A homologous series of regioselectively tetradeprotonated group 8 metallocenes: new inverse crown ring compounds synthesized via a mixed sodium-magnesium tris(diisopropylamide) synergic base

Subjecting ferrocene, ruthenocene, or osmocene to the synergic amide base sodium-magnesium tris(diisopropylamido) affords a unique homologous series of metallocene derivatives of general formula [(M(C(5)H(3))(2))Na(4)Mg(4)(i-Pr(2)N)(8)] (where M = Fe (1), Ru (2), or Os (3)). X-ray crystallographic studies of 1-3 reveal a common molecular "inverse crown" structure comprising a 16-membered [(NaNMgN)(4)](4+) "host" ring and a metallocenetetraide [M(C(5)H(3))(2)](4-) "guest" core, the cleaved protons of which are lost selectively from the 1, 1', 3, and 3'-positions. Variable-temperature NMR spectroscopic studies indicate that 1, 2, and 3 each exist as two distinct interconverting conformers in arene solution, the rates of exchange of which have been calculated using coalescence and EXSY NMR measurements.     

Photoproduction of the omega meson on the proton at large momentum transfer

The differential cross section, dsigma/dt, for omega meson exclusive photoproduction on the proton above the resonance region (2.6相似文献   

New measurements of G using the measurement standards laboratory torsion balance

This Letter presents the results of a series of measurements of the Newtonian gravitational constant G using the compensated torsion balance developed at the Measurement Standards Laboratory. Since our last published result using the torsion balance in the compensated mode of operation [Meas. Sci. Technol. 10, 439 (1999)]], several improvements have been made to reduce the uncertainty in the final result. The new measurements have used both stainless steel and copper large masses. The values of G for the two sets of masses are in good agreement. After combining all of the measurements we get a value of G=6.673 87(0.000 27) x 10(-11) m3 kg(-1) s(-2). This new value is 5 parts in 10(5) smaller than our previous published values.     

Regularized resolvent transform for direct calculation of 45 degrees projections of 2D J spectra

The regularized resolvent transform (RRT) has been applied in a novel way to J-resolved spectra. This involves the direct calculation of the 45 degrees projection without constructing the 2D spectrum. The results show a significant resolution enhancement over that obtained by the 45 degrees projection of a 2D Fourier spectrum, even for much larger signals. In particular, RRT is able to resolve peaks that belong to different overlapping multiplets in a very crowded spectral region, where the conventional technique fails for any signal size. The resolving power of this method along with the significantly shorter signals required, make this method a powerful tool in spectral assignment.     

Instantaneous, stoichiometric generation of powerfully reducing states of protein active sites using Eu(II) and polyaminocarboxylate ligands

Addition of an equivalent of a polyaminocarboxylate ligand (L) to a solution of a redox protein and the aqua Eu2+ ion results in the instantaneous in situ generation of a very powerful reductant Eu(II)-L that can rapidly drive an electron stoichiometrically onto a redox centre having an extremely negative reduction potential (lower than -1 V): this is exemplified by straightforward generation of the super-reduced state of the Fe-protein of nitrogenase.     

A new class of oxo-bridged high-valent hexamanganese clusters supported by sterically hindered carboxylate ligands

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel (Mn6O8)6+ core, [MnIV(4)MnIII2(mu-O)4(mu3-O)4(dmb)6(O2CR)2]4+ (where dmb=4,4'-dimethyl-2,2'-bipyridine, and RCO2=2,6-di(p-tolyl)benzoate (Ar(Tol)CO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2=Ar(Tol)CO2- (1) or Ar(4-tBuPh)CO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn2(mu-O)2(dmb)4)](3+/4+). The compound [MnIV4MnIII2(mu-O)4(mu3-O)4(dmb)6(Ar(Tol)CO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a=15.447(1) A, b=15.077(2) A, c=27.703(2) A, beta=91.68(2) degrees, V=6449.3(6) A3, and Z=2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu3, and "T-shaped" mu3. Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic ST=0 ground spin state with a large number of low-lying, thermally accessible states with ST>0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.     

Studying the Complexity of Global Verification for NP-Hard Discrete Optimization Problems

This paper examines the complexity of global verification for MAX-SAT, MAX-k-SAT (for k3), Vertex Cover, and Traveling Salesman Problem. These results are obtained by adaptations of the transformations that prove such problems to be NP-complete. The class of problems PGS is defined to be those discrete optimization problems for which there exists a polynomial time algorithm such that given any solution , either a solution can be found with a better objective function value or it can be concluded that no such solution exists and is a global optimum. This paper demonstrates that if any one of MAX-SAT, MAX-k-SAT (for k3), Vertex Cover, or Traveling Salesman Problem are in PGS, then P=NP.     

嵌入式POL DC/DC转换器设计

大多数嵌入式系统都是通过48V背板供电的。这个电压通常要降为更低的12V或5V中间电压,以向系统内的电路板供电。然而,在这些电路板上,大多数电路都要求供电电压范围在0.8V～3.3V,电流范围为几十mA至几十A。因此,需要负载点(POL)DC/DC转换器对12V或5V降压,以获得所需的电压和电流值。     

半导体绿色节能技术趋势1你的产品有多"绿"?

引言 我们来看一下支撑互联网运行、传送成百上千万份YouTube视频、保持经济运转的大功率计算机服务器的功耗,从2000-2005年,服务器使用的电能翻了一番,到2010年可能再增长75%.换一个角度来看,在2005年,保持Google、微软、雅虎等互联网巨擘的数据中心运转所需的电量相当于14个1000 MW的发电厂发出的电量.