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121.
122.
Microstructural and magnetic changes induced by ball milling in Nd2Fe17 alloy have been investigated. X-ray and neutron powder diffraction studies have shown that the main crystalline phase present in the as-cast Nd2Fe17 compound is the rhombohedral Th2Zn17-type crystal structure. Contrary to other materials, the crystal structure does not change after milling, and the crystal lattice parameters slightly increases while the induced strain is less than 0.1%. It has been observed from SEM and TEM images that the microstructure consists of agglomerates of nanoparticles with a mean size around 20 nm, and from magnetic measurements a broadening of the temperature range in which ferromagnetic to paramagnetic transition takes place.  相似文献   
123.
We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π–π interactions or hydrogen bonding) in yields of generally 60–80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et3N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs2CO3 and room temperature).  相似文献   
124.
Cellulose - In this work a wide sample analysis, under similar conditions, has been carried out and a calibration strategy based on a careful selection of input variables combined with sensitivity...  相似文献   
125.
Near-infrared spectroscopy (NIRS) has been widely used in the pharmaceutical field because of its ability to provide quality information about drugs in near-real time. In practice, however, the NIRS technique requires construction of multivariate models in order to correct collinearity and the typically poor selectivity of NIR spectra. In this work, a new methodology for constructing simple NIR calibration models has been developed, based on the spectrum for the target analyte (usually the active principle ingredient, API), which is compared with that of the sample in order to calculate a correlation coefficient. To this end, calibration samples are prepared spanning an adequate concentration range for the API and their spectra are recorded. The model thus obtained by relating the correlation coefficient to the sample concentration is subjected to least-squares regression. The API concentration in validation samples is predicted by interpolating their correlation coefficients in the straight calibration line previously obtained. The proposed method affords quantitation of API in pharmaceuticals undergoing physical changes during their production process (e.g. granulates, and coated and non-coated tablets). The results obtained with the proposed methodology, based on correlation coefficients, were compared with the predictions of PLS1 calibration models, with which a different model is required for each type of sample. Error values lower than 1-2% were obtained in the analysis of three types of sample using the same model; these errors are similar to those obtained by applying three PLS models for granules, and non-coated and coated samples. Based on the outcome, our methodology is a straightforward choice for constructing calibration models affording expeditious prediction of new samples with varying physical properties. This makes it an effective alternative to multivariate calibration, which requires use of a different model for each type of sample, depending on its physical presentation.  相似文献   
126.
Styrene and montmorillonite organically modified with imidazolium surfactants (MMT) at various alkyl chain lengths (C12, C16 and C18) were used to prepare the corresponding PS/MMT/C12, PS/MMT/C16 and PS/MMT/C18 nanocomposites by in situ polymerization. XRD and TEM analyses evidenced the formation of both intercalated and exfoliated structures. The glass transition temperatures (T g) of nanocomposites, as well as that of neat PS, were obtained by DSC measurements. The thermal degradations were carried out in the scanning mode, in both inert and oxidative environments, and the initial temperatures of decomposition (T i) and the apparent activation energies of degradation (E a) were determined. Due to an oxidative degradation mechanism, the T i and E a values in air atmosphere were lower than those under nitrogen. The results indicated that nanocomposites are more thermally stable than polystyrene, and suggested an increasing degree of exfoliation as a function of alkyl chain length of surfactant, associated with enhancing thermal stability.  相似文献   
127.
A simple one-pot and efficient method is described for the synthesis of pyrazinothienopyrimidines 6 by domino processes involving aza-Wittig/intermolecular nucleophilic addition/intramolecular cyclization. A tandem aza-Wittig reaction of phosphazenes 7, derived from 6, with heterocumulenes (isocyanates, carbon disulfide or carbon dioxide) generates the pyrazinothienotriazolopyrimidinones 9, 11 and 12, respectively. Pyrazino[2′,3′:4,5]thieno[3,2-d]-1,2,4-triazolo[1,5-a]pyrimidin-4(3H)-ones 15 and bis(pyrazinothienotriazolopyrimidinones) 17 were synthesized by the intermolecular aza-Wittig reaction of phosphazenes 7 with acyl chlorides or α,ω-dichlorides followed by heterocyclization via imidoyl chloride intermediate 16. Further S-alkylation of 11 and reaction of 6 with phosgeniminium chloride produce 2-alkylthio- and 2-N,N-dimethylaminopyrazinothienotriazolopyrimidinones 13 and 19, respectively.  相似文献   
128.
A convenient and straightforward procedure has been developed for the preparation of functionalized 3,4-ethylenedithiathiophene (EDTT) derivatives by using a new bromomethyl-EDTT derivative as a versatile synthon. Based on this procedure a novel EDTT derivative endowed with 9,10-anthraquinone moieties has been synthesized and electrochemically polymerized to yield a donor-acceptor PEDTT derivative.  相似文献   
129.
An efficient chromium-promoted alkyl- or silylcyclopropanation of alpha,beta-unsaturated amides is described. These reactions can be carried out on (E)- or (Z)-alpha,beta-enamides in which the C-C double bond is di-, or trisubstituted. This process takes place with total stereospecificity and the new stereogenic center is generated with high or total stereoselectivity. Some synthetic applications of the obtained silylcyclopropyl amides are also reported. Two mechanisms based on the generation of carbenoid or carbene complexes have been proposed to explain this cyclopropanation reaction.  相似文献   
130.
We report the synthesis and characterization of a novel type of nanohoop, consisting of a cycloparaphenylene derivative incorporating a curved heptagon-containing π-extended polycyclic aromatic hydrocarbon (PAH) unit. We demonstrate that this new macrocycle behaves as a supramolecular receptor of curved π-systems such as fullerenes C60 and C70, with remarkably large binding constants (ca. 107 M−1), as estimated by fluorescence measurements. Nanosecond and femtosecond spectroscopic analysis show that these host-guest complexes are capable of quasi-instantaneous charge separation upon photoexcitation, due to the ultrafast charge transfer from the macrocycle to the complexed fullerene. These results demonstrate saddle-shaped PAHs with dibenzocycloheptatrienone motifs as structural components for new macrocycles displaying molecular receptor abilities and versatile photochemical responses with promising electron-donor properties in host-guest complexes.  相似文献   
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