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731.
Taylor MT Blackman ML Dmitrenko O Fox JM 《Journal of the American Chemical Society》2011,133(25):9646-9649
Computation was used to design a trans-cyclooctene derivative that displays enhanced reactivity in the tetrazine-trans-cycloctene ligation. The optimized derivative is an (E)-bicyclo[6.1.0]non-4-ene with a cis-ring fusion, in which the eight-membered ring is forced to adopt a highly strained 'half-chair' conformation. Toward 3,6-dipyridyl-s-tetrazine in MeOH at 25 °C, the strained derivative is 19 and 27 times more reactive than the parent trans-cyclooctene and 4E-cyclooct-4-enol, respectively. Toward 3,6-diphenyl-s-tetrazine in MeOH at 25 °C, the strained derivative is 160 times more reactive than the parent trans-cyclooctene. 相似文献
732.
A new technique for the rapid quantification of orientation-dependent etch rates, which uses micromachined test patterns and optical microscopy, has been developed. The etching of silicon in KOH etchants with and without isopropanol was studied. Etch rates measured with this technique are in good agreement with conventionally measured rates. In most cases, the etch rate anisotropies are well described by a simple model that is based on step-flow etching. Kinetic Monte Carlo simulations of etching were used to test the simple model and to generate approximate morphologies of the etched surfaces. Vicinal Si(110) surfaces display unusual, orientation-dependent etch rates in some etchants; the functional form of the etch rate anisotropy suggests that a morphological transition occurs on these highly reactive faces. In moderately concentrated KOH solutions where isopropanol is readily soluble, the measured etch rate anisotropies suggest that isopropanol stabilizes step-flow etching. 相似文献
733.
Let M be a compact manifold of dimension n, P=P(h) a semiclassical pseudodifferential operator on M, and u=u(h) an L
2 normalized family of functions such that P(h)u(h) is O(h) in L
2(M) as h↓0. Let H⊂M be a compact submanifold of M. In a previous article, the second-named author proved estimates on the L
p
norms, p≥2, of u restricted to H, under the assumption that the u are semiclassically localized and under some natural structural assumptions about the principal symbol of P. These estimates are of the form Ch
−δ(n,k,p) where k=dim H (except for a logarithmic divergence in the case k=n−2, p=2). When H is a hypersurface, i.e., k=n−1, we have δ(n,n−1, 2)=1/4, which is sharp when M is the round n-sphere and H is an equator. 相似文献
734.
The synthesis of a series of 7-alkoxy-4-trifluoromethylcoumarins via the von Pechmann reaction catalyzed by molecular iodine is described. The reaction protocol is simple, inexpensive and leads to the formation of the corresponding coumarin derivatives in good yield and high purity. A key intermediate as well as several iodo byproducts were isolated. 相似文献
735.
Melissa S. Caetano Teodorico C. Ramalho Douglas F. Botrel Elaine F. F. da Cunha Walclee Carvalho de Mello 《International journal of quantum chemistry》2012,112(15):2752-2762
Glyphosate is the active component of one of the top‐selling herbicides, which is also a potent EPSP synthase inhibitor. The herbicide is absorbed by living tissue and translocated via the phloem, to plant roots and rhizomes. When applied directly into the soil it has low activity, due to the high adsorption by soil constituents. Understanding the specific interactions between metals in the soil and glyphosate is the main step in understanding the low activity of the herbicide when applied directly into the ground and not pulverized. We can observe there is a stability order for both tetrahedral and octahedral complexes between glyphosate and metals: Zn>Cu>Co>Fe>Cr>Al>Ca>Mg. © 2012 Wiley Periodicals, Inc. 相似文献
736.
Melissa A. Vandiver James L. Horan Yuan Yang Emily T. Tansey Söenke Seifert Matthew W. Liberatore Andrew M. Herring 《Journal of polymer science. Part A, Polymer chemistry》2013,51(24):1761-1769
In this study, new alkaline exchange membranes were prepared from the perfluorinated 3M ionomer with various quaternary ammonium cations attached with sulfonamide linkage. The degree of functionalization varied depending on the cation species, resulting in different ion exchange capacities (IECs), 0.33–0.72 meq g−1. There was evidence of polymer degradation when the films were exposed to hydroxide, and hence all membrane characterization was performed in the chloride form. Conductivity was dependent on cation species and IEC, Ea = 36–59 kJ mol−1. Diffusion of water through the membrane was relatively high 1.6 × 10−5 cm2 s−1 and indicated restriction over a range of diffusion times, 6–700 ms. Water uptake (WU) in the membranes was generally low and the hydration level varied based on cation species, λ = 6–11. Small-angle scattering experiments suggested ionic aggregation, 37–42 Å, independent of cation species but slight differences in long-range order with cation species. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1761–1769, 2013 相似文献
737.
Melissa Manuszak Guerrini Bernadette Charleux Jean‐Pierre Vairon 《Macromolecular rapid communications》2000,21(10):669-674
The application of atom transfer radical polymerization (ATRP) to the homopolymerization of 2‐hydroxyethyl acrylate and 2‐(methacryloyloxy) ethyl trimethylammonium chloride at the surface of a crosslinked polystyrene latex functionalized with alkyl bromide groups is reported. Polymerization was carried out using the surface groups of the dialyzed latex as initiators. The resulting hydrophobic core, hydrophilic shell latexes, were analyzed by FTIR, 13C‐NMR spectroscopy, and dynamic light scattering. 相似文献
738.
Fernando Galembeck Carlos Alberto Paula Leite Maria do Carmo V.M. da Silva Amauri José Keslarek Carlos Alberto Rodrigues Costa Érico Teixeira-Neto Márcia Maria Rippel Melissa Braga 《Macromolecular Symposia》2002,189(1):15-26
This work reviews new probe and electron microscopy approaches for the detection of charged domains in insulating polymers, as well as for the identification of the charge-bearing species: scanning electric potential microscopy (SEPM), electric force microscopy (EFM) and energy-loss spectroscopy imaging in the transmission electron microscope (ESI-TEM). The SEPM and EFM micrographs show patterned domains bearing excess electric charges and extending for tens of nanometers, in polymer latex particles and films. The charged species are identified by ESI-TEM as emulsion polymerization initiator and surfactant residues, as well as the associated counter-ions. Charged domains are also observed in common thermoplastic polymers, producing unexpectedly large electric potential gradients. 相似文献