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排序方式: 共有957条查询结果,搜索用时 15 毫秒
1.
Pedro lvarez‐Boo Jos Sergio Casas Alfonso Castieiras María Delfina Couce Eduardo Freijanes Eva Novoa Jos Sordo 《应用有机金属化学》2003,17(9):725-729
Reaction of dichloro‐ and dibromodimethyltin(IV) with 2‐(pyrazol‐1‐ylmethyl)pyridine (PMP) afforded [SnMe2Cl2(PMP)] and [SnMe2Br2(PMP)] respectively. The new complexes were characterized by elemental analysis and mass spectrometry and by IR, Raman and NMR (1H, 13C) spectroscopies. Structural studies by X‐ray diffraction techniques show that the compounds consist of discrete units with the tin atom octahedrally coordinated to the carbon atoms of the two methyl groups in a trans disposition (Sn? C = 2.097(5), 2.120(5) Å and 2.110(6), 2.121(6) Å in the chloro and in the bromo compounds respectively), two cis halogen atoms (Sn? Cl = 2.4908(16), 2.5447(17) Å; Sn? Br = 2.6875(11), 2.7464(9) Å) and the two donor atoms of the ligand (Sn? N = 2.407(4), 2.471(4) Å and 2.360(5), 2.455(5) Å). In both cases, the Sn? N(pyridine) bond length is markedly longer than the Sn? N(pyrazole) distance. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
Marino JP Rubio MB Cao G de Dios A 《Journal of the American Chemical Society》2002,124(45):13398-13399
A new strategy was developed for the enantiospecific synthesis of aspidosperma alkaloids. The key steps involve a novel ketene-lactonization reaction of a chiral vinyl sulfoxide to efficiently set up the quaternary carbon center, and a tandem Michael addition-alkylation reaction sequence to form the polycyclic core structure. This new strategy was employed in the total synthesis of natural product (+)-aspidospermidine. 相似文献
3.
The reactivity of the fluorescent reagent calcein with the trivalent cations of the rare earths has been spectrofluorimetrically studied in aqueous solution. Optimum excitation and emission wavelengths were 492-497 and 519-522 nm, respectively. Optimum pH was in the range 6.0-9.2. The stoichiometry of the complexes was 1:1. A direct, rapid and sensitive method for the determination of rare earth mixtures has been proposed with a detection limit of 4.49x10(-8) M and a coefficient of variation of 0.82%. 相似文献
4.
Bandini M Fagioli M Garavelli M Melloni A Trigari V Umani-Ronchi A 《The Journal of organic chemistry》2004,69(22):7511-7518
A new catalytic system for the first example of enantioselective Friedel-Crafts-type (FC) addition of indoles to simple enones is described. The use of an equimolar amount of chiral [Al(salen)Cl] and 2,6-lutidine (10 mol %) was found to be effective in promoting the conjugate addition of indoles to (E)-arylcrotyl ketones, furnishing the corresponding beta-indolyl ketones in excellent yield and high enantioselectivity (ee up to 89%). The role of the base was investigated through spectroscopic as well as computational analyses, which suggested that in situ formation of a new chiral (base.[Al(salen)]) complex was operating under our reaction conditions. In particular, a stable cationic [Al(salen)] hexacoordinate trans complex with the additive base and the enone is suggested as being responsible for the stereocontrolled reaction. Finally, detailed monitoring of the reaction course was carried out showing that a conventional FC pathway induced by [Al(salen)Cl] acting as a Lewis acid is operating. 相似文献
5.
Souček [1, 2] discovered an intriguing connection between the standard twistor correspondence and the biquaternionic projective
line
The biquaternionic projective point,
also has twistor structure corresponding to the collection of α- or β-planes passing through the origin in spacetime. The
duality between α- or β-planes is shown to correspond to the choice of left vs. right scalar action. Moreover, we find that
is homeomorphic to the scheme
相似文献
6.
