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951.
Krüger O  Wille U 《Organic letters》2001,3(10):1455-1458
[reaction: see text] Photochemically generated nitrate radicals (NO(3)(*)) cleave the stereoisomeric N,N-dimethyl-substituted uracil cyclobutane dimers 1a-d into the monomeric uracil derivative 2 as the major reaction pathway. A preferred splitting of the syn dimers 1a,b was observed. The reaction is expected to proceed through initial one-electron oxidation with formation of an intermediate cyclobutane radical cation 11. In addition to cycloreversion, competing reaction steps of 11, which lead to the observed byproducts, are suggested.  相似文献   
952.
Imitations of amber have been prepared since a long time, but the number of imitations is increasing since modern polymers are available. At present, many imitations are based on a few synthetic polymers and even experts have some problems in distinguishing between real and falsed amber without destruction of the material. IR-Spectroscopy or pyrolysis combined with gas chromatography/mass spectrometry are very efficient methods in identifying amber and imitations but these methods need a sample preparation. By means of near infrared reflection spectroscopy it is possible to identify amber and imitations without any sample preparation in a short time. Optimization of the method and the application are discussed.  相似文献   
953.
Zusammenfassung Wird die gesamte Energietönung eines Vorganges vom TypusA starr +B gasförmig AB starr mitE 1 bezeichnet, die Energie der Chemoadsorption vonB anAB mitE 2, die der Oberflächenspannung entsprechende, auf 1 Mol sich beziehende Oberflächenarbeit bei der PhaseA mitE 3 und bei der PhaseAB mitE 4, so besteht die BeziehungE 1 +E 3E 2E 4 =0 . Der isotherme Abbau nach derGibbsschen Funktion kann nur stattfinden, wennE 3 vergleichsweise sehr groß (unendlich) ist, derjenige nach derLangmuirschen Funktion, wennE 3 sehr klein (Null) ist.  相似文献   
954.
The oxidation of yellow Cr(CO)5NH2R complexes (NH2R = aniline, m-toluidine, 3,5-xylidine, m-anisidine) with Pbac4 gives deep blue to deep purple coloured compounds, which have been identified as the respective [Cr(CO)5(N-phenyl-1,4-benzochinon-diimine)] complexes. Oxidation of the p-phenylenediamine complex yields [(OC)5CrHNC6H4NHCr(CO)5], which is also deep blue. The binuclear blue complex [{Cr(CO)5}2HNC6H4NC6H5] is obtained by treating Cr(CO)5THF with the free ligand in THF/hexane; it dissociates rapidly in acetone to form [Cr(CO)5HNC6H4NC6H5] and Cr(CO)5. Analogous Mo(CO)5 and W(CO)5 complexes could not be obtained. The oxidation of [W(CO)5(m-anisidine)] by I2 yields [W(CO)4I]2. All the compounds were characterized by spectroscopic methods as well as by elemental analysis.  相似文献   
955.
The ESR. sprectra of DPPH in mixed organic solvents have been measured with a K-band superheterodyne spectrometer and self-locking magnetometer with high precision. Approximately 150 lines have been resolved in the spectra. Both the zero and first order derivative spectra were analyzed by a line shape analysis program for determination of the coupling constants. The smallest coupling constant was found to be 377 ± 5 mG, all other coupling parameter are multiples of this constant. A set of most probable coupling constants is given. The two nitrogen coupling constants are found to be equal within experimental error, and there exist several rotational relations and several possible assignments.  相似文献   
956.
[reaction: see text] The enzymatic degradation of starch can be monitored electronically using single-walled carbon nanotubes (SWNTs) as semiconducting probes in field-effect transistors (FETs). Incubation of these devices in aqueous buffer solutions of amyloglucosidase (AMG) results in the removal of the starch from both the silicon surfaces and the side walls of the SWNTs in the FETs, as evidenced by direct imaging and electronic measurements.  相似文献   
957.
In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992  相似文献   
958.
The adsorption of 4,4-bipyridyl by natural sepiolite and smectite group clay minerals (bentonite, hectorite and saponite) from Anatolia (Turkey) has been studied using vibrational spectroscopy. Investigation of Fourier-transform infrared and Fourier-transform Raman spectra of adsorbed 4,4-bipyridyl indicate the presence of chemisorbed species. However, any evidence for the generation of anionic species on the surface of the phyllosilicates has not been detected. It is proposed that the adsorbed bipyridyl molecules on sepiolite are centrosymmetric and H-bonded to the surface hydroxyls through both the nitrogen lone pairs as bidentate ligands. The adsorbed bipyridyl molecules on the smectite group clays are coordinated to exchangeable cations both directly and also indirectly through water as monodentate ligands. XRD patterns of the clays studied are also recorded.  相似文献   
959.
Müller P  Nury P 《Organic letters》2000,2(18):2845-2847
[reaction: see text] ++-Phenyl-1,3-dioxolanes (1) react with organolithium reagents (2), associated with (-)-sparteine, in the presence of BF(3).OEt(2) to afford chiral monosubstitution products 3. Enantioselectivity is highest if both 1 and 2 carry alkyl substituents in the ortho position. However, the enantioselectivity decreases in the case of very bulky substituents such as tert-butyl or phenyl.  相似文献   
960.
A new system for palladium-catalyzed arylation of a broad spectrum of free (NH)-heteroarenes has been developed (indole, pyrrole, pyrazole, 2-phenylimidazole, imidazole, benzimidazole, and purine). Remarkable selectivity has been achieved in the presence of MgO base, providing single C-arylation products, while no N-arylation and no bis-arylation products have been detected. In the case of free imidazole, exclusive C-4 arylation may be switched to exclusive 2-arylation by the addition of CuI to the Pd/Ph3P/MgO system. When free aryl-(NH)-azoles are desired, direct arylation eliminates three steps in comparison to standard methods, including N-protection, stoichiometric metalation or halogenation, and N-deprotection.  相似文献   
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