From an Icosahedron to a Plane: Flattening Dodecaiodo‐dodecaborate by Successive Stripping of Iodine

It has been shown by electrospray ionization–ion‐trap mass spectrometry that B₁₂I₁₂^2? converts to an intact B₁₂ cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B₁₂I_n^? species (n=11 to 1) determined by means of first‐principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B₁₂I₁₂^2?, and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B₁₂I₁₂^2? through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B₁₂ unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral.     

2-Halopyridines in the triple reaction in the Pn/KOH/DMSO system to form tri(2-pyridyl)phosphine: Experimental and quantum-chemical dissimilarities

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Magnesium Hexafluoridozirconates MgZrF6·5H2O,MgZrF6·2H2O,and MgZrF6: Structures,Phase Transitions,and Internal Mobility of Water Molecules

The MgZrF₆ · n H₂O (n = 5, 2 and 0) compounds were studied by the methods of X‐ray diffraction and ¹⁹F, MAS ¹⁹F, and ¹H NMR spectroscopy. At room temperature, the compound MgZrF₆ · 5H₂O has a monoclinic C‐centered unit cell and is composed of isolated chains of edge‐sharing ZrF₈ dodecahedra reinforced with MgF₂(H₂O)₄ octahedra and uncoordinated H₂O molecules and characterized by a disordered system of hydrogen bonds. In the temperature range 259 to 255 K, a reversible monoclinic ? two‐domain triclinic phase transition is observed. The phase transition is accompanied with ordering of hydrogen atoms positions and the system of hydrogen bonds. The structure of MgZrF₆ · 2H₂O comprises a three‐dimensional framework consisting of chains of edge‐sharing ZrF₈ dodecahedra linked to each other through MgF₄(H₂O)₂ octahedra. The compound MgZrF₆ belongs to the NaSbF₆ type and is built from regular ZrF₆ and MgF₆ octahedra linked into a three‐dimensional framework through linear Zr–F–Mg bridges. The peaks in ¹⁹F MAS spectra were attributed to the fluorine structural positions. The motions of structural water molecules were studied by variable‐temperature ¹H NMR spectroscopy.     

Photodynamics of the bacteriochlorophyll-carotenoid system. 2. Influence of central metal,solvent and beta-carotene on photobleaching of bacteriochlorophyll derivatives

Bacteriochlorophyll (BChl) derivatives (with central Mg replaced by metal "M") ([M]-BChl with M = 2H, Mg, Zn, Pd, Cu) have been investigated for their photodynamic capacity and stability toward photodegradation in organic solvents and aqueous micellar solution. A protocol has been developed for screening new sensitizers. BChl and [Zn]-BChl are efficient sensitizers, but they are also quickly degraded by the reactive oxygen species (ROS) produced by autosensitization, as well as by hetero-sensitization with 17(4)-methyl-13(2)-demethoxycarbonyl-pheophorbide a (MPP). Photostable [Cu]-BChl is a poor sensitizer, whereas [Pd]-BChl and bacteriopheophytin a are not only very efficient sensitizers but are also very stable toward ROS. beta-Carotene is no efficient physical quencher of ROS in the system; rather, it acts as a photochemical quencher that competes with [M]-BChl and undergoes photooxygenation at high rates. Photolability seems to depend on the pigment oxidation potential and, in parallel, on the presence of central metals preferring coordination numbers higher than 4, whereas photodynamic capacity depends on long excited state life-times of the pigment or efficient intersystem crossing (or both).     

Unfolding Features of Bovine Testicular Hyaluronidase Studied by Fluorescence Spectroscopy and Fourier Transformed Infrared Spectroscopy

Chemical unfolding of bovine testicular hyaluronidase (HAase) has been studied by fluorescence spectroscopy and Fourier transformed infrared spectroscopy (FTIR). Thermodynamic parameters were determined for unfolding HAase from changes in the intrinsic fluorescence emission intensity and the formations of several possible unfolding intermediates have been identified. This was further confirmed by representation of fluorescence data in terms of ‘phase diagram’. The secondary structures of HAase have been assigned and semiquantitatively estimated from the FTIR. The occurrence of conformational change during chemical unfolding as judged by fluorescence and FTIR spectroscopy indicated that the unfolding of HAase may not follow the typical two-state model.