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81.
Mixtures of methane and olefins (ethylene, propylene, butenes, butadiene, and styrene) have been polymerized over HSO3 F-SbF3 to yield an oily oligomer with a molecular weight ranging from 100 to 700. The NMR spectra of each polymer showed a sharp peak at or near 1.25 &, suggesting the presence of block methylene in the polymer. The formation of block methylene is surprising considering the fact that the polymerization reaction is carbonium ion in nature. A primary cation has been invoked to explain the results. The formation of this primary cation must involve some extraordinary stabilization by some component in the acid.  相似文献   
82.
A directional microphone system for field recording of sounds in the air often involves a parabolic reflector to focus the sound waves on the microphone (transducer) element. Some deficiencies of such a system are noted with respect to reproduction of spectra. The reflector system, involving as it does a structure comparable to a wavelength in linear dimension, is not susceptible to traditional high- or low-frequency approximate methods of computation. Modern numerical techniques now permit precise calculation of the directional responses of small reflectors of various shapes. One result is a proposal for a very economical and effective system involving a plane reflector. Other baffle shapes are also investigated, which may be of interest in special applications.  相似文献   
83.
Graft copolymers of acrylamide and yellow dextrin were prepared using cerium(IV) as initiator. The yellow dextrin had a very broad molecular weight distribution but was fractionated utilizing dialysis and ultrafiltration membranes. Initiator efficiencies were determined using size exclusion chromatography and were found to be between 2.4 and 34%. Initiator efficiency increased with acrylamide concentration at constant cerium (IV) and yellow dextrin concentrations, and decreased with increasing cerium(IV) concentration at constant acrylamide and yellow dextrin concentrations. Plots of acrylamide conversion and intrinsic viscosity vs initial acrylamide concentration at constant yellow dextrin and ceric ion concentrations showed a maximum at about 2.0 M.  相似文献   
84.
85.
This paper is concerned with the construction and analysis ofcompact finite difference approximations to the model linearsource problem –(pu')' + qu = f where the functions p,q, and f can have jump discontinuities at a finite number ofpoints. Explicit formulae that give O(h2) O(h3) and O(h4) accuracyare derived, and a procedure for computing three-point schemesof any prescribed order of accuracy is presented. A rigoroustruncation and discretization error analysis is offered. Numericalresults are also given.  相似文献   
86.
The use of peptides in medicine is limited by low membrane permeability, metabolic instability, high clearance, and negligible oral bioavailability. The prediction of oral bioavailability of drugs relies on physicochemical properties that favor passive permeability and oxidative metabolic stability, but these may not be useful for peptides. Here we investigate effects of heterocyclic constraints, intramolecular hydrogen bonds, and side chains on the oral bioavailability of cyclic heptapeptides. NMR‐derived structures, amide H–D exchange rates, and temperature‐dependent chemical shifts showed that the combination of rigidification, stronger hydrogen bonds, and solvent shielding by branched side chains enhances the oral bioavailability of cyclic heptapeptides in rats without the need for N‐methylation.  相似文献   
87.
Abstract

The complex and important role of the propellant binder in solid-propellant rockets is described. The severe weight problems of space exploratrion cause high propellant performance to be of major concern in a highly competitive field. Binders contribute to performance, not only in terms of fuel value, but by being compatible with energetic components and by providing good mechanical properties. Both sterilization required for planetary landings and compatibility problems of new energetic oxidizers generate new requirements too stringent for existing binders; only binders composed essentially of saturated aliphatic hydrocarbon are predicted to be suitable. Several saturated-binder developments are in progress. The achievements of one of these programs, based on free-radical synthesis, are evaluated against the goals of the new binder.  相似文献   
88.
The modular synthesis of glycosaminoglycans requires straightforward methods for the production of large quantities of protected uronic acid building blocks. In particular, the preparation of fully differentiated iduronic acids has proven particularly challenging. An efficient route to methyl 3-O-benzyl-1,2-O-isopropylidene-alpha-l-idopyranosiduronate 6 from diacetone glucose in nine steps and 36% overall yield is described. Idopyranosiduronate 6 is useful as a glycosyl acceptor and as an intermediate that may be further elaborated into iduronic acid trichloroacetimidate glycosyl donors for the assembly of glycosaminoglycan structures as illustrated here.  相似文献   
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90.
Magnetic field induced quadrupolar line splittings in the 2H NMR spectra of pyridine and metadinitrobenzene are reported. From the splittings the anisotropy in the magnetic susceptibility Δx has been deduced. The concentration dependence of the quadrupolar magnetic field effect, and hence of Δx, of nitrobenzene, metadinitrobenzene and naphthalene has been investigated. The effect of different solvents has also been studied. Environmental effects of Δx have been found to be substantial in several cases. The relation of the present experiments with the Cotton-Mouton effect is discussed in detail.  相似文献   
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