant, the Kinetic Isotope Effect of elementary steps, and the evolution probabilities of each PCET pathway. Further temperature-dependent kinetic experiments revealed an efficient tunneling-mediated route over the classical thermodynamic pathway, which is further supported by the analysis of the microscopic PCET kinetics. The population of the tunneling route increased from 13% to 42% under varied conditions, indicating a tunable modulation of the reaction at room temperature. Our single-molecule method establishes a general mechanistic platform for quantifying reaction mechanisms in room-temperature multistep electrocatalysis. It provides the opportunity to evaluate the promotion of proton tunneling. This advancement can guide the development of energy-efficient electrocatalytic systems that leverage quantum tunneling.