The Impact of Huge Structural Changes on Electron Transfer and Measurement of Redox Potentials: Reduction of ortho-12-Carborane

• Published in: The journal of physical chemistry. B 2019-11, Vol.123 (45), p.9668-9676
• Main Authors: Miller, John R, Cook, Andrew R, Šimková, Ludmila, Pospíšil, Lubomír, Ludvík, Jiří, Michl, Josef
• Format: Article
• Language: English
• Subjects: carborane; cyclic voltammetry; free energy relation; Frumkin Effect; Homogeneous vs heterogeneous electron transfer; overpotential; pulse radiolysis; reorganization energy; SOLAR ENERGY
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.9b08151

A massive structural change accompanies electron capture by the 1,2-dicarba-closo-dodecaborane cage molecule (1). Bimolecular electron transfer (ET) by pulse radiolysis found a reduction potential of E 0 = −1.92 V vs Fc+/0 for 1 and rate constants that slowed greatly for ET to or from 1 when the redox partner had a potential near this E 0. Similarly, two electrochemical techniques could detect no current at potentials near E 0, finding instead peaks or polarographic waves near −3.1 V, which is 1.2 V more negative than E 0. Voltammetry could determine rate constants, but only near −3.1 V. DigiSim simulations can describe the irreversible voltammograms but require electrochemical rate constants near 1 × 10–10 cm/s at E 0, a factor of 10–10 relative to molecules undergoing facile ET. This factor of 10–10 compared to ∼10–5 for bimolecular ET presents a puzzle. This puzzle can be understood as a manifestation of one of the “Frumkin Effects” in which only part of the applied voltage is available to drive ET at the electrode.