Deciphering Proton Tunneling in Single-Molecule Chemical Reactions

Area of Science:

• Physical Chemistry
• Quantum Mechanics
• Electrocatalysis

Background:

• Proton tunneling is crucial in proton-coupled electron transfer (PCET) reactions but challenging to study under ambient conditions.
• Understanding the contribution of quantum effects like proton tunneling is vital for designing efficient catalysts.

Purpose of the Study:

• To develop and apply a single-molecule technique for quantifying the role of proton tunneling in PCET reactions.
• To investigate the mechanistic pathways and kinetics of PCET on a molecular catalyst at room temperature.

Main Methods:

• Utilized an on-chip electrochemical mechanically controllable break junction to immobilize a single benzothiadiazole molecule.
• Monitored PCET reactions via conductance tracking and correlated with Raman spectroscopy to identify intermediates.
• Performed temperature-dependent kinetic experiments to analyze reaction pathways and calculate rate constants and Kinetic Isotope Effects.

Main Results:

• Identified four key PCET intermediates and quantified elementary step kinetics and pathway evolution probabilities.
• Revealed an efficient tunneling-mediated pathway competing with the classical thermodynamic pathway.
• Demonstrated tunable modulation of the tunneling route's population (13% to 42%) at room temperature.

Conclusions:

• Established a single-molecule platform for mechanistic quantification in room-temperature electrocatalysis.
• Quantified the significant contribution of proton tunneling, showing its potential for energy-efficient catalysis.
• This method enables evaluation and promotion of proton tunneling in catalytic systems.