A recently developed density functional approach has been used to carry out a systematic computational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreement with high-field EPR data in frozen 2-propanol is achieved only after taking into account the significant reduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison of various model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol molecules to each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of the methoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thus influences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproduced quantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformations of methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of the interrelations between structure, bonding, and spectroscopic data are provided. The relevance of the computational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reaction centers is discussed.

Understanding the bonding in transition metal complexes with redox-active ligands is a major challenge, for example in redox catalysis or in bioinorganic chemistry. In this work, electronic g-tensors, spin-density distributions, and electronic structure have been studied by different density functional methods for an extended series of complexes [Ru(acac)2(L)]n (n = -1, 0, +1; L = redox-active o-quinonoid ligand). Comparison is made with experimental g-tensors and g-tensor-based oxidation-state assignments for a number of experimentally studied examples, using both gradient-corrected (BP86) and hybrid functionals (B3LYP, BHLYP) representing a range of exact-exchange admixtures. Reasonable, albeit not perfect, agreement with experimental g-tensors is obtained in one-component DFT calculations with hybrid functionals. Analyses of spin densities confirm the assignment of the cationic complexes as predominantly d5-Ru(III) with a neutral quinonoid ligand. However, this conclusion is obtained only after inclusion of the appreciable spin polarization of the unrestricted determinant, while the singly occupied molecular orbital (SOMO) is localized more on the acac ligands. The anionic complexes turn out to be approximately halfway between a d6-Ru(II)/semiquinone and a d5-Ru(III)/catecholate formulation, but again only after taking into account the extensive spin polarization. Even the previous assignment of the neutral parent systems as d5-Ru(III)/semiquinone is not accurate, as a d6-Ru(II)/quinone resonance structure contributes to some extent. Very unusual trends in the spin contamination of the Kohn-Sham determinant with increasing exact-exchange admixture in some of the cationic complexes have been traced to an interplay between spin delocalization and spin polarization.

A coupled-perturbed Kohn−Sham treatment for the calculation of hyperfine tensors has been implemented into the MAG-ReSpect program. It treats spin−orbit contributions to hyperfine tensors by a combination of accurate and efficient approximations to the one- and two-electron spin−orbit Hamiltonians:  (a) by the all-electron atomic mean-field approximation, and (b) by spin−orbit pseudopotentials. In contrast to a previous implementation, the code allows the use of hybrid functionals and lifts restrictions in the orbital and auxiliary basis sets that may be employed. Validation calculations have been performed on various transition metal complexes, as well as on a series of small diatomic molecules. In the case of a series of copper(II) complexes, the spin−orbit contributions are large, and their inclusion is essential to achieve agreement with experiment. Calculations with spin−orbit pseudopotentials allow the efficient simultaneous introduction of scalar relativistic and spin−orbit effects in the case of light nuclei in the neighborhood of heavy atoms.