Gañán-Calvo AM 《Physical review letters》2007,98(13):134503
Electro-flow focusing, a technique combining the features of electrospray (ES) and flow focusing (FF), provides a reliable tool to reach parametrical microjetting ranges not attainable by ES or FF alone under specific operational regimes (liquid properties and flow rate). In this Letter, we provide not only a closed theoretical model predicting the diameter of a high electrical conductivity electro-flow focused liquid microjet, but also its convective or absolute instability, linked to the jetting-to-dripping transition and the minimum liquid flow rate that can be ejected in steady jetting regime, in which the smallest droplets are issued. Good agreement is found with experimental values. 相似文献
7.
Arezou Rezazadeh Josep Font-Segura Alfonso Martinez Albert Guilln i Fbregas 《Entropy (Basel, Switzerland)》2021,23(5)
This paper studies a generalized version of multi-class cost-constrained random-coding ensemble with multiple auxiliary costs for the transmission of N correlated sources over an N-user multiple-access channel. For each user, the set of messages is partitioned into classes and codebooks are generated according to a distribution depending on the class index of the source message and under the constraint that the codewords satisfy a set of cost functions. Proper choices of the cost functions recover different coding schemes including message-dependent and message-independent versions of independent and identically distributed, independent conditionally distributed, constant-composition and conditional constant composition ensembles. The transmissibility region of the scheme is related to the Cover-El Gamal-Salehi region. A related family of correlated-source Gallager source exponent functions is also studied. The achievable exponents are compared for correlated and independent sources, both numerically and analytically. 相似文献
8.
Alfonso Romero 《manuscripta mathematica》1987,59(3):261-276
The rigidity for full holomorphic isometric immersions of an indefinite Kähler manifold into an indefinite complex space form is proved. All such immersions between indefinite complex projective (and hyperbolic) spaces are founded and examples of non-congruents holomorphic isometric immersions are exposed. 相似文献
9.
Rafael Hernández García Moni Behar Johnny Ferraz Dias Margarita Edelia Villanueva Tagle María Dolores Durruthy Rodríguez Francisco de Paula Calderón Piñar Mario Simeón Pomares Alfonso 《X射线光谱测定》2012,41(3):156-163
A particle‐induced X‐ray emission (PIXE) technique has been used in the determination of the principal components Pb, Zr and Ti and the substituting elements Sr, Cr, Nb and La in lead zirconate titanate ceramics. In general, precision of analysis was concentration dependent from each element under study. For Pb, precision varied between 0.13% and 0.16%, at higher concentration of 59.32–64.5%. It was around 6–9% for Sr, Cr and Nb at concentrations of 1% or lower. Particular attention was devoted to the estimation of the analysis trueness. With this purpose, three methods were applied: (1) comparison of PIXE and inductively coupled plasma optical emission spectrometry concentrations, (2) recovery study and (3) comparison with a laboratory standard. Trueness of analysis was around 100 ± 10% for the evaluated elements Pb, Zr, Ti, Sr and Cr. The expected stoichiometry and elemental composition homogeneity of a wide group of produced ceramics were confirmed by the PIXE technique. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
10.
Marcolan M Martins PA Pedrosa VA Rodrigues MR de Oliveira HP Codognoto L 《Journal of fluorescence》2011,21(2):733-738
A simple, rapid and effective analytical method based on fluorescence spectroscopy for the determination of coumarin in pharmaceutical
formulations without pre-treatment or pre-concentration step was development. Coumarin had maximum excitation and emission
at 310 nm and 390 nm, respectively. Optimum conditions for the detection of coumarin were investigated. Under optimized conditions,
we observed a linear behavior for the sign of coumarin in the concentration range of 2.5 × 10−6 to 1.0 × 10−4 mol L−1, with linearity of 0.998 and sensitivity of 2.9 × 1010 u.a/mol L−1. The proposed method was validated in terms of accuracy, precision and specificity of coumarin using the standard addition
and external calibration. It was noted that the results support (P < 0.05), indicating that the matrices were not an interference in the determination of coumarin by fluorescence spectroscopy.
The results were favorable compared with those obtained by reference chromatographic methods. 相似文献