Does Koopmans’ Paradigm for 1-Electron Oxidation Always Hold? Breakdown of IP/E_ox Relationship for p-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/Eox, and IP/Eox. In cases where a molecule undergoes dramatic structural reorganization upon 1-electron oxidation, the IP/Eox 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 p-hydroquinone ethers ((T)HE and (M)HE) showed that they undergo electron-transfer-induced conformational reorganization and show breakdown of the IP/Eox relationship. DFT calculations revealed that Koopmans' paradigm still holds true because the electron-transfer-induced subtle conformational reorganization, responsible for the breakdown of IP/Eox relationship, is also responsible for the reordering of HOMO and HOMO-1. Perceived failure of Koopmans' paradigm in cases of (T)HE and (M)HE 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.