Infrared spectrum and bonding in uranium methylidene dihydride, CH2=UH2

Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U+CH4 reaction products are identified through isotopic substitution (13CH4, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C1 ground-state structure for the triplet CH2=UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2=UH2 complex is distorted more than CH2=ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes.     

Basis- and partition identification for quadratic programming and linear complementarity problems

Optimal solutions of interior point algorithms for linear and quadratic programming and linear complementarity problems provide maximally complementary solutions. Maximally complementary solutions can be characterized by optimal partitions. On the other hand, the solutions provided by simplex–based pivot algorithms are given in terms of complementary bases. A basis identification algorithm is an algorithm which generates a complementary basis, starting from any complementary solution. A partition identification algorithm is an algorithm which generates a maximally complementary solution (and its corresponding partition), starting from any complementary solution. In linear programming such algorithms were respectively proposed by Megiddo in 1991 and Balinski and Tucker in 1969. In this paper we will present identification algorithms for quadratic programming and linear complementarity problems with sufficient matrices. The presented algorithms are based on the principal pivot transform and the orthogonality property of basis tableaus. Received April 9, 1996 / Revised version received April 27, 1998? Published online May 12, 1999     

Abläufe der Ammonolysen der Ammoniumhexafluorometallate des Aluminiums,Galliums und Indiums, (NH4)3MF6 (M = Al,Ga, In)

The Courses of the Ammonolyses of the Ammonium Hexafluorometalates of Aluminum, Gallium, and Indium, (NH₄)₃MF₆ (M = Al, Ga, In) The courses of the ammonolysis reactions of the ammonium hexafluorometalates (NH₄)₃MF₆ (M = Al, Ga, In) were investigated with the aid of in‐situ powder diffractometry and differential thermal analysis. Under these conditions, the reaction of (NH₄)₃AlF₆ with gaseous ammonia yields at about 360 °C AlF₃ via the intermediates NH₄AlF₄, Al(NH₃)₂F₃ and Al(NH₃)F₃. The ammonolysis of (NH₄)₃GaF₆ produces GaN at about 400 °C. Depending upon the actual reaction conditions, the intermediates NH₄GaF₄ and Ga(NH₃)F₃ as well as their ammonia adducts NH₄GaF₄ · NH₃ and Ga(NH₃)₂F₃ and the amide‐ammoniate Ga(NH₃)(NH₂)F₂ are observed. In the case of (NH₄)₃InF₆ the intermediates (NH₄)₃InF₆ · NH₃ and In(NH₃)F₃ may exist; there are also indications for the reduction of In(III) to In(I) and for the existence of In(NH₃)₂F and InF as products of the ammonolysis of (NH₄)₃InF₆.     

A new kernel function yielding the best known iteration bounds for primal-dual interior-point algorithms

Kernel functions play an important role in defining new search directions for primal-dual interior-point algorithm for solving linear optimization problems. In this paper we present a new kernel function which yields an algorithm with the best known complexity bound for both large- and small-update methods.     

A comparison study of multishot vs. single-shot DWI-EPI in the neonatal brain: reduced effects of ghosting compared to adults

In the neonatal brain, it is important to use a fast imaging technique to acquire all diffusion weighted images (DWI) for apparent diffusion coefficient (ADC) calculation. Taking into account the occurrence of typical echo planar imaging (EPI) artifacts, we have investigated whether single-shot (SSh) or multishot (MSh) DWI-EPI should be preferred. In 14 neonates, 17 adult patients and 5 adult volunteers, DWIs are obtained both with SSh and MSh EPI. The occurrence of artifacts and their influence on the ADC are explored and further quantified using simulations and phantom studies. Two radiologists scored overall image quality and diagnosability of all images. Single-shot and MSh DWI-EPI scored equally well in neonates with respect to overall image quality and diagnosability. In newborns, more motion artifacts in MSh can be noticed while N/2-ghost artifacts in SSh occur less frequently than in adults. Both N/2-ghost and motion artifacts result in significant ADC abnormalities. There is a serious risk that these artifacts will be mistaken for genuine diffusion abnormalities. N/2-ghost artifacts are hardly noticed in the neonatal brain, which might be due to smaller cerebrospinal fluid (CSF) velocity than in adults. Apparent diffusion coefficient values in MSh are unreliable if motion occurs. We conclude that for ADC calculations in neonates SSh DWI-EPI is more reliable than MSh.     

