Explaining Kinetic Isotope Effects in Proton-Coupled Electron Transfer Reactions

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Theoretical Chemistry

Background:

• Proton-coupled electron transfer (PCET) is fundamental to numerous chemical and biological processes.
• Understanding PCET mechanisms is crucial for controlling and optimizing these reactions.
• Kinetic isotope effects (KIEs) are experimental probes for PCET, but their interpretation is often complex.

Purpose of the Study:

• To present a theoretical framework for interpreting KIEs in concerted PCET reactions.
• To classify PCET reactions based on vibronic and electron-proton nonadiabaticities.
• To provide a method for computing KIEs using computational approaches.

Main Methods:

• Classification of PCET reactions into vibronically adiabatic and nonadiabatic regimes.
• Application of transition state theory for adiabatic reactions.
• Utilization of golden rule formalism for nonadiabatic reactions, employing the nuclear-electronic orbital (NEO) method for calculating vibronic coupling and energy surfaces.

Main Results:

• Vibronically adiabatic PCET reactions exhibit moderate KIEs due to zero-point energy and shallow tunneling.
• Vibronically nonadiabatic PCET reactions can display a wide range of KIEs (unity to >500) attributed to competing tunneling pathways between vibronic states.
• The framework successfully explains KIEs in diverse systems, including enzymes and surface reactions, by considering vibronic couplings and donor-acceptor distances.

Conclusions:

• The proposed theoretical framework provides a robust method for interpreting KIEs in PCET reactions.
• Reaction KIEs are highly dependent on the dominant vibronic states and their couplings.
• This work offers significant implications for the experimental interpretation of PCET mechanisms across various scientific disciplines.