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911.
Fisher GL Szakal C Wetteland CJ Winograd N 《The journal of physical chemistry. B》2006,110(4):1820-1829
The chemical evolution of poly(tetrafluoroethylene) (PTFE) that is brought about by increasing levels of irradiation with alpha particles is accompanied by the emergence and proliferation of functionalized moieties. Families of reaction products specifically identified in the alpha-irradiated polymer matrix include hydride-, hydroxide-, and oxide-functionalized fluorocarbons. The data also indicate the emergence of hydrogen peroxide (H2O2) and hydrazine (N2H4), but no distinct evidence suggesting the formation of perfluorinated amines, amides, or cyanogens is found. In this article we substantiate the speciation of emergent species and reveal evidence of intercalated molecular gases with which alpha particle-generated radicals may react to form the observed products. Furthermore, we present evidence to suggest that the kinetics of alpha particle-induced reaction is limited by the diffusion of radicals within the polymer matrix. That is to say, chemical additives in the polymer matrix are shown to be scavengers of H*, O*, and F* radicals and limit the rates of reaction that produce functionalized fluorocarbon moieties. Above a threshold dose of alpha particles, the concentration of radicals exceeds that of the scavenger species, and free radical diffusion commences as evidenced by a sudden increase in the yield of reaction products. Samples of PTFE were irradiated to alpha doses in the range of 10(7) to 5 x 10(10) rad with 5.5 MeV 4He2+ ions from a tandem accelerator. Residual gas analysis (RGA) was utilized to monitor the liberation of molecular gases from PTFE during alpha particle irradiation of samples in vacuum. Static time-of-flight SIMS (TOF-SIMS), equipped with a 20 keV C60+ source, was employed to probe chemical changes as a function of alpha particle irradiation. Chemical images and high-resolution mass spectra were collected in both the positive and negative polarities. 相似文献
912.
Guan Y Yang S Zhang Y Xu J Han CC Kotov NA 《The journal of physical chemistry. B》2006,110(27):13484-13490
We observed Fabry-Perot fringes in the absorption spectra of hydrogen-bonded layer-by-layer (LBL) films of poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA), which stem from the interferences between beams transmitted and partially reflected at the highly smooth film-air interface and film-quartz interface. The appearance and disappearance of Fabry-Perot fringes can be used to evaluate the homogeneity of the film. They also provide information about the film thickness. Using this optical phenomenon, with a minimal requirement of instrumentation, we studied the effect of several experimental conditions on the film buildup and structure. The film grows linearly with dipping cycles. Films fabricated from higher molecular weight polymers tend to be thicker. Increasing the concentration of the assembly solutions can also make thicker films. However, films from high molecular weight polymers or high concentration assembly solutions may be heterogeneous and do not display Fabry-Perot fringes in their absorption spectra. The defects in these heterogeneous films can be healed by a postannealing in water or diluted HCl to allow the chain rearrangement to complete. We further found the PVPON/PAA films can be eroded by long-term annealing in water or diluted HCl by monitoring the movement of the Fabry-Perot fringes. In most cases, the erosion rate is constant with annealing time. The erosion rate decreases with a decrease in the pH of the media and an increase in the molecular weight of the polymers. 相似文献
913.
Michael W. Stadelmaier Nicholas J. Rose George D. Poole Carl D. Meyer 《Linear algebra and its applications》1982
If A is a nonsingular M-matrix, the elements of the sequence {A?k} all have the same zero pattern. Using the Drazin inverse, we show that a similar zero pattern invariance property holds for a class of matrices which is larger than the generalized M-matrices. 相似文献
914.
A base-catalyzed, direct synthesis of 3,5-disubstituted 1,2,4-triazoles from nitriles and hydrazides
Kap-Sun Yeung Michelle E. Farkas John F. Kadow Nicholas A. Meanwell 《Tetrahedron letters》2005,46(19):3429-3432
A convenient and efficient one step, base-catalyzed synthesis of 3,5-disubstituted 1,2,4-triazoles by the condensation of a nitrile and a hydrazide is presented. A diverse range of functionality and heterocycles are tolerated under the reaction conditions developed, and the reactivity of the nitrile partner is relatively insensitive to electronic effects. 相似文献
915.
