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851.
852.
The reaction of enone 1, bearing an internal nucleophilic moiety, i.e., furan or pyrrole (X = O, NR'), with isocyanides is presented. The formation of products resulting from the reaction of the zwitterionic intermediate 2 with a second equivalent of isocyanide prior to cyclization to give 3, as well as the direct formation of 4 from 2, is described. 相似文献
853.
854.
Rangan S Bournel F Gallet JJ Kubsky S Le Guen K Dufour G Rochet F Sirotti F Piaszenski G Funke R Kneppe M Köhler U 《The journal of physical chemistry. B》2005,109(26):12899-12908
Using a combination of local -- scanning tunneling microscopy -- and spatially integrated, but chemically sensitive probes -- X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy -- we have examined how 3-butenenitrile reacts with the Si(001)-2 x 1 surface at room temperature. Electron spectroscopies indicate three different nitrogen chemical bonds: a Si-C=N-Si bond, a C=C=N cumulative double bond, and a CN moiety datively bonded to a silicon atom. All molecular imprints detected by scanning tunneling microscopy (STM) involve two adjacent silicon dimers in the same row. The three geometries we propose -- a double di-sigma bonding via the CN and the C=C, a cumulative double bond formation associated with alphaC-H bond dissociation, and a di-sigma vinyl bonding plus a CN datively bonded to a silicon atom -- are all compatible with electron spectroscopies and data. Real-time Auger yield kinetic measurements show that the double di-sigma bonding geometry is unstable when exposed to a continuous flux of 3-butenenitrile molecules, as the Si-C=N-Si unit transforms into a CN moiety. A model is proposed to explain this observation. 相似文献
855.
856.
J. Magnes C. Goldberg Sylvie Magnier Monique Aubert-Frécon Li Li 《Journal of Molecular Spectroscopy》2003,221(1):72-79
This paper reports for the first time both, an experimental observation and theoretical calculations of the K2 43Δg state. For the experiment we used cw perturbation-facilitated optical-optical double resonance (PFOODR) spectroscopy. A single mode Ti-sapphire laser and a dye laser served as the pump and probe lasers, respectively. A total of 55 PFOODR signals have been assigned to the 43Δg ← b3Πu transitions. Absolute vibrational numbering was determined by using quantum defect analysis combined with comparing observed intensities with calculated Franck-Condon factors (FCF). For the former we used known parameters from the 23Δg state since the 23Δg and the 43Δg states belong to the same Rydberg series. We report here our experimental and calculated spectroscopic constants, the corresponding RKR potential energy curve, the Franck-Condon table for the 43Δg ↔ b3 Πu system, as well as a comparison with the theoretical potential energy curve. The Te value is found to be 28408.938(52) cm−1. 相似文献
857.
A Bayesian network (BN) is a knowledge representation formalism that has proven to be a promising tool for analyzing gene expression data. Several problems still restrict its successful applications. Typical gene expression databases contain measurements for thousands of genes and no more than several hundred samples, but most existing BNs learning algorithms do not scale more than a few hundred variables. Current methods result in poor quality BNs when applied in such high-dimensional datasets. We propose a hybrid constraint-based scored-searching method that is effective for learning gene networks from DNA microarray data. In the first phase of this method, a novel algorithm is used to generate a skeleton BN based on dependency analysis. Then the resulting BN structure is searched by a scoring metric combined with the knowledge learned from the first phase. Computational tests have shown that the proposed method achieves more accurate results than state-of-the-art methods. This method can also be scaled beyond datasets with several hundreds of variables. 相似文献
858.
Bonose-Crosnier de Bellaistre M Nowik W Tchapla A Heron S 《Journal of chromatography. A》2011,1218(6):778-786
Chromatographic study of natural products helps to determine their molecular composition and to identify their sources (biological, geographical, etc.). However, identifying anthraquinoids is still a challenge because this chemical family is composed of more than half a thousand molecules. In the present work, a series of C18 stationary phases were systematically studied to evaluate their separation potential for these compounds in high performance reversed phase liquid chromatography (RPLC). The stationary phases with various physicochemical properties were evaluated with respect to the separation of 30 standards. Selectivity of the studied stationary phases and symmetry of the obtained peaks were chosen as the evaluation criteria. Calculations of symmetry and separation capacity parameters have been proposed. These parameters are not dependent on the dimensions of the columns and were used for the classifications of phases. In conclusion, this study is intended to be a guide as to which stationary phases analysts can use for resolving separation of a complex mixture of anthraquinoids. 相似文献
859.
Chiang GC Bond AD Ayscough A Pain G Ducki S Holmes AB 《Chemical communications (Cambridge, England)》2005,(14):1860-1862
The enantioselective synthesis of a simplified eleutherobin analogue by ring closing metathesis (RCM) of the 2,9-divinyl-substituted tetrahydro-oxonin is described; the analogue and an advanced intermediate revealed microtubule stabilising properties in the micromolar range. 相似文献
860.
Sylvie Chardon-Noblat Philippe Da Costa Alain Deronzier Matti Haukka Tapani A. Pakkanen Raymond Ziessel 《Journal of Electroanalytical Chemistry》2000,490(1-2)
The mono-bipyridine bis carbonyl complex [Ru(bpy)(CO)2Cl2] exists in two stereoisomeric forms having a trans(Cl)/cis(CO) (1) and cis(Cl)/cis(CO) (2) configuration. In previous work we reported that only the trans(Cl)/cis(CO) isomer 1 leads by a two-electron reduction to the formation of [Ru(bpy)(CO)2]n polymeric film on an electrode surface. This initial statement was overstated, as both isomers allowed the build up of polymers. A detailed comparison of the electropolymerization of both isomers is reported here, as well as the reduction into dimers of parent stereoisomer [Ru(bpy)(CO)2(C(O)OMe)Cl] complexes 3 and 4 obtained as side products during the synthesis of 1 and 2. 相似文献