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21.
We demonstrate that the giant spin contrast observed by scanning tunneling microscopy for double-layer Coislands on Pt(111) is caused by adsorbates at the apex of the Cr-coated W tip. The most likely candidate, in ab initio simulations, is hydrogen. Here, the electron charge is highly polarized by the adjacent Cr layers. The hydrogen adsorption site is shown to change from hollow to on top due to the electric field at the tip apex, created by the tunnel voltage. 相似文献
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Tóth G Kordás K Vähäkangas J Uusimäki A George TF Nanai L 《The journal of physical chemistry. B》2005,109(15):6925-6928
Gold micropatterns are deposited from aqueous solutions of NaAuCl(4) on boron-doped Si(100) surfaces (rho = 1.5 x 10(-4) Omega m) using a focused Ar(+) laser beam (TEM(00), lambda = 488 nm, w(0) = 1.5 mum, P = 20-80 mW). The finite-element method employed for computing the surface temperature profiles reveals that the maximum temperature at the precursor/silicon interface increases only to the range 316-372 K, which is not high enough for chemical reactions with formaldehyde in the precursor. This suggests a different mechanism to be responsible for the reduction of gold ions, namely, changes in the surface potential of Si caused by the Dember and Seebeck effects. 相似文献
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Dynamic kinetic resolution of allylic alcohols to allylic acetates followed by copper-catalyzed allylic substitution gave alkenes in high yields and high optical purity. Subsequent oxidative C-C double bond cleavage afforded pharmaceutically important alpha-methyl substituted carboxylic acids in high ee. 相似文献
25.
Approaching the Kinetic Inertness of Macrocyclic Gadolinium(III)‐Based MRI Contrast Agents with Highly Rigid Open‐Chain Derivatives
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Dr. Gyula Tircsó Dr. Martín Regueiro‐Figueroa Viktória Nagy Zoltán Garda Tamás Garai Dr. Ferenc Krisztián Kálmán Dr. David Esteban‐Gómez Dr. Éva Tóth Dr. Carlos Platas‐Iglesias 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(3):896-901
A highly rigid open‐chain octadentate ligand (H4cddadpa) containing a diaminocylohexane unit to replace the ethylenediamine bridge of 6,6′‐[(ethane‐1,2 diylbis{(carboxymethyl)azanediyl})bis(methylene)]dipicolinic acid (H4octapa) was synthesized. This structural modification improves the thermodynamic stability of the Gd3+ complex slightly (log KGdL=20.68 vs. 20.23 for [Gd(octapa)]?) while other MRI‐relevant parameters remain unaffected (one coordinated water molecule; relaxivity r1=5.73 mm ?1 s?1 at 20 MHz and 295 K). Kinetic inertness is improved by the rigidifying effect of the diaminocylohexane unit in the ligand skeleton (half‐life of dissociation for physiological conditions is 6 orders of magnitude higher for [Gd(cddadpa)]? (t1/2=1.49×105 h) than for [Gd(octapa)]?. The kinetic inertness of this novel chelate is superior by 2–3 orders of magnitude compared to non‐macrocyclic MRI contrast agents approved for clinical use. 相似文献
26.
High-efficiency aminocarbonylation by introducing CO to a pressurized continuous flow reactor 总被引:1,自引:0,他引:1
Csajági C Borcsek B Niesz K Kovács I Székelyhidi Z Bajkó Z Urge L Darvas F 《Organic letters》2008,10(8):1589-1592
Halogenated aryl carboxylic acids were efficiently converted to the corresponding dicarboxylic acid monoamides by a one-step Pd-catalyzed aminocarbonylation in a micro/meso fluidic continuous flow reactor (X-Cube) operated at high pressure and high temperature with CO gas introduction. Reaction parameters (solvent, base, catalyst, pressure, temperature) were rapidly optimized in the reactions, which required less than 2 min. The method gave improved results over comparable batch techniques and is also suited to automated parallel syntheses of compound libraries. 相似文献
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Dynamic kinetic resolution (DKR) of a series of sterically hindered allylic alcohols has been conducted with Candida antarctica lipase B (CALB) and ruthenium catalyst 1. The optically pure allylic acetates obtained were subjected to oxidative cleavage to give the corresponding acylated acyloins in high yields without loss of chiral information. 相似文献
28.
We review the recently developed three-dimensional (3D) atom-superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS) based on ab initio electronic structure data. In the method, contributions from individual electron tunneling transitions between the tip apex atom and each of the sample surface atoms are summed up assuming the one-dimensional (1D) Wentzel–Kramers–Brillouin (WKB) approximation in all these transitions. This 3D WKB tunneling model is extremely suitable to simulate spin-polarized STM and STS on surfaces exhibiting a complex noncollinear magnetic structure, i.e., without a global spin quantization axis, at very low computational cost. The tip electronic structure from first principles can also be incorporated into the model, that is often assumed to be constant in energy in the vast majority of the related literature, which could lead to a misinterpretation of experimental findings. Using this approach, we highlight some of the electron tunneling features on a prototype frustrated hexagonal antiferromagnetic Cr monolayer on Ag(111) surface. We obtain useful theoretical insights into the simulated quantities that is expected to help the correct evaluation of experimental results. By extending the method to incorporate a simple orbital dependent electron tunneling transmission, we reinvestigate the bias voltage- and tip-dependent contrast inversion effect on theW(110) surface. STM images calculated using this orbital dependent model agree reasonably well with Tersoff-Hamann and Bardeen results. The computational efficiency of the model is remarkable as the k-point samplings of the surface and tip Brillouin zones do not affect the computational time, in contrast to the Bardeen method. In a certain case we obtain a relative computational time gain of 8500 compared to the Bardeen calculation, without the loss of quality. We discuss the advantages and limitations of the 3D WKB method, and show further ways to improve and extend it. 相似文献
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Zsolt E. Horváth Antal A. Koós Krisztián Kertész György Molnár Gábor Vértesy Márton C. Bein Tamás Frigyes Zoltán Mészáros József Gyulai László P. Biró 《Applied Physics A: Materials Science & Processing》2008,93(2):495-504
The electrical resistance of 24 different carbon nanotube (CNT) thin film samples in blowing ambient air and 10 different
analyte vapor environments was measured. The effects of the CNT growth method, different chemical treatments, ball milling,
sample preparation conditions and Ar+-ion irradiation are compared. Significant differences in the response signal curves as a function of time in the case of
the studied sensor/vapor combinations show the important role of the defect structure and attached functional groups in the
chemical sensing properties of CNTs. 相似文献
30.
Krisztián Albitz Dr. Dániel Csókás Dr. Zoltán Dobi Prof. Imre Pápai Dr. Tibor Soós 《Angewandte Chemie (International ed. in English)》2023,62(13):e202216879
Prenylation is a ubiquitous late-stage modification in nature that often confers significantly improved bioactivity for secondary metabolites. While this lipophilic modification renders enhanced potency, the lipophilic tag(s) can diminish bioavailability and adversely alter drug transportation and metabolism. Thus, a functional-group-tolerant, mild, and selective late-stage C−H functionalization of prenyl tags would present a great potential in drug discovery programs but could also impact other fields, such as agrochemistry and chemical biology. Herein we report an exocyclic-strain-driven cross-metathesis reaction of prenyl tags, a formal double C−H oxidation protocol, that can be used for the selective late-stage derivatization of prenylated compounds and natural products. This methodology avoids the need for prefunctionalization of target molecules and affords ready access to an unprecedented library of oxo- and aza-prenylated complex molecules. Thus, in a broader context, this methodology extends late-stage functionalization beyond that available to nature. 相似文献