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31.
Designs, Codes and Cryptography - In this paper, we give a matrix construction method for designing DNA codes that come from group matrix rings. We show that with our construction one can obtain... 相似文献
32.
E. Blasius K.-P. Janzen W. Adrian W. Klein H. Klotz H. Luxenburger E. Mernke V.B. Nguyen T. Nguyen-Tien R. Rausch J. Stockemer A. Toussaint 《Talanta》1980,27(2):127-141
Exchangers with cyclic polyethers as anchor groups have a large range of applications such as separations of cations with a common anion, of anions with a common cation, and of neutral organic compounds, and the determination of water by elution chromatography. Some crown ether monomers, especially 4- and 4,4′-alkyl-substituted benzo-derivatives are suitable for extractions and their adducts with heteropoly acids are used as liquid ion-exchangers. The exchangers are also applied in thin-layer chromatography and thin-layer electrophoresis. Furthermore the exchangers are successfully used in preparative chemistry, e.g., in salt conversions in order to isolate salts which are difficult to prepare by other means, in isolation and purification of organic compounds, and for anion activation in organic reactions. 相似文献
33.
Walter Fischer Cyril A. Grob Reinhard Hanreich Georg von Sprecher Adrian Waldner 《Helvetica chimica acta》1981,64(7):2298-2311
The solvolysis rates and products of the 6-exo-substituted 2-exo- 1a - 1u , and 2-endo-norbornyl p-toluenesulfonates 2a - 2u , have been determined. In general, the rate constants for 1 and 2 (log k) correlate well with the inductive constants σ of the substitutents at C(6); however, their sensitivity to σ is much larger in the 2-exo-series 1 than in the 2-endo-series 2 . This differential transmission of polar effects is the cause of decreasing 2-exo/2-endo rate ratios from 2388 for R = t-C4H9 to 0.37 for R = Br, i. e. with increasing electron attraction by the substituent. The high sensitivity of the rate constants for the 2-exo-p-toluenesulfonates 1 to σ indicates an unusually strong inductive interaction between C(6) and the incipient cationic center at C(2). This interaction is ascribed to the participation of the pentacoordinate C(6)-atom, i. e. to 1,3-bridging, a consequence of steric hindrance of nucleophilic solvent participation in norbornanes. Donor substituents enhance 1,3-bridging, lead to faster reactions and to the formation of 2-exo substitution products. Conversely, acceptor substituents reduce 1,3-bridging, decrease rates and facilitate the formation of 2-endo substitution products. Graded 1,3-bridging is discussed in the light of Winstein's nonclassical ion concept. 相似文献
34.
35.
We propose and test a pair potential that is accurate at all relevant distances and simple enough for use in large-scale computer simulations. A combination of the Rydberg potential from spectroscopy and the London inverse-sixth-power energy, the proposed form fits spectroscopically determined potentials better than the Morse, Varnshi, and Hulburt-Hirschfelder potentials and much better than the Lennard-Jones and harmonic potentials. At long distances, it goes smoothly to the London force appropriate for gases and preserves van der Waals's "continuity of the gas and liquid states," which is routinely violated by coefficients assigned to the Lennard-Jones 6-12 form. 相似文献
36.
When the Hammett-Taft equation log (k/ko)=ρq · σ is applied to the solvolysis of the 3-substituted propyl bromides 6a-6i in ethanol/water 4:1 (v/v) log k correlates linearly with σ except in cases where R exerts an anchimeric effect. The reaction constant ρq for 6 is ? 0.12 and is typical for a nucleophilic solvent-assisted ks process at a primary C-atom. The tertiary halides 1 and 3 , however, which react with little or no nucleophilic solvent assistance, i.e. by kc processes, lead to larger ρq values of ?0.71 and ?1.14, respectively. The reaction constant pq is therefore a sensitive gauge for charge development in the transition state for solvolysis of saturated compounds. 相似文献
37.
Eshelby J Goessman M Parsons PJ Pennicott L Highton A 《Organic & biomolecular chemistry》2005,3(16):2994-2997
The construction of the fourteen membered ring present in galbonolide B 1 is reported. The 10,11-diene system present in the southern portion of has been constructed using an ester enolate rearrangement/silicon mediated fragmentation cascade, whilst the macrocycle has been synthesised following a Johnson rearrangement/mercury assisted ring closure protocol. 相似文献
38.
Crespo A Martí MA Estrin DA Roitberg AE 《Journal of the American Chemical Society》2005,127(19):6940-6941
A novel technique for computing free energy profiles in enzymatic reactions using the multiple steering molecular dynamics approach in the context of an efficient QM-MM density functional scheme is presented. The conversion reaction of chorismate to prephenate catalyzed by the Bacillus subtilis enzyme chorismate mutase has been chosen as an illustrative example. 相似文献
39.
An improvement of the signal-to-noise ratio in atomic absorption spectroscopy may be obtained by each of the following three methods of signal averaging: increase of the time constant digital signal averaging true integration of the signal The most adequate of these methods appears to be true integration. Two examples are presented to demonstrate the improved signal-to-noise ratio due to the application of this method: trace analysis of mercury in water by means of flameless AAS. The detection limit appears to be 0.2 ng which corresponds to a concentration of 2 · 10?13 g/ml. trace analysis of copper in human urine by means of flame AAS: 1,3 · 10?8 g/ml copper could be detected without any solvent extraction process. 相似文献
40.
Horgan A Saunders B Vincent B Heenan RK 《Journal of colloid and interface science》2003,262(2):548-559
A series of water-soluble, amphiphilic graft copolymers has been prepared by free-radical copolymerization of methoxypoly(ethylene glycol) macromonomers, with either methyl methacrylate or butyl methacrylate as the comonomers, in water/ethanol solvent mixtures. Lower molecular weight copolymers were obtained by increasing the concentration of the initiator, azobisisobutyronitrile (AIBN), used in the polymerization reaction. However, the route used also led to the formation of significant quantities of tetramethylsuccinodinitrile, a toxic byproduct resulting from the cage reaction of AIBN. Static fluorescence measurements using pyrene as a probe, along with 1H NMR experiments, showed that the graft copolymers form aggregates in water at very low concentrations (approximately 0.01 g l(-1)) with the pendant hydrophilic graft chains forming a stabilizing shell around the hydrophobic backbone. An increase in the hydrophile-lipophile balance of the graft copolymers was found to lead to smaller aggregates with lower aggregation numbers and highly swollen hydrophilic shells, as revealed by small angle neutron scattering (SANS). 相似文献