Structural characterisation ofp-semiquinone radical in a crystal: X-ray structure and EPR evidence

Abstract

The aim of this work is the study of weak intermolecular interactions in molecular crystals, by both diffraction methods and ab initio calculations using the new version of the CRYSTAL [1] program, which is designed to treat infinite periodic systems. Two molecular systems were chosen: i) formamide [2]: each molecule is linked to three other molecules by two types of hydrogen bonds; ii) boric acid [3]: each molecule forms six almost equivalent hydrogen bonds with three neighbouring molecules. The CRYSTAL03 calculations were carried out employing the HF and DFT (B3LYP) methods with the 6-31G(d,p) basis set, also using the new geometry optimisation options. The electron density, the electrostatic potential, the binding energies and the structure factors were derived. In formamide the cooperative effects of packing on the hydrogen bond interactions leading to the formation of linear chains and on those connecting the chains were analyzed. Hydrogen bonded molecules of boric acid form layers held together by weak B-O interactions, which may play a role in dictating the inter-layer packing. The comparison of the experimental and theoretical results and the most recent results obtained using the geometry optimisation facilities in CRYSTAL03 will be presented. [1] Saunders V.R., et al., CRYSTAL03, www.crystal.unito.it [2] Stevens E.D., Acta Cryst., 1978, B34, 544. [3] Gajhede M., Larsen S., Rettrup S., Acta Cryst., B42, 545, 1986. Keywords: hydrogen bond, weak interactions, ab-initio calculations P.08.08.16 Acta Cryst. (2005). A61, C332 Many properties of cytochromes and model systems depend on orientations of axial ligands. In this work we elucidated role of substituents of porphyrin on orientation of axial ligands in model systems of cytochromes. The orientations of axially coordinated imidozoles and pyridines in crystal structures of model systems of cytochromes were analyzed and data were compared with previous quantum-chemical calculations. The results show that eight ethyl groups on porphyrin ring strongly favor parallel orientation, hence, in all these complexes axial ligands, pyridines or imidazoles, are mutually parallel. Four phenyl or mesityl groups at meso-carbons also favor parallel orientation but less strong. Hence, in most of the bis-imidazole complexes orientation is parallel, while in bis-pyridine complexes orientation of pyridines depends on oxidation state of Fe. In Fe(II) complexes orientation is parallel, in Fe(III) it is orthogonal. In Fe(III) complexes influence of the interaction of pyridine with porphyrin ring, dominates over influence of phenyl or mesityl groups on porphyrin ring, and the orientation is parallel. Namely, by previous quantum chemical calculations it was shown that in bis-pyridine Fe(III) porphyrinato complexes without substituents orthogonal orientation is favored by 16 kcal/mol. By coordinating to metal atom with acetylacetonato ligand makes planar chelate ring, with delocalized bonds. Hence, coordinated acetylacetonato ligand could be involved in CH/ interactions with phenyl ...