Summary: Laser‐induced fluorescence spectroscopy of the optical probe Nile Blue A in polymer clay nanocomposites is described. Concentration quenching of the fluorescence dominates the probe behavior until the clay platelets are physically separated by polymer intercalation. Further separation into an exfoliated structure results in an intense increase in probe fluorescence. Preliminary results indicate the ability to discriminate between intercalated and exfoliated structures in nanocomposites formed by melt processing.
Zusammenfassung Nach einer Einleitung betreffend den Stand des Problems zur Bestimmung der Acidität von Cellulosen wird über die Experimente der Anfärbung von chemisch unbehandelter Baumwolle mit der Pseudobase des Kristallvioletts berichtet. Die Umsetzung fand im wasserfremden, organischen Medium (Benzol) statt und wurde bezüglich ihrer Mengen-und Zeitabhängigkeit untersucht.Anschließend werden die Resultate der Anfärbung von vier standardisierten Baumwollproben mitgeteilt.Mit 2 AbbildungenHerrn Prof. Dr.F. Wessely zu seinem 60. Geburtstag gewidmet. 相似文献
One‐electron oxidation of the disilicon(0) compound Si2(Idipp)2 ( 1 , Idipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) with [Fe(C5Me5)2][B(ArF)4] (ArF=C6H3‐3,5‐(CF3)2) affords selectively the green radical salt [Si2(Idipp)2][B(ArF)4] ( 1 ‐[B(ArF)4). Oxidation of the centrosymmetric 1 occurs reversibly at a low redox potential (E1/2=?1.250 V vs. Fc+/Fc), and is accompanied by considerable structural changes as shown by single‐crystal X‐ray structural analysis of 1 ‐B(ArF)4. These include a shortening of the Si?Si bond, a widening of the Si‐Si‐CNHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1+ . Comparative quantum chemical calculations of 1 and 1+ indicate that electron ejection occurs from the symmetric (n+) combination of the Si lone pairs (HOMO). EPR studies of 1 ‐B(ArF)4 in frozen solution verified the inequivalency of the two Si sites observed in the solid‐state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar 29Si hyperfine coupling tensors in 1+ . EPR studies of 1 ‐B(ArF)4 in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1+ . 相似文献
Structural studies of HIV‐1 Gag, the primary structural polyprotein involved in retroviral assembly, have been challenging, owing to its flexibility and conformational heterogeneity. Using residual dipolar couplings, we show that the four structural units of the capsid (CA)–spacer peptide 1 (SP1)–nucleocapsid (NC) fragment of HIV‐1 Gag (namely, the N‐ and C‐terminal domains of capsid, and the N‐ and C‐terminal Zn knuckles of nucleocapsid) have the same structures as their individually isolated counterparts, and tumble semi‐independently of one another in the absence of nucleic acids. Nucleic acids bind exclusively to the nucleocapsid domain and fix the orientation of the two Zn knuckles relative to one another so that the nucleocapsid domain/nucleic acid complex behaves as a single structural unit. The low 15N–{1H} heteronuclear NOE values (≤0.4), the close to zero values for the residual dipolar couplings of the backbone amides, and minimal deviations from random‐coil chemical shifts for the C‐terminal tail of capsid and SP1, both in the absence and presence of nucleic acids, indicate that these regions are intrinsically disordered in the context of CA–SP1–NC. 相似文献
We report an efficient and highly stereoselective strategy for the synthesis of Aspidosperma alkaloids based on the transannular cyclization of a chiral lactam precursor. Three new stereocenters are formed in this key step with excellent diastereoselectivity due to the conformational bias of the cyclization precursor, leading to a versatile pentacyclic intermediate. A subsequent stereoselective epoxidation followed by a mild formamide reduction enabled the first total synthesis of the Aspidosperma alkaloids (?)‐mehranine and (+)‐(6S,7S)‐dihydroxy‐N‐methylaspidospermidine. A late‐stage dimerization of (?)‐mehranine mediated by scandium trifluoromethanesulfonate completed the first total synthesis of (?)‐methylenebismehranine. 相似文献
The self‐assembly of organic TCNQF.? radicals (2‐fluoro‐7,7,8,8‐tetracyano‐p‐quinodimethane) and the anisotropic [Tb(valpn)Cu]3+ dinuclear cations produced a single‐chain magnet (SCM) involving stacking interactions of TCNQF.? radicals (H2valpn is the Schiff base from the condensation of o‐vanillin with 1,3‐diaminopropane). Static and dynamic magnetic characterizations reveal that the effective energy barrier for the reversal of the magnetization in this hetero‐tri‐spin SCM is significantly larger than the barrier of the isolated single‐molecule magnet based on the {TbCu} dinuclear core. 相似文献
