jz5b01532_si_001.pdf (28.27 MB)
Does Koopmans’ Paradigm for 1‑Electron Oxidation Always Hold? Breakdown of IP/Eox Relationship for p‑Hydroquinone Ethers and the Role of Methoxy Group Rotation
journal contribution
posted on 2015-12-17, 09:27 authored by Marat
R. Talipov, Anitha Boddeda, Sergey V. Lindeman, Rajendra RathoreKoopmans’
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 (THE and MHE) 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 THE and MHE 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.