Semiquinone (p-benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90 K) and EPR data support a dynamical disorder of the proton. A low-barrier O-H...O hydrogen bond stabilizes the radical. The C-O bond length is 1.297 (4) A, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs.

A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples).

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 ...