Measurement of the parity-violating asymmetry in inclusive electroproduction of π- near the Δ0 resonance

The parity-violating (PV) asymmetry of inclusive π- production in electron scattering from a liquid deuterium target was measured at backward angles. The measurement was conducted as a part of the G0 experiment, at a beam energy of 360 MeV. The physics process dominating pion production for these kinematics is quasifree photoproduction off the neutron via the Δ0 resonance. In the context of heavy-baryon chiral perturbation theory, this asymmetry is related to a low-energy constant d(Δ)- that characterizes the parity-violating γNΔ coupling. Zhu et al. calculated d(Δ)- in a model benchmarked by the large asymmetries seen in hyperon weak radiative decays, and predicted potentially large asymmetries for this process, ranging from A(γ)-=-5.2 to +5.2 ppm. The measurement performed in this work leads to A(γ)-=-0.36±1.06±0.37±0.03 ppm (where sources of statistical, systematic and theoretical uncertainties are included), which would disfavor enchancements considered by Zhu et al. proportional to V(ud)/V(us). The measurement is part of a program of inelastic scattering measurements that were conducted by the G0 experiment, seeking to determine the N-Δ axial transition form factors using PV electron scattering.     

Ring‐Extended Gramicidin S Analogs Containing cis δ‐Sugar Amino Acid Turn Mimetics with Varying Ring Size

This article presents a series of ring‐extended gramicidin S derivatives, 9 – 14 , that have four ornithine residues as polar protonated side chains and one modified turn region containing a mono‐functionalized cis‐δ‐oxetane, δ‐furanoid, or δ‐pyranoid sugar amino acid residue. Of the GS analogs evaluated, we identified compound 7 , which contains the mono‐benzyloxy cis‐δ‐pyranoid sugar amino acid, as having a better biological profile than the clinically applied topical antibiotic gramicidin S.     

Plutonium Isotopes in Sediments from the Sudanese Coast of the Red Sea

This paper presents the activity concentrations of Pu isotopes in surface marine sediments collected from the Sudanese coast of the Red Sea. The following concentration ranges were determined: ²³⁸Pu, 4.7–28.6 mBq/kg; ^239+240Pu, 53–343 mBq/kg dry weight. The average activity ratios of ²³⁸Pu/^239+240Pu (0.075±0.045 mBqk/kg) and ^239+240Pu/¹³⁷Cs (0.026±0.025 mBq/kg) are appropriately comparable to the literature values that are characteristic of the global fallout from the atmospheric nuclear weapon tests. However, ^239+240Pu/¹³⁷Cs ratio in sediments collected from the biologically rich fringing reef is an order of magnitude higher compared to other sampling locations.     

A theoretical study of the copper–cysteine bond in blue copper proteins

 The accuracy of theoretical calculations on models of the blue copper proteins is investigated using density functional theory (DFT) Becke's three-parameter hybrid method with the Lee–Yang–Parr correlation functional (B3LYP) and medium-sized basis sets. Increasing the basis set to triple-zeta quality with f-type functions on all heavy atoms and enlarging the model [up to Cu(imidazole-CH₃)₂(SC₂H₅) (CH₃SC₂H₅)^0/+] has only a limited influence on geometries and relative energies. Comparative calculations with more accurate wave-function–based methods (second-order M?ller–Plesset perturbation theory, complete-active-space second-order perturbation theory, coupled-cluster method, including single and double replacement amplitudes and in addition triple replacement perturbatively) and a variety of basis sets on smaller models indicate that the DFT/B3LYP approach gives reliable results with only a small basis set dependence, whereas the former methods strongly depend on the size of the basis sets. The effect of performing the geometry optimizations in a continuum solvent is quite small, except for the flexible Cu-S_Met bond. The results of this study confirm the earlier results that neither the oxidized nor the reduced copper site in the blue proteins is strained to any significant degree (in energy terms) by the protein surrounding. Received: 7 July 2000 / Accepted: 17 November 2000 / Published online: 21 March 2001