Carbon-13 solid-state NMR is used to determine, via the temperature dependence of paramagnetic shifts, the spin ladder of magnetic levels and the related spin exchange parameters in [4Fe−4S]2+ clusters. This study has been carried out between 180 and 330 K in three model compounds of the active sites of 4Fe−4S proteins:  [(C2D5)4N]2[Fe4S4(S13CD2C6D5)4], [(C2H5)4N]2[Fe4S4(SC(CH3)3)4], and [(C2H5)4N]2[Fe4Se4(SC(CH3)3)4]. The temperature dependencies of both the isotropic and anisotropic parts of the 13C paramagnetic shift tensors have been measured and analyzed. Antiferromagnetic exchange coupling constants among iron atoms of J ≈ 430, 380, and 370 cm-1 have been deduced from fits of their temperature dependencies. Additional contributions of the double-exchange term B favoring electronic delocalization and of a ΔJ term associated with the compression of the cubane geometry are also discussed. We also show that connections between these measurements and those of magnetic susceptibility are especially close in the case of the [4Fe−4S]2+ redox state.

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The Joint Research Centre, in cooperation with the Commissariat à l’Energie Atomique et aux Energies Alternatives, produced a novel 243Am spike reference material for mass spectrometry. Americium solution with an isotopic composition of 88% 243Am and 12% 241Am was used as the source for the preparation of the spike material. The certified value of 5.696 (11) nmol g−1 for the amount content of 243Am and 0.136138 (54) for the n(241Am)/n(243Am) amount ratio were assigned. The assigned values from mass spectrometry were confirmed by alpha-particle spectrometry, alpha-particle counting at a defined solid angle, and high-resolution gamma-ray spectrometry. Furthermore, an external validation of the certified values was obtained from the results of an interlaboratory comparison exercise, using this americium reference solution as the test sample.

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The photoinduced addition of 0, onto the unsaturated cluster Pd,(dppm),C~~+ (as a CF,CO,-salt) in acetonitrile is reported. The final product ~d , ( d~~m ) , ( O , )~ (~( 0 , ) = 838 and 866 cm-') is formed in a multistep fashion in which two photochemical intermediates are observed (presumably ~d,(dppm),(0,)(~0)~+ and ~d , ( d~~m ) , ( 0 , )~+ ) .No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that 0, binds the Pd, center as a peroxo-02, and acts as a two-electron donor that triply bridges the metal atoms (forming a 44-electron cluster). The very small excited state lifetimes (between 25 and 35 ? 10 ps) obtained by picosecond flash photolysis indicate that the primary photoreaction is unimolecular, and demonstrate that the first photochemically added 0, molecule must be preassembled in the excited state prior to any photoinduced transformation. This ~d , ( d~~m ) , C 0~+ . . . 0 , ground state complex is responsible for the photoinduced production of the bisdioxygen compound and can be observed by UV-visible spectroscopy. The low efficiency of the photoreaction (quantum yield (0) = 0.033 ? 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of 0, with solvent molecules to occupy the unsaturated site of the empty cavity in ~d , ( d~~m ) ,~0~+ (i.e., ground state guest-host chemistry). The binding constant for 0, with ~d , ( d~~m ) ,~0~+ is roughly estimated to range between 1 and 730 M-I in the ground state and is considered to be weak.