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31.
A highly enantioselective epoxidation of cyclic enones with hydrogen peroxide has been developed that is catalyzed by chiral primary amine salts. 相似文献
32.
DNA--metal base pairs 总被引:1,自引:0,他引:1
Recent developments show encouraging results for the use of DNA as a construction material for nanometer-sized objects. Today, however, DNA-based molecular nanoarchitectures are constructed with mainly unmodified or at best end-modified oligonucleotides, thus shifting the development of functionalized DNA structures into the limelight. One of most recent developments in this direction is the substitution of the canonical Watson-Crick base pairs by metal complexes. In this way "metal-base pairs" are created, which could potentially impart magnetic or conductive properties to DNA-based nanostructures. This review summarizes research which started almost 45 years ago with the investigation of how metal ions interact with unmodified DNA and which recently culminated in the development of artificial ligand-like nucleobases so far able to coordinate up to ten metal ions inside a single DNA duplex in a programmable fashion. 相似文献
33.
Reisinger CM Nowack RJ Volkmer D Rieger B 《Dalton transactions (Cambridge, England : 2003)》2007,(2):272-278
A route to various substituted phosphine phosphonic acid compounds of the general form Ar(2)PC(6)H(4)PO(OH)(2) (Ar = Ph, o-MeC(6)H(4), o-MeOC(6)H(4)) has been investigated. These compounds were employed as bidentate anionic [P,O] ligands in neutral palladium complexes. The [P,O] chelating coordination was determined by X-ray crystallography of a representative palladium complex. Furthermore, the bifunctional ligand Ph(2)PC(6)H(4)PO(OH)Ph represents the first example of a chelating anionic [P,O] ligand resulting from the combination of a phosphine and a phosphinate moiety. 相似文献
34.
Ce3Cl5[SiO4] and Ce3Cl6[