Electronic structure of nickel(II) and zinc(II) borohydrides from spectroscopic measurements and computational modeling

The previously reported Ni(II) complex, Tp*Ni(κ(3)-BH(4)) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate anion), which has an S = 1 spin ground state, was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy as a solid powder at low temperature, by UV-vis-NIR spectroscopy in the solid state and in solution at room temperature, and by paramagnetic (11)B NMR. HFEPR provided its spin Hamiltonian parameters: D = 1.91(1) cm(-1), E = 0.285(8) cm(-1), g = [2.170(4), 2.161(3), 2.133(3)]. Similar, but not identical parameters were obtained for its borodeuteride analogue. The previously unreported complex, Tp*Zn(κ(2)-BH(4)), was prepared, and IR and NMR spectroscopy allowed its comparison with analogous closed shell borohydride complexes. Ligand-field theory was used to model the electronic transitions in the Ni(II) complex successfully, although it was less successful at reproducing the zero-field splitting (zfs) parameters. Advanced computational methods, both density functional theory (DFT) and ab initio wave function based approaches, were applied to these Tp*MBH(4) complexes to better understand the interaction between these metals and borohydride ion. DFT successfully reproduced bonding geometries and vibrational behavior of the complexes, although it was less successful for the spin Hamiltonian parameters of the open shell Ni(II) complex. These were instead best described using ab initio methods. The origin of the zfs in Tp*Ni(κ(3)-BH(4)) is described and shows that the relatively small magnitude of D results from several spin-orbit coupling (SOC) interactions of large magnitude, but with opposite sign. Spin-spin coupling (SSC) is also shown to be significant, a point that is not always appreciated in transition metal complexes. Overall, a picture of bonding and electronic structure in open and closed shell late transition metal borohydrides is provided, which has implications for the use of these complexes in catalysis and hydrogen storage.     

Constructing metrics on a 2-torus with a partially prescribed stable norm

A result of Bangert states that the stable norm associated to any Riemannian metric on the 2-torus T ² is strictly convex. We demonstrate that the space of stable norms associated to metrics on T ² forms a proper dense subset of the space of strictly convex norms on ${{\mathbb R}^2}$ . In particular, given a strictly convex norm || · ||_∞ on ${{\mathbb R}^2}$ we construct a sequence ${\langle {\| \cdot \|}_j \rangle_{j=1}^{\infty}}$ of stable norms that converge to || · ||_∞ in the topology of compact convergence and have the property that for each r > 0 there is an ${N \equiv N(r)}$ such that || · || _j agrees with || · ||_∞ on ${{\mathbb Z}^2 \cap \{(a,b) : a^2 + b^2 \leq r \}}$ for all j ≥ N. Using this result, we are able to derive results on multiplicities which arise in the minimum length spectrum of 2-tori and in the simple length spectrum of hyperbolic tori.     

LCZ696: a dual-acting sodium supramolecular complex

LCZ696 displays several surprising features, such as an intrinsic stabilization of the supramolecular complex by monovalent multi-coordinated sodium cations and water molecules. In addition, to the best of our knowledge, the compound is the first example of a dual-acting pharmaceutical built as a supramolecular complex delivering two pharmacologic effects—angiotensin receptor 1 (AT1) blockage and neprilysin (NEP) inhibition.     

Effect of off-frequency sampling in magnetic resonance elastography

In magnetic resonance elastography (MRE), shear waves at a certain frequency are encoded through bipolar gradients that switch polarity at a controlled encoding frequency and are offset in time to capture wave propagation using a controlled sampling frequency. In brain MRE, there is a possibility that the mechanical actuation frequency is different from the vibration frequency, leading to a mismatch with encoding and sampling frequencies. This mismatch can occur in brain MRE from causes both extrinsic and intrinsic to the brain, such as scanner bed vibrations or active damping in the head. The purpose of this work was to investigate how frequency mismatch can affect MRE shear stiffness measurements. Experiments were performed on a dual-medium agarose gel phantom, and the results were compared with numerical simulations to quantify these effects. It is known that off-frequency encoding alone results in a scaling of wave amplitude, and it is shown here that off-frequency sampling can result in two main effects: (1) errors in the overall shear stiffness estimate of the material on the global scale and (2) local variations appearing as stiffer and softer structures in the material. For small differences in frequency, it was found that measured global stiffness of the brain could theoretically vary by up to 12.5% relative to actual stiffness with local variations of up to 3.7% of the mean stiffness. It was demonstrated that performing MRE experiments at a frequency other than that of tissue vibration can lead to artifacts in the MRE stiffness images, and this mismatch could explain some of the large-scale scatter of stiffness data or lack of repeatability reported in the brain MRE literature.     

Multi-kHz mixture fraction imaging in turbulent jets using planar Rayleigh scattering

In this study, we describe the development of two-dimensional, high repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent flows. In particular, we report what we believe to be the first sets of high-speed 2D Rayleigh scattering images in turbulent non-reacting jets, yielding temporally correlated image sequences of the instantaneous mixture fraction field. Results are presented for turbulent jets of propane issuing into a low-speed co-flow of air at jet-exit Reynolds numbers of 10,000, 15,000, and 30,000 at various axial positions downstream of the jet exit. The quantitative high-speed mixture fraction measurements are facilitated by the use of a calibrated, un-intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition rate pulse-burst laser system (PBLS) at Ohio State, which yields output energies of ∼200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The quality, accuracy, and resolution of the imaging system and the resulting image sets are assessed by (1) comparing the mean mixture fraction results to known scaling laws for turbulent jets, (2) comparing instantaneous images/mixture fraction profiles acquired simultaneously with the high-speed CMOS camera and a well-characterized, high-quantum efficiency CCD camera, and (3) comparing statistical quantities such as the probability density function of the mixture fraction results using the high-speed CMOS camera and the CCD camera. Results indicate accurate mixture fraction measurements and a high potential for accurately measuring mixture fraction gradients in both time and space.