Ligand polarization contributions to lanthanide 4f → 4f magnetic dipole transition moments and rotatory strengths

Radiation-induced (dynamic) polarizations of electronic charge distributions on ligands may contribute to both the electric- and magnetic-dipole moments of 4f → 4f transitions in lanthanide complexes. These ligand-polarization effects on magnetic-dipole transition moments are examined in the present study, and their possible influence on the rotatory strengths of 4f → 4f transitions in chiral lanthanide systems is given special attention. The ligand-polarization mechanism for magnetic-dipole transition moments is not likely to contribute significantly to the total dipole strengths of 4f → 4f transitions in noncentrosymmetric lanthanide systems. However, this mechanism may contribute significantly (or detectably) to the rotatory strengths of certain 4f → 4f transitions in chiral lanthanide systems. The relative importance of these contributions will be greatest for transitions with predominantly |ΔJ| ≥ 2 character and with relatively large U ^λ(λ = 2, 4, and 6) matrix elements.     

Chiroptical spectra of molecules with nearly degenerate electronic states

The influence of vibronic interactions on the chiroptical spectra associated with pairs of nearly degenerate electronic transitions in chiral systems is examined on a formal theoretical model. We consider the special case in which two nearly degenerate electronic states are coupled by a single non-totally symmetric vibrational mode. Formal expressions are developed for the rotatory strengths of individual vibronic transitions in this coupled system. Calculations based on these expressions are carried out for a large number of parameter sets appropriate for various energy spacings between the unperturbed electronic states, vibronic coupling strengths, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. The calculated results demonstrate the profound influence of vibronic interactions on the sign patterns and intensity distributions within the rotatory strength spectrum associated with the two coupled electronic states. The implications of these results for interpretations of circular dichroism spectra are discussed.     

Internal stresses in cold-deformed Cu–Ag and Cu–Nb wires

The co-deformation of Cu–Ag or Cu–Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic–plastic response, which is related to both magnet design and service life. The second derivative ?² σ/?² ε of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. The internal stresses in Cu–Nb are higher than in Cu–Ag and, consequently, the absolute values of (?² σ/?² ε)_Cu–Nb are higher than those of (?² σ/?² ε)_Cu–Ag at low strains.     

Thermal expansion and atomic vibrations of zirconium carbide to 1600 K

We have measured the lattice constants and Debye–Waller factors of ZrC from 12 to 1600?K using neutron diffraction. The data have been analyzed with the Grüneisen equation of state using an existing procedure for the Debye–Waller factors and a new procedure for the CTE that takes the temperature dependence of the elastic stiffness into account. The results of these measurements are in good agreement with previous measurements and with recent calculations. We have used the results of our measurements to estimate the melting point of ZrC with the Lindemann rule and obtain an estimate of 4000?K, in good agreement with the measured melting point of 3700?K of this highly refractory material. This last result demonstrates the importance of anharmonic effects in determining the melting point.     

Non-Gaussian diffusion MRI assessment of brain microstructure in mild cognitive impairment and Alzheimer’s disease

We report the first application of a novel diffusion-based MRI method, called diffusional kurtosis imaging (DKI), to investigate changes in brain tissue microstructure in patients with mild cognitive impairment (MCI) and AD and in cognitively intact controls. The subject groups were characterized and compared in terms of DKI-derived metrics for selected brain regions using analysis of covariance with a Tukey multiple comparison correction. Receiver operating characteristic (ROC) and binary logistic regression analyses were used to assess the utility of regional diffusion measures, alone and in combination, to discriminate each pair of subject groups. ROC analyses identified mean and radial kurtoses in the anterior corona radiata as the best individual discriminators of MCI from controls, with the measures having an area under the ROC curve (AUC) of 0.80 and 0.82, respectively. The next best discriminators of MCI from controls were diffusivity and kurtosis (both mean and radial) in the prefrontal white matter (WM), with each measure having an AUC between 0.77 and 0.79. Finally, the axial diffusivity in the hippocampus was the best overall discriminator of MCI from AD, having an AUC of 0.90. These preliminary results suggest that non-Gaussian diffusion MRI may be beneficial in the assessment of microstructural tissue damage at the early stage of MCI and may be useful in developing biomarkers for the clinical staging of AD.     

Zur Bestimmung der Minerals?uren im Essig

Ohne Zusammenfassung     

Untersuchung der Milch

Ohne Zusammenfassung     

Asymmetric Ring Opening in a Tetrazine-Based Ligand Affords a Tetranuclear Opto-Magnetic Ytterbium Complex

We report the formation of a tetranuclear lanthanide cluster, [Yb₄(bpzch)₂(fod)₁₀] ( 1 ), which occurs from a serendipitous ring opening of the functionalised tetrazine bridging ligand, bpztz (3,6-dipyrazin-2-yl-1,2,4,5-tetrazine) upon reacting with Yb(fod)₃ (fod⁻=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octandionate). Compound 1 was structurally elucidated via single-crystal X-ray crystallography and subsequently magnetically and spectroscopically characterised to analyse its magnetisation dynamics and its luminescence behaviour. Computational studies validate the observed M_J energy levels attained by spectroscopy and provides a clearer picture of the slow relaxation of the magnetisation dynamics and relaxation pathways. These studies demonstrate that 1 acts as a single-molecule magnet (SMM) under an applied magnetic field in which the relaxation occurs via a combination of Raman, direct, and quantum tunnelling processes, a behaviour further rationalised analysing the luminescent properties. This marks the first lanthanide-containing molecule that forms by means of an asymmetric tetrazine decomposition.