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111.
Barnakov YA Yu MH Rosenzweig Z 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7524-7527
The paper describes the synthesis and characterization of the magnetic properties of magnetite/silica nanocomposites using a modified Stober method. Magnetite nanoparticles averaging 8-10 nm in diameter and stabilized with oleic acid in toluene were used as the magnetic component of the nanocomposites. SQUID magnetic measurements and ferromagnetic resonance spectroscopy measurements were performed at each stage of the synthesis to understand the properties of the formed composites. Changes of blocking temperature in ZFC/FC SQUID curves correlated with corresponding changes of the resonance field in the ferromagnetic spectra of the sample at each stage of formation. The paper concludes that it is possible to manipulate the magnetic properties of silica/magnetite composite materials by controlling their surface properties and silica coating thickness. 相似文献
112.
Zeev Tashma 《Tetrahedron》1982,38(24):3745-3747
Thiocarbamoyl phosphonates 1 did not react with alkylating agents to give the S-alkyl derivatives 2, but gave zwitterions 3a–e in which the phosphonate ester moiety was dealkylated. In some starting material could be recovered. A mechanism is suggested in order to explain the relationship between the alkylation and the dealkylation steps of the reaction. 相似文献
113.
Lesot P Merlet D Sarfati M Courtieu J Zimmermann H Luz Z 《Journal of the American Chemical Society》2002,124(34):10071-10082
We describe the enantiomeric and enantiotopic analysis of the NMR spectra of compounds derived from the functionalized cone-shaped core, cyclotriveratrylenes (CTV), dissolved in weakly oriented lyotropic chiral liquid crystals (CLCs) based on organic solutions of poly-gamma-benzyl-L-glutamate. The CTV core lacks prostereogenic as well as stereogenic tetrahedral centers. However, depending on the pattern of substitution, chiral and achiral compounds with different symmetries can be obtained. Thus, symmetrically nonasubstituted CTVs (C(3) symmetry) are optically active and exhibit enantiomeric isomers, while symmetrically hexasubstituted (C(3v) symmetry) derivatives are prochiral and possess enantiotopic elements. In the first part we use (2)H and (13)C NMR to study two nonasubstituted (-OH or -OCH(3)) CTVs, where the ring methylenes are fully deuterated, and show for the first time that the observation of enantiomeric discrimination of chiral molecules with a 3-fold symmetry axis is possible in a CLC. It is argued that this discrimination reflects different orientational ordering of the M and P isomers, rather than specific chiral short-range solvent-solute interactions that may affect differently the magnetic parameters of the enantiomers or even their geometry. In the second part we present similar measurements on hexasubstituted CTV with flexible side groups (-OC(O)CH(3) and the, partially deuterated bidentate, -OCH(2)CH(2)O-), having on the average C(3v) symmetry. No spectral discrimination of enantiotopic sites was detected for the -OC(O)CH(3) derivative. This is consistent with a recent theoretical work (J. Chem. Phys. 1999, 111, 6890) that indicates that in C(3v) molecules no chiral discrimination between enantiotopic elements, based on ordering, is possible. In contrast, a clear splitting was observed in the (2)H spectra of the enantiotopic deuterons of the side groups in the tri(dioxyethylene)-CTV. It is argued that this discrimination reflects different ordering characteristics of the various, rapidly (on the NMR time scale) interconverting conformers of this compound. Assuming two twisted structures for each of the dioxyethylene side groups, four different conformers are expected, comprising two sets of enantiomeric pairs with, respectively, C(3) and C(1) symmetries. Differential ordering and/or fractional population imbalance of these enantiomeric pairs leads to the observed spectral discrimination of sites in the side chains that on average form enantiotopic pairs. 相似文献
114.
Stavitski E Berg A Ganguly T Mahammed A Gross Z Levanon H 《Journal of the American Chemical Society》2004,126(22):6886-6890
Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state. 相似文献
115.
Sarah Shtelzer Moshe Weitzberg Juliette Jeries Tuvia Sheradsky Zeev Aizenshtat Jochanan Blum 《Journal of heterocyclic chemistry》1984,21(1):1-3
Phenanthrene 9,10-imine ( 1 ) was shown to undergo N-alkylation without aziridine ring cleavage by (a) metallation with sodium methylsulfinylmethide followed by addition of an alkyl halide at −20° (b) reaction of 1 , sodium hydride and the halide in dimethylformamide at 40° (c) treatment of a dichloromethane solution of 1 , the halide and triethylbenzylammonium chloride with aqueous sodium hydroxide under phase transfer conditions. The syntheses of N-isopropyl-, N-butyl-, N-pentyl-, N-allyl- and N-benzylphenanthrene 9,10-imine ( 2–6 ) are described. 相似文献
116.
Saltsman I Mahammed A Goldberg I Tkachenko E Botoshansky M Gross Z 《Journal of the American Chemical Society》2002,124(25):7411-7420
This work demonstrates the feasibility and power of electrophilic substitution on the peripheral carbon atoms of triarylcorroles as a synthetic tool to new derivatives. The large difference in the reactivity of the various carbon atoms on the macrocycle was shown to be of electronic rather than steric origin. A careful choice of reagents and a delicate control of reaction conditions allowed the selective syntheses of novel derivatives, in all of which substitution took place selectively in only the directly joined pyrrole rings of the macrocycle. This was proven by a combination of X-ray crystallography of the various products and detailed analysis of their NMR spectra. 相似文献
117.
A thorough mechanistic investigation has been performed on the reactions of primary and secondary amines with diazoacetates, which proceed uniquely quickly and efficiently when catalyzed by iron(III) corroles and porphyrins. Two major differences in relation to other metal-based catalysts are that the iron complexes are not poisoned by excess amine and that metal-carbene intermediates are apparently not involved in the reaction pathway. The results instead point towards nitrogen ylide intermediates formed by nucleophilic attack of the amines on diazoacetate-coordinated iron complexes. Nitrogen ylides are also formed when allyl- and propargyl-substituted tertiary amines react with diazoacetates, a scenario that smoothly leads to 2,3-rearrangement reaction products with catalytic amounts of the iron(III) complexes. Similar findings regarding the superiority of the iron(III) complexes (in terms of catalyst loading, chemical yields, and reaction conditions) were obtained with thiols (S--H insertion) and sulfides (2,3-rearrangement reactions), which suggest similar mechanisms operate in these cases. 相似文献
118.
119.
Zeev Nutov 《Discrete Mathematics》2008,308(12):2533-2543
Let G be a minimally k-connected graph with n nodes and m edges. Mader proved that if n?3k-2 then m?k(n-k), and for n?3k-1 an equality is possible if, and only if, G is the complete bipartite graph Kk,n-k. Cai proved that if n?3k-2 then m?⌊(n+k)2/8⌋, and listed the cases when this bound is tight.In this paper we prove a more general theorem, which implies similar results for minimally k-outconnected graphs; a graph is called k-outconnected from r if it contains k internally disjoint paths from r to every other node. 相似文献
120.