Introduction to Electron Transfer: Theoretical Foundations and Pedagogical Examples

• Published in: Journal of chemical education 2019-11, Vol.96 (11), p.2450-2466
• Main Authors: Piechota, Eric J, Meyer, Gerald J
• Format: Article
• Language: English
• Subjects: Chemistry; College Science; College students; Coupling; Coupling (molecular); Education & Educational Research; Electron transfer; Electrons; Energy; Gibbs free energy; Graduate Study; Hookes law; Kinetics; Learning; Normal distribution; Organic chemistry; Parameters; Rate constants; Regeneration; Science education; Science Instruction; Scientific Concepts; Theories; Transfer Rates (College); Undergraduate Study; Wave functions
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/acs.jchemed.9b00489

A central theme in introductory and advanced chemical education courses pertains directly to the transfer of electrons between atoms, ions, or molecules. This article presents theoretical treatments of electron transfer with specific attention toward applying these principles to experiment. The goal is to revitalize teaching electron transfer at the undergraduate and graduate levels. Central theoretical aspects are presented through the construction of Gibbs free (potential) energy surfaces with definitions and semiquantitative descriptions of the three key parameters necessary to compute electron transfer rate constants with Marcus theory: (1) the Gibbs free energy change, ΔG°; (2) the reorganization energy, λ; and (3) the electronic coupling between D and A wave functions, H DA. A simplified description of this theory is presented with classical free energy surfaces for the electron donor and acceptor wherein the force constant in Hooke’s Law is replaced by λ. Variation of ΔG° results in a Gaussian distribution of activation energies that give rise to Marcus normal, activationless, and inverted kinetic behaviors. Classical and contemporary experimental examples that have tested and utilized Marcus theory are described, including the first validation of the inverted kinetic region. It is shown that, as the donor–acceptor coupling increases, adiabatic electron transfer may result where it becomes more difficult to decouple the Marcus parameters through experiment. The trials and tribulations of doing so are described that provide context and enable readers to understand the prior electron transfer literature and use the pedagogical foundations presented herein for their own learning and pleasure.