A newchola lariat ether (1, a 21-crown-6) was constructed fromreadily available precursors. The association constant of compound1 with alkali metal picrates was measured using Cram’s extraction protocol. Evidence is presented for the involvement of the
3-methoxy group for the complexation. Energy minimised structures show that the A-ring gets slightly distorted upon metal
ion binding.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
In this paper, we report for the first time finite size effects on Néel temperature (TN) of magnetoelectric BiFeO3 system. Novel wet chemical route has been developed to produce fine particles of BiFeO3 with controlled size and size distribution. Unlike other oxide systems, lattice volume contraction has been observed with
decrease in particle size. The decrease in TN is co-related to unit cell volume contraction occurring with reduction in particle size. 相似文献
17-Chlorosteroids from template-directed halogenation can be converted to 17(20) olefins with DBU or DBN. The dependence of this reaction on base-structure and reaction conditions suggests geometric control by liquid packing effects related to those operating in liquid crystals. 相似文献
The aggregation properties of novel bile acid analogs-phosphonobile salts (PBS)-have been studied. The critical micellar concentration of 23 and 24-phosphonobile salts were measured using fluorescence and 31P NMR methods. All the ten synthesized phosphonobile salts formed gels at different pH ranges in water. The pH range at which individual PBSs could gelate water was narrow and influenced by the number and conformation of hydroxyl groups. A reversible thermochromic system has been developed (with 23-phosphonodeoxycholate at pH 3.3), which changes color upon gelation. The investigation of the first hydrogels derived from trihydroxy bile acid analogs 1 and 6 was made using fluorescence, 31P NMR, X-ray crystallography, circular dichroism and SEM. The present studies reveal that the gel network consists of a chiral, fibrous structure possessing hydrophobic interiors. 相似文献
We show that (27)Al triple-quantum magic angle spinning (3Q-MAS) experiments alleviate the second-order quadrupolar broadening to reveal the structure-building units of nonequivalent aluminum octahedra in the most extensively studied aluminum hydroxides, namely, gibbsite, bayerite, and boehmite. Further, aided by ab initio calculations of the electric field gradient tensors, the 3Q-MAS/MAS results are shown to lead to the assignment of (27)Al isotropic resonances to the aluminum positions in their X-ray-determined structures. The present work paves the way for future studies on various structurally transformed materials derived from these basic aluminum hydroxides. 相似文献
In situ non-invasive 3D characterization of membrane fouling was achieved using femtosecond near infrared non-linear optical imaging together with a novel crossflow filtration module. Washed fluorophore-labelled yeast suspensions were filtered through Millipore 0.22 μm mixed cellulose ester membranes and the fouling layer was imaged at different times throughout the experiment.
Based on the 3D femtosecond images, it has been possible to identify fine structural features of the cake and to measure the thickness of the filter cake formed on the microfiltration (MF) membranes. Our findings reveal that low concentration feeds result in the initial formation of a patchy monolayer of cells leading to a multilayered cake, whilst at higher concentrations a multilayer cake forms rapidly. For patchy cakes, the technique offers greater resolution than that which is achievable with the direct observation through membrane technique. Deposited cell aggregates and broken fragments of cells can clearly be imaged. For thick cakes, it has been possible to image up to depths 45 μm below the cake surface in the present work. 相似文献
Tripodal cholamide 1 is a supergelator of aqueous fluids. A variety of physical techniques, including cryo-transmission electron microscopy (TEM), circular dichroism (CD), steady-state fluorescence, time-resolved fluorescence, and dynamic light-scattering, were employed to understand the structure and dynamics of the gel. Fluorescent probes [ANS (8-anilinonaphthalene-1-sulfonic acid) and pyrene] reported two critical aggregation concentrations (CAC(1) and CAC(2)) of 1 in predominantly aqueous media, with the minimum gel concentration (MGC) being close to CAC(2). Fluorescence lifetime measurements with pyrene revealed ineffective quenching of pyrene fluorescence by oxygen, possibly caused by slower Brownian diffusion due to the enhanced viscosity in the gel phase. The study of the gelation kinetics by monitoring the ultrafast dynamics of ANS revealed a progressive increase in the aggregate size and the microviscosity of the aqueous pool encompassed by the self-assembled fibrillar network (SAFIN) during the gelation. The striking difference between microviscosity and bulk (macroscopic) viscosity of the gel is also discussed. 相似文献
The Maitra group has explored a variety of chemistry with bile acids during the past 15 years and these experiments have covered a wide variety of chemistry-asymmetric synthesis, molecular recognition, ion receptors/sensors, dendrimers, low molecular mass organo and hydrogelators, gel-nanoparticle composites, etc. Some of what excites us in this field is highlighted in this perspective article. 相似文献