Does Koopmans’ Paradigm for 1‑Electron Oxidation Always Hold? Breakdown of IP/<i>E</i><sub>ox</sub> Relationship for <i>p</i>‑Hydroquinone Ethers and the Role of Methoxy Group Rotation

Koopmans’ paradigm states that electron loss occurs from HOMO, thus forming the basis for the observed linear relationships between HOMO/IP, HOMO/<i>E</i><sub>ox</sub>, and IP/<i>E</i><sub>ox</sub>. In cases where a molecule undergoes dramatic structural reorganization upon 1-electron oxidation, the IP/<i>E</i><sub>ox</sub> relationship does not hold, and the origin of which is not understood. For example, X-ray crystallography of the neutral and cation radicals of bicyclo[2.2.1]­heptane-annulated <i>p</i>-hydroquinone ethers (<sup>T</sup><b>HE</b> and <sup>M</sup><b>HE</b>) showed that they undergo electron-transfer-induced conformational reorganization and show breakdown of the IP/<i>E</i><sub>ox</sub> relationship. DFT calculations revealed that Koopmans’ paradigm still holds true because the electron-transfer-induced subtle conformational reorganization, responsible for the breakdown of IP/<i>E</i><sub>ox</sub> relationship, is also responsible for the reordering of HOMO and HOMO-1. Perceived failure of Koopmans’ paradigm in cases of <sup>T</sup><b>HE</b> and <sup>M</sup><b>HE</b> assumes that both vertical and adiabatic electron detachments involve the same HOMO; however, this study demonstrates that the vertical ionization and adiabatic oxidation occur from different molecular orbitals due to reordering of HOMO/HOMO-1. The underpinnings of this finding will spur widespread interest in designing next-generation molecules beyond HQEs, whose electronic structures can be modulated by electron-transfer-induced conformation reorganization.