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排序方式: 共有798条查询结果,搜索用时 15 毫秒
791.
Danyal Turkoglu Joshua Burke Radoslaw Lewandowski Lei R. Cao 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(2):321-327
A thermal neutron beam facility has been designed and implemented at the Ohio State University Research Reactor. A project is underway to construct a large vacuum chamber such that the facility could have neutron depth profiling and neutron radiography capabilities as intended. The neutron beam is extracted from the reactor through a neutron collimator emplaced in Beam Port #2. The neutron spectrum entering the neutron collimator was unfolded from foil activation analysis results and also simulated with a full reactor core model in the MCNP Monte Carlo code. The neutron collimator uses polycrystalline bismuth as a gamma ray filter and single-crystal sapphire as a fast neutron filter. The beam is defined by multiple 3.0 cm diameter apertures made of borated aluminum. Characterization of the beam was performed using foil activation to find the flux and a low-budget neutron imaging apparatus to see the beam profile. The modulation transfer function was calculated to offer insight into the resolution of the imaging system and the collimation of the beam. The neutron collimator delivers the filtered thermal neutron beam with a ~4 cm diameter and a thermal equivalent flux of (1.27 ± 0.03) × 107 n/(cm2s) at 450 kW power at the end of the collimator. 相似文献
792.
793.
Within the adiabatic approximation, time-dependent density functional theory yields only single excitations. Near states of double excitation character, the exact exchange-correlation kernel has a strong dependence on frequency. We derive the exact frequency-dependent kernel when a double excitation mixes with a single excitation, well separated from the other excitations, in the limit that the electron--electron interaction is weak. Building on this, we construct a nonempirical approximation for the general case, and illustrate our results on a simple model. 相似文献
794.
Eileen G. Burke Prof. Jennifer M. Schomaker 《Angewandte Chemie (International ed. in English)》2015,54(41):12097-12101
Regioselectivity in the aziridination of silyl‐substituted homoallenic sulfamates is readily diverted to the distal double bond of the allene to yield endocyclic bicyclic methyleneaziridines with excellent stereocontrol. Subsequent reaction with electrophilic oxygen sources initiates facile rearrangement to densely functionalized, fused azetidin‐3‐ones in excellent d.r., effectively transferring the axial chirality of the allene to central chirality in the products. The steric nature of the silyl group dictates which of the two rings of the fused azetidin‐3‐one will undergo further functionalization, providing an additional element of diversity for the preparation of enantioenriched azetidine scaffolds with potential biological activity. 相似文献
795.
Dr. Lisa J. Enman Ashlee E. Vise Dr. Michaela Burke Stevens Prof. Shannon W. Boettcher 《Chemphyschem》2019,20(22):3089-3095
FeOxHy and Fe-containing Ni/Co oxyhydroxides are the most-active catalysts for the oxygen evolution reaction (OER) in alkaline media. However, the activity of Fe sites appears strongly dependent on the electrode-substrate material and/or the elemental composition of the matrix in which it is embedded. A fundamental understanding of these interactions that modulate the OER activity of FeOxHy is lacking. We report the use of cyclic voltammetry and chronopotentiometry to assess the substrate-dependent activity of FeOxHy on a number of commonly used electrode substrates, including Au, Pt, Pd, Cu, and C. We also evaluate the OER activity and Tafel behavior of these metallic substrates in 1 M KOH aqueous solution with Fe3+ and other electrolyte impurities. We find that the OER activity of FeOxHy varies by substrate in the order Au>Pd≈Pt≈Cu>C. The trend may be caused by differences in the adsorption strength of the Fe oxo ion on the substrate, where a stronger adhesion results in more adsorbed Fe at the interface during steady-state OER and possibly a decreased charge-transfer resistance at the FeOxHy-substrate interface. These results suggest that the local atomic and electronic structure of [FeO6] units play an important role in catalysis of the OER as the activity can be tuned substantially by substrate interactions. 相似文献
796.
Cooper TM Hall BC McLean DG Rogers JE Burke AR Turnbull K Weisner A Fratini A Liu Y Schanze KS 《The journal of physical chemistry. A》2005,109(6):999-1007
As part of an effort to develop a spectroscopic structure-property relationship in platinum acetylide oligomers, we have prepared a series of mesoionic bidentate Pt(PBu3)2L2 compounds containing sydnone groups. The ligand is the series o-Syd-(C6H4-C[triple bond]C)n-H, where n = 1-3, designated as Syd-PEn-H. The terminal oligomer unit consists of a sydnone group ortho to the acetylene carbon. We synthesized the platinum complex (Syd-PEn-Pt), the unmodified ligands (PEn-H), and the unmodified platinum complexes (PEn-Pt). The compounds were characterized by various methods, including X-ray diffraction, 13C NMR, ground-state absorption, fluorescence, phosphorescence, and laser flash photolysis. From solving the structure of Syd-PE1-Pt, we find the angle between the sydnone group and the phenyl group is 45 degrees . By comparison of the 13C NMR spectra of the sydnone-containing ligands, the sydnone complexes with the corresponding unmodified ligands and complexes not containing the sydnone group, the sydnone group is shown to polarize the nearest acetylenes and have a charge-transfer interaction with the platinum center. Ground-state absorption spectra of the complexes in various solvents give evidence that the Syd-PE1-Pt complex has an excited state less polar than the ground state, while the PE1-Pt complex has an excited state more polar than the ground state. In all the higher complexes the excited state is more polar than the ground state. The phosphorescence spectrum of the Syd-PE1-Pt complex has an intense vibronic progression distinctly different from the PE1-Pt complex. The sydnone effect is small in Syd-PE2-Pt and negligible in Syd-PE3-Pt. From absorption and emission spectra, we measured the singlet-state energy E(S), the triplet-state energy E(T), and the singlet-triplet splitting Delta E(ST). By comparison with energies obtained from the unmodified complexes, attachment of the sydnone lowers E(S) by approximately 0.1 eV and raises E(T) by approximately 0.1 eV. As a result, the sydnone group lowers Delta E(ST) by approximately 0.2 eV. The trends suggest one of the triplet-state singly occupied molecular orbitals (SOMOs) is localized on the sydnone group, while the other SOMO resides on the rest of the ligand. 相似文献
797.
James E. Guillet Nicholas A. D. Burke Maria Nowakowska Howard Reese David M. Gravett 《Macromolecular Symposia》1995,98(1):53-72
Photozymes are novel polyelectrolytes made from mixtures of hydrophobic and hydrophilic comonomers. They often contain a relatively high concentration of bound chromophores as photon harvesting or antenna groups, which transport excitation energy to organic substrates located in the interior of the macromolecular coil in aqueous solution. Previous studies have shown that the photocatalytic behavior of these polymers has many similarities to natural enzymes which carry out thermal reactions in biological systems. Mechanistic studies of four photozyme reactions are discussed: (1) the oxidation of perylene to perylenequinones; (2) the dehalogenation of hexachloro- and pentachlorobenzene; (3) the high-yield synthesis of Vitamin D; and (4) the selective photolysis of aliphatic and aromatic ketones. 相似文献
798.