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Density functional theory (DFT) was employed to study the impact of Mg(2+) ions on the o-semiquinone radical anions of different aromaticity in protic and aprotic solvents. After the geometry optimization of ligands and complexes, their g tensors were computed at the UBP86/TZVP and UB3LYP/TZVP theory levels. The suitability of various model systems, assuming continuum dielectric approaches, different Mg(2+) coordination spheres (completed by solvent molecules), and inclusion of additional solvent molecules H-bonded to the ligands, was tested in terms of correlation between the experimental and calculated g-shifts. The effects of complexation, ligands aromaticity, and solvents on the electron spin density for o-semiquinones are discussed. To recognize clearly the changes in the nature of the g tensor components, the contributions from particular excited states were analyzed. A structural characterization of the tested complexes is expected to be helpful in investigations on the complicated biosystems in which the similar paramagnetic units are present.

A diindenophenanthrene biradicaloid, formally derived from Chichibabin's hydrocarbon, is obtained in a short, scalable synthesis.

X-band (9.76 GHz) and high field (416.00 GHz) electron paramagnetic resonance spectroscopy (EPR) was used to study the interactions between Pb(II) ions and semiquinone radicals of natural humic acids and their simple models. The EPR experiments were performed on powder samples. The formation of Pb(II) complexes with the radicals was accompanied by a significant decrease of g parameters as compared to those observed for parent radicals. Two types of complexes were identified depending on the initial concentration of Pb(II) ions. For one of them the anisotropic hyperfine coupling with the (207)Pb nucleus was observed. Systematic DFT calculations were carried out for complexes with different forms of radical ligands (L(2)(-*), HL(-*), and H(2)L*) derived from 3,4-dihydroxybenzoic acid representing different ligation schemes. The g parameters calculated for the structure characterized by a significant accumulation of spin density on the Pb atom are strongly deviated from the values observed experimentally. Moreover, a decrease of the spin population on all oxygen atoms as a result of complexation of Pb(II) via carboxyl oxygens and protonation of hydroxyl oxygens is required to reproduce the experimental g parameters.

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The g matrices (g tensors) of various phosphinyl radicals (R2 P(.) ) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless PO or PS bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with PO bonds, this contribution to the g2 component was dominant.

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