Partially Covalent Two-Electron/Multicentric Bonding\nbetween Semiquinone Radicals

X-ray\ncharge density was determined and analyzed for two polymorphs\nof the N-methylpyridinium salt of the tetrachlorosemiquinone\nradical anion and its analogous closed-shell relatives, tetrachloroquinone\n(chloranil) and tetrachlorohydroquinone. The study, which was combined\nwith calculations of electron delocalization, electrostatic potentials,\nand aromaticity, presents details of electronic structure of the semiquinoid\nring. This comparative study reveals that the negative charge is delocalized\nover the entire semiquinone radical and that the chlorine substituents\nplay a crucial role in its stabilization through induction effect.\nIn general, the semiquinoid ring has partially delocalized π-electrons\nand is approximately halfway between a quinoid and an aromatic ring.\nIn the orthorhombic polymorph with stacks of equidistant radicals\nelectron density between the rings of almost 0.05 e Å–3 and four (3,–1) saddle points between the contiguous rings\nwere found. In the diamagnetic triclinic polymorph, comprising strongly\nbound radical dimers (with significant covalent character“pancake\nbond”), maximum electron density between the rings exceeds\n0.095 e Å–3, and multiple (3,–1) critical\npoints are found. However, only negligible electron density is observed\nbetween the dimers. Thus, in the radical anion stacks spin coupling,\nalong with dispersive and polarization effects, defines interplanar\ndistance and magnetic behavior, whereas intermolecular electrostatic\npotential determines the ring offset.