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781.
[reaction: see text] The stereoselectivity of Lewis acid-induced endo-regioselective oxacyclizations of 1,4-diepoxides is dependent upon the nature of the terminating nucleophile. For instance, the tert-butyl carbonate-substituted diepoxide of 3,6-dimethylhepta-2,5-dien-1-ol provides a cis-fused bicyclic product, whereas the N,N-dimethylcarbamate derivative affords the trans-fused diastereomer. Stereospecific and regioselective conversion of the tertiary carbamate-terminated 1,4,7-triepoxide (I) to tricyclic all-trans-fused polypyran (II) is also demonstrated. 相似文献
782.
I. McDonald M. Tredoux R. J. Hart 《Journal of Radioanalytical and Nuclear Chemistry》1995,198(1):169-178
The determination of ultratrace concentrations of Pt in rock samples using nickel sulphide fire assay combined with neutron activation analysis (NiS-NAA) is influenced by the presence of Te in the irradiated noble metal concentrate. Interference caused by123mTe on the 158 keV peak of199Au (produced from198Pt) is discussed with reference to a noble metal standard (Wits-1) and sedimentary rocks which generally have low Pt concentrations and high Te/Pt ratios. The importance of a rigorously controlled dissolution of the fire assay button to achieve complete removal of Te is emphasized. 相似文献
783.
P.R. Laity P.M. Glover J. Godward P.J. McDonald J. N. Hay 《Cellulose (London, England)》2000,7(3):227-246
NMR imaging and spatially resolved diffusometry have been used to study the distribution of water within swollen cellophane and measure its diffusion coefficient. Water concentration and diffusion coefficient were found to be essentially constant across most of the film thickness. However, significantly slower diffusion was indicated for water near the film surface (D=0.5×10–9m2s–1) compared with water in the centre of the film (D=0.88×10–9m2s–1). This was also reflected in lower T
2 values at the edge of the film indicating water with more restricted motion. These observations were interpreted in terms of dense surface regions of cellulose (skin) over a more porous interior (core). 相似文献
784.
Robert J. Hinkle Robert McDonald 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(2):o117-o121
The crystal structures of [(Z)‐2‐methylbut‐1‐en‐1‐yl][4‐(trifluoromethyl)phenyl]iodonium trifluoromethanesulfonate, C12H13F3I+·CF3O3S?, (I), (3,5‐dichlorophenyl)[(Z)‐2‐methylbut‐1‐en‐1‐yl]iodonium trifluoromethanesulfonate, C11H12Cl2I+·CF3O3S?, (II), and bis{[3,5‐bis(trifluoromethyl)phenyl][(Z)‐2‐methylbut‐1‐en‐1‐yl]iodonium} bis(trifluoromethanesulfonate) dichloromethane solvate, 2C13H12F6I+·2CF3O3S?·CH2Cl2, (III), are described. Neither simple acyclic β,β‐dialkyl‐substituted alkenyl(aryl)idonium salts nor a series containing electron‐deficient aryl rings have been described prior to this work. Compounds (I)–(III) were found to have distorted square‐planar geometries, with each I atom interacting with two trifluoromethanesulfonate counter‐ions. 相似文献
785.
McDonald WH Tabb DL Sadygov RG MacCoss MJ Venable J Graumann J Johnson JR Cociorva D Yates JR 《Rapid communications in mass spectrometry : RCM》2004,18(18):2162-2168
As the speed with which proteomic labs generate data increases along with the scale of projects they are undertaking, the resulting data storage and data processing problems will continue to challenge computational resources. This is especially true for shotgun proteomic techniques that can generate tens of thousands of spectra per instrument each day. One design factor leading to many of these problems is caused by storing spectra and the database identifications for a given spectrum as individual files. While these problems can be addressed by storing all of the spectra and search results in large relational databases, the infrastructure to implement such a strategy can be beyond the means of academic labs. We report here a series of unified text file formats for storing spectral data (MS1 and MS2) and search results (SQT) that are compact, easily parsed by both machine and humans, and yet flexible enough to be coupled with new algorithms and data-mining strategies. 相似文献
786.
787.
Jonkmans G Ahmad S Armstrong DS Azuelos G Bertl W Blecher M Chen CQ Depommier P Doyle BC von Egidy T Gorringe TP Gumplinger P Hasinoff MD Healey D Larabee AJ Macdonald JA McDonald SC Munro M Poutissou J Poutissou R Robertson BC Sample DG Saettler E Sigler CM Taylor GN Wright DH Zhang NS 《Physical review letters》1996,77(22):4512-4515
788.
789.
Jones John G. Farrington Dennis J. McDonald Fiona M. Mooney Peter M. Twigg Martyn V. 《Transition Metal Chemistry》1998,23(6):693-699
Reactions of ferrous phthalocyanine with 4–aminopyridine, imidazole or pyridine in dimethylsulphoxide have been re-investigated using exacting stopped-flow techniques focusing attention on the rapid first stage forming the intermediate FePc(dmso)(base). Effects of impurities in ferrous phthalocyanine samples formed during storage, and the presence of -oxo dimer, on the observed kinetic behaviour with base were examined. The correctness of the findings of an Italian group that kinetic data can be obtained for the first step was affirmed. New results and reassigned published data enabled rate and equilibrium constants to be assigned to the first and second steps for reactions of ferrous phthalocyanine with each of the three nitrogenous bases employed. 相似文献
790.