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排序方式: 共有209条查询结果,搜索用时 46 毫秒
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We investigate the statistical mechanics of polymers with bending and torsional elasticity described by the helical wormlike model. Noticing that the energy function is factorizable, we provide a numerical method to solve the model using a transfer matrix formulation. The tangent-tangent and binormal-binormal correlation functions have been calculated and displayed rich profiles which are sensitive to the combination of the temperature and the equilibrium torsion. Their behaviors indicate that there is no finite temperature Lifshitz point between the disordered and helical phases. The asymptotic behavior at low temperature has been investigated theoretically and the predictions fit the numerical results very well. Our analysis could be used to understand the statics of dsDNA and other chiral polymers. 相似文献
64.
Numerical wave packet simulations are performed for studying coherent anti-Stokes Raman scattering (CARS) for CN radicals. Electronic coherence is created by femtosecond laser pulses between the X(2)Σ and B(2)Σ states. Due to the large energy separation of vibrational states, the wave packets are superpositions of rotational states only. This allows for a specially detailed inspection of the second- and third-order coherences by a two-dimensional imaging approach. We present the time-frequency domain images to illustrate the intra- and intermolecular interferences, and discuss the procedure to rationally control and experimentally detect the interferograms in solid Xe environment. 相似文献
65.
Lin Z Ramezani H Eichelkraut T Kottos T Cao H Christodoulides DN 《Physical review letters》2011,106(21):213901
Parity-time (PT) symmetric periodic structures, near the spontaneous PT-symmetry breaking point, can act as unidirectional invisible media. In this regime, the reflection from one end is diminished while it is enhanced from the other. Furthermore, the transmission coefficient and phase are indistinguishable from those expected in the absence of a grating. The phenomenon is robust even in the presence of Kerr nonlinearities, and it can also effectively suppress optical bistabilities. 相似文献
66.
Low‐Temperature Oxidation of Carbon Monoxide with Gold(III) Ions Supported on Titanium Oxide 下载免费PDF全文
Prof. Wolfgang Grünert Dr. Dennis Großmann Dr. Heshmat Noei Dr. Marga‐Martina Pohl Dr. Ilya Sinev Dr. Andrea De Toni Dr. Yuemin Wang Prof. Martin Muhler 《Angewandte Chemie (International ed. in English)》2014,53(12):3245-3249
Au/TiO2 catalysts prepared by a deposition–precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature‐independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion–temperature characteristics to the well‐known S‐shape, with activation energies slightly below 30 kJ mol?1. Sample characterization by XAFS and electron microscopy and a low‐temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas‐phase O2 at 90 K already over the freeze‐dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII‐containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. 相似文献
67.
Synthesis of a DNA Promoter Segment Containing All Four Epigenetic Nucleosides: 5‐Methyl‐, 5‐Hydroxymethyl‐, 5‐Formyl‐, and 5‐Carboxy‐2′‐Deoxycytidine 下载免费PDF全文
Arne S. Schröder Jessica Steinbacher Barbara Steigenberger Felix A. Gnerlich Stefan Schiesser Toni Pfaffeneder Prof. Dr. Thomas Carell 《Angewandte Chemie (International ed. in English)》2014,53(1):315-318
A 5‐formyl‐2′‐deoxycytidine (fdC) phosphoramidite building block that enables the synthesis of fdC‐containing DNA with excellent purity and yield has been developed. In combination with phosphoramidites for 5‐methyl‐dC, 5‐hydroxymethyl‐dC, and carboxy‐dC, it was possible to prepare a segment of the OCT‐4 promoter that contains all four epigenetic bases. Because of the enormous interest in these new epigenetic bases, the ability to insert all four of them into DNA should be of great value for the scientific community. 相似文献
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Gebrenegus Yohannes Susanne K. Wiedmer Toni Andersson Marja-Liisa Riekkola 《Analytica chimica acta》2005,542(2):222-229
Polyelectrolyte complex (PEC) formation between cationic poly(methacryloxyethyl trimethylammonium chloride) (PMOTAC) and anionic poly(ethylene oxide)-block-poly(sodium methacrylate) (PEO-b-PMANa) was studied by asymmetrical flow field-flow fractionation and dynamic light scattering. The influence of ionic strength and mixing ratios of the charged units of the polyelectrolytes on the complex formation was evaluated. The diffusion coefficients and the hydrodynamic diameter distributions of the free and complexed polyelectrolytes were measured. In the absence of salt, the weight averaged hydrodynamic diameters were 48 and 28 nm for PMOTAC and PEO-b-PMANa, respectively. In the presence of salt, the particles were smaller, with weight averaged hydrodynamic diameters of 44-45 and 8-10 nm, respectively. In salt-free solution, at 1:1 mixing ratio of the charged monomer units of PMOTAC and PEO-b-PMANa, polydisperse particles with diameters of 2000-4000 nm were formed. In the presence of 20, 80, and 160 mM of sodium chloride, the 1:1 complexes were relatively monodisperse particles with weight averaged hydrodynamic diameters of 93, 124, and 120 nm, respectively. 相似文献
70.
Toni El-Hajj Jean-Claude Martin Grard Descotes 《Journal of heterocyclic chemistry》1983,20(1):233-235
Addition of α, β-unsaturated sulfones or ketones to 5-hydroxmethyl-furaldehyde or related furanaldehydes in the presence of a thiazolium salt gives di or tetraketonic furan compounds. Dehydrative cyclizationled to di- and terfuryl derivatives. 相似文献