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1.
ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-? part is relaxed to the ?Mn(III)4?…?O(5)- structure and the cation radical [O(6)=Mn(V)1]+ ? part is relaxed to the +O(6)–Mn(IV)1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism. 相似文献
2.
Hiroaki Miyagawa Michael J. Rich Lawrence T. Drzal 《Journal of Polymer Science.Polymer Physics》2004,42(23):4391-4400
The thermophysical and mechanical properties of a nanocomposite material composed of amine‐cured diglycidyl ether of bisphenol A (DGEBA) reinforced with organomontmorillonite clay are reported. The storage modulus at 100 °C, which was above the glass‐transition temperature (Tg), increased approximately 350% with the addition of 10 wt % (6.0 vol %) of clay. Below the Tg, the storage modulus at 30 °C increased 50% relative to the value of unfilled epoxy. It was determined that the Tg linearly increased as a function of clay volume percent. The tensile modulus of epoxy at room temperature increased approximately 50% with the addition of 10 wt % of clay. The reinforcing effect of the organoclay nanoplatelets is discussed with respect to the Tandon–Weng and Halpin–Tsai models. A pseudoinclusion model is proposed to describe the behavior of randomly oriented, uniformly dispersed platelets in nanocomposite materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4391–4400, 2004 相似文献
3.
The possibility of radioisotope X-ray absorptiometry to determine the particle size of powder in conjunction with sedimentation was investigated. The experimental accuracy was primarily determined by Cow and X-ray intensity. where Co'=weight concentration of the particle in the suspension w'=(micron/rho)l/(mu/rho)s-rhol/rhos rho; density micron/rho; mass absorption coefficient, suffix l and s indicate dispersion and particle, respectively. The radiosiotopes, Fe-55, Pu-238 and Cd-109 have high w-values over the wide range of the atomic number. However, a source of high micron value such as Fe-55 is not suitable because the optimal X-ray transmission length, Lopt is decided by the expression, micronlLopt approximately 2/(1+C'ow') by using Cd-109 AgKX-ray source, the weight size distribution of particles from the heavy elements such as PbO2 to light elements such as Al2O3 or flyash was determined. 相似文献
4.
Toyoharu Kobayashi Masako Ishida Kazuhiro Imaida Hideki Abe Hisanaka Ito 《Tetrahedron letters》2017,58(33):3294-3295
The stereoselective total synthesis of pleurolactone has been accomplished in 7 steps. The key synthetic features were the construction of four contiguous stereocenters using an endo-selective Diels-Alder reaction and high diastereoselective dihydroxylation. 相似文献
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7.
The pressure-temperature phase diagram of the organic Mott insulator kappa-(ET)2Cu2(CN)3, a model system of the spin liquid on triangular lattice, has been investigated by 1H NMR and resistivity measurements. The spin-liquid phase is persistent before the Mott transition to the metal or superconducting phase under pressure. At the Mott transition, the spin fluctuations are rapidly suppressed and the Fermi-liquid features are observed in the temperature dependence of the spin-lattice relaxation rate and resistivity. The characteristic curvature of the Mott boundary in the phase diagram highlights a crucial effect of the spin frustration on the Mott transition. 相似文献
8.
Novel spirocyclic trichothecanes, spirotenuipesine A and B, isolated from entomopathogenic fungus, Paecilomyces tenuipes 总被引:8,自引:0,他引:8
Kikuchi H Miyagawa Y Sahashi Y Inatomi S Haganuma A Nakahata N Oshima Y 《The Journal of organic chemistry》2004,69(2):352-356
Entomopathogenic fungi forming fruiting bodies have been employed as tonics and antitussives from ancient times. Paecilomyces tenuipes, which is also called Isaria japonica, is a very popular entomopathogenic fungus and is often considered a health food in northeast Asian countries such as China, Korea, and Japan. We cultivated the fruiting bodies of Paecilomyces tenuipes. Among the large-scale cultivations, fruiting body grown in barley grain contained two novel spirocyclic trichothecane derivatives, spirotenuipesine A (1) and B (2), and known trichothecane mycotoxins. Compounds 1 and 2 showed potent activity in neurotrophic factor biosynthesis in glial cells. The isolation of these compounds indicated that P. tenuipes is a promising source for producing various biologically active substances including trichothecanes. It is noteworthy that trichothecane mycotoxins are present in Paecilomyces tenuipes, which is typically used in medicinal health food. 相似文献
9.
A finite element method for treating two-dimensional electron systems with Rashba spin–orbit interaction is developed. The Rashba spin–orbit interaction removes spin degeneracy, so that each spin contributes to the conductance differently. By accounting for the connection between a system and leads, this method yields the conductance of a nanoscale quantum device for each spin state. As an example, this calculation method is applied to a model of a quantum point contact. The results of this calculation indicate conductance quantization and a large spin polarization. We discuss the estimated accuracies of this calculation. 相似文献
10.
Nanocomposite materials consisting of an epoxy matrix and silicate clay particles have been processed and characterized mechanically.
The clay material used was a modified natural montmorillonite. The clay particles consisted of 1 nm thick layers with aspect
ratios in the range of 100–1000. The clay particles were mixed with acetone and sonicated, then mixed with the polymer, deaerated
and cured. The ultimate objective of processing was to produce a polymer/clay nanocomposite with separated (exfoliated) platelets,
dispersed as uniformly as possible. Samples were prepared with clay concentrations of up to 10 wt%. The process used resulted
in limited exfoliation but mostly intercalation, i.e., infusion of polymer between the silicate layers and increase of interlayer
spacing. The characteristics of the nanocomposite were assessed by transmission electron microscopy and x-ray diffraction.
Results from these observations show that the basal spacing of clay platelets increased from an initial pre-processing value
of 1.85 nm to 4.5 nm. Enhancement of mechanical properties was measured by tensile testing of coupons. Stiffness increases
of up to 50% over that of the unfilled epoxy were measured for clay concentrations of 5 wt%. Strength increases were also
measured for low clay concentrations and low strain rate loading. Micromechanics modeling of mechanical behavior is discussed
as a function of clay platelet dispersion. 相似文献