916.
Michael Krivelevich Benny Sudakov Van H. Vu Nicholas C. Wormald 《Random Structures and Algorithms》2001,18(4):346-363
Random d‐regular graphs have been well studied when d is fixed and the number of vertices goes to infinity. We obtain results on many of the properties of a random d‐regular graph when d=d(n) grows more quickly than . These properties include connectivity, hamiltonicity, independent set size, chromatic number, choice number, and the size of the second eigenvalue, among others. ©2001 John Wiley & Sons, Inc. Random Struct. Alg., 18: 346–363, 2001. 相似文献
917.
We present a functorial computation of the equivariant intersection cohomology of a hypertoric variety, and endow it with
a natural ring structure. When the hyperplane arrangement associated with the hypertoric variety is unimodular, we show that
this ring structure is induced by a ring structure on the equivariant intersection cohomology sheaf in the equivariant derived
category. The computation is given in terms of a localization functor which takes equivariant sheaves on a sufficiently nice
stratified space to sheaves on a poset.
T. Braden’s research was supported in part by NSF grant DMS-0201823.
N. Proudfoot’s research was supported in part by an NSF Postdoctoral Research Fellowship and NSF grant DMS-0738335. 相似文献
918.
Marleen de Veij Peter Vandenabeele Krystyn Alter Hall Facundo M. Fernandez Michael D. Green Nicholas J. White Arjen M. Dondorp Paul N. Newton Luc Moens 《Journal of Raman spectroscopy : JRS》2007,38(2):181-187
During the last decade there has been an apparent increase in the prevalence of counterfeit medicines in developing as well as developed countries. The pivotal antimalarial artesunate has been counterfeited on a large scale in SE Asia. In this work, the possibilities of Raman spectroscopy are explored as a fast and reliable screening method for the detection of counterfeit artesunate tablets. In this study, 50 ‘artesunate tablets’, purchased in SE Asia, were examined. This spectroscopic method was able to distinguish between genuine and counterfeit artesunate and to identify the composition of the counterfeit tablets. These contained no detectable levels of artesunate, but consisted mostly of starch, calcite (CaCO3), and paracetamol (4‐acetamidophenol). In one particular case an admixture of rutile (TiO2) and artesunate was detected. The results of the investigation by Raman spectroscopy were in agreement with those of colorimetric tests and of liquid chromatography‐mass spectrometry on the artesunate. Moreover, principal components analysis (PCA) was combined with hierarchical cluster analysis to establish an automated approach for the discrimination between different groups of counterfeits and genuine artesunate tablets. These results demonstrate that Raman spectroscopy combined with multivariate analysis is a promising and reliable methodology for the fast characterization of genuine and counterfeit artesunate antimalarial tablets. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
919.
Turro NJ 《Chemical communications (Cambridge, England)》2002,(20):2279-2292
The reactions of carbon centered radical pairs often involve diffusion controlled combination and/or disproportionation reactions which are non-selective. A triplet geminate pair of radicals is produced by the photolysis of suitable ketones. The reactions of such geminate pairs can be controlled though the application of supramolecular concepts which emphasize non-covalent interaction to "steer" the geminate pair toward a selected pathway. In addition, "superdupermolecular" concepts, which emphasize the control of radical pair reactions through the orientation of electron spins, can be employed to further control the course of geminate pair reactions. Examples of control of a range of the selectivity of geminate radical combinations, which form strong covalent bonds, through supramolecular and superdupermolecular effects will be presented for the photolysis of ketones adsorbed in the supercages of zeolites. 相似文献
920.
Gregg A Hogan August A Gallo Kenneth M Nicholas Radhey S Srivastava 《Tetrahedron letters》2002,43(52):9505-9508
The copper(I) complex [Cu(CH3CN)4]PF6 catalyzes the allylic amination of alkenes by aryl hydroxylamine in fair to moderate yields. Unsymmetrical alkenes react with high regioselectivity with N-functionalization occuring at the less substituted vinylic carbon. Trapping experiments indicate that free PhNO is not an intermediate in these reactions. 相似文献