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81.
Synthesis and Structure of two Mixed Substituted Dialanes Al2X2{Si(SiMe3)3}2 · 2 THF (X = Cl, Br) The syntheses of tris(trimethylsilyl)silyl (hypersilyl) and halide substituted dialanes Al2X2{Si(SiMe3)3}2 · 2 THF (X = Cl, Br) are presented. The results of the X‐ray diffraction experiments are presented and discussed in comparison to the AlIII compounds AlBr2Si(SiMe3)3 · THF and AlBr3 · OPh2. 相似文献
82.
Michael Arend Bernhard Westermann Nikolaus Risch 《Angewandte Chemie (International ed. in English)》1998,37(8):1044-1070
Important building blocks for the synthesis of drugs or natural products are found in Mannich bases and their derivatives. Modern variants of the Mannich reaction that expand the potential of the classical intermolecular reaction significantly and enable efficient control of the regioselectivity and stereoselectivity are therefore the topic of intensive research. Intramolecular reactions, in particular as part of domino reaction sequences, often afford astoundingly simple and elegant approaches to complex target compounds. 相似文献
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84.
Matthias Albert Dieter Seebach Elke Duchardt Harald Schwalbe 《Helvetica chimica acta》2002,85(2):633-658
Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph2Si and C‐terminal PhCH2 protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe2), and Leu (CH2CHMe2). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl2](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H2 (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)2, CH2Cl2, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF3CH2OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl3 solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 21‐ or a 31‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na+, K+, or Ba2+ are alluded to (Figs. 7–9). 相似文献
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88.
Synthesis and Structure of Mo2NCl7 The reaction of VN with MoCl5 at 175 °C in a sealed glass ampoule yields the molybdenum(V) nitride chloride Mo2NCl7 in form of air sensitive black crystals with the triclinic space group P1¯ and a = 905.7(8); b = 975.4((6); c = 1283.4(8) pm, α = 103.13(4)°; β = 109.83(5)° und γ = 98.58(5)°. The crystal structure is built up from dinuclear units [Mo2N2Cl7]3— and [Mo2Cl7]3+, which are connected by asymmetric nitrido bridges to form endless chains. Within both dinuclear units the Mo atoms are bridged by three Cl atoms resulting in a Mo‐Mo distance of 349.2(3) pm in the unit [Mo2N2Cl7]3—. In case of [Mo2Cl7]3+, however, a shorter Mo‐Mo distance of 289.4(3) pm is observed, which can be interpreted by a single bond. Correspondingly a reduced magnetic moment of 0.95 B.M. per Mo atom is observed. 相似文献
89.
Alexander Donchev Andreas Schnepf Elke Baum Gregor Stßer Hansgeorg Schnckel 《无机化学与普通化学杂志》2002,628(1):157-161
[Ga6R8]2– (R = SiPh2Me): A Metalloid Cluster Compound with an Unexpected Ga6‐Frame The reaction of a metastable solution of GaBr with a solution of LiSiPh2Me in a toluene/THF mixture results in orange coloured crystals of [Ga6(SiPh2Me)8]2– · 2 [Li(THF)4]+ ( 1 ). The unexpected structure of the planar Ga6 frame (C2h) could also be realized with the help of DFT calculation. DFT calculations furthermore show that 1 is energetically favoured against an octahedral Ga6R62– species and R2. In contrast calculations for the similar Al and B species show that in these cases the octahedral entities are favoured. These results demonstrate that even for similar compounds of B, Al, and Ga Wade rules are too general and that they cannot predict the correct structure. Moreover the atomic arrangement within 1 shows that a structure is preferred which is also present in allotropic β‐Ga and that therefore clusters of this type should be called metalloid or more general elementoid. 相似文献
90.