Structural and density functional studies of uranium(III) and lanthanum(III) complexes with a neutral tripodal N-donor ligand suggesting the presence of a U-N back-bonding interaction

The crystal structures of two trisiodide octacoordinated uranium(III) complexes of tris[(2-pyrazinyl)methyl]amine (tpza), which differ only by the ligand occupying the eighth coordination site (thf or MeCN), and of their lanthanum(III) analogues have been determined. In the acetonitrile adducts the M-N(pyrazine) distances are very similar for U(III) and La(III), while the U-N(acetonitrile) distance is 0.05 A shorter than the La-N(acetonitrile) distance. In the [M(tpza)I(3)(thf)] complexes in which the monodentate acetonitrile ligand, a weak pi-acceptor ligand, is replaced by a thf molecule, a sigma-donor only, the mean value of the distance U-N(pyrazine) is 0.05 A shorter than the mean value of the La-N(pyrazine) distance. Since we are comparing isostructural compounds of ions with very similar ionic radii, these differences indicate the presence of a stronger M-N interaction in the U(III) complexes and therefore suggest the presence of a covalent contribution to the U-N bonding. The selectivity of the tpza ligand toward U(III) complexation (with respect to that of La(III)) in the presence of sigma-donor-only ligands has been quantified by the value of K(U(tpza))/K(La(tpza)) measured to be 3.3 +/- 0.5. The analysis of the metal-N-donor ligand bonding was carried out by a quasi-relativistic density functional theory study on small model compounds, of formula I(3)M-L (M = La, Nd, U; L = acetonitrile, pyrazine) and I(3)M-(pyrazine)(3) (M = La, U). The structural data obtained from geometry optimizations on these systems reproduce experimental trends, i.e., a decrease in the M-N distance from La to U, combined with an increase of the C-N distance in the acetonitrile derivatives. A detailed orbital analysis carried out on the resulting optimized complexes did not reveal any orbital interaction between the trivalent lanthanide cations (Ln(3+)) and the N-donor ligands. In contrast, a back-donation electron transfer from 5f U(3+) orbitals to the pi* virtual orbital of the ligand was observed for both acetonitrile and pyrazine. Evaluation of the total bonding energy between the MI(3) and L fragments shows that this orbital interaction leads to a stabilization of the uranium(III) system compared to the lanthanide species.     

Catalytic aziridination of olefins and amidation of thioanisole by a non-heme iron complex

A non-heme iron complex catalyses the aziridination of various olefins and the amidation of thioanisole in good yields at the expense of an aryl iodinane.     

Study of ferroelectric transitions of vinylidene fluoride–trifluoroethylene copolymers

Thermally stimulated depolarization (TSD) measurements with superimposed “piezoelectric noise” allow us to identify the ferroelectric-to-paraelectric phase transition in vinylidene fluoride–trifluoroethylene (VDF/TrFE) copolymers with different molar ratios. In addition, a new transition which did not change the ferroelectric features of the material has also been detected. Long relaxation processes always accompany these transitions. By the initial-rise method we calculated the activation energies for these relaxations as 0.73 eV for the ferroelectric-to-paraelectric transition and 0.33 eV for the ferroelectric-to-ferroelectric one.     

Search for isovector giant monopole resonances via the Pb(3He,tp) reaction

The (nat)Pb(3He,tp) reaction at E(3He) = 177 MeV was studied to identify 2Planck's over 2piomega isovector monopole strength in Bi isotopes. Monopole strength was found in the region -45相似文献   

Analogy between unconventional superconductivity and unconventional states of liquid crystals

An analogy is stressed between the order-parameter symmetries of the two-dimensional d-pairing wave superconductors and of liquid-crystal mesophases formed from achiral bent-shaped molecules. It leads to a definition of a class of liquid-crystal states which are the analogs of the unconventional superconducting states, and are characterized by a loss of discrete symmetry operations of the parent state.     

Creatine kinase B deficient neurons exhibit an increased fraction of motile mitochondria

Background  

Neurons require an elaborate system of intracellular transport to distribute cargo throughout axonal and dendritic projections. Active anterograde and retrograde transport of mitochondria serves in local energy distribution, but at the same time also requires input of ATP. Here we studied whether brain-type creatine kinase (CK-B), a key enzyme for high-energy phosphoryl transfer between ATP and CrP in brain, has an intermediary role in the reciprocal coordination between mitochondrial motility and energy distribution. Therefore, we analysed the impact of brain-type creatine kinase (CK-B) deficiency on transport activity and velocity of mitochondria in primary murine neurons and made a comparison to the fate of amyloid precursor protein (APP) cargo in these cells, using live cell imaging.     

Critical petermann K factor for intensity noise squeezing

We investigate the impact of the Petermann-excess-noise factor K>/=1 on the possibility of intensity noise squeezing of laser light below the standard quantum limit. Using an N-mode model, we show that squeezing is limited to a floor level of 2(K-1) times the shot noise limit. Thus, even a modest Petermann factor significantly impedes squeezing, which becomes impossible when K>/=1.5. This appears as a serious limitation for obtaining sub-shot-noise light from practical semiconductor lasers. We present experimental evidence for our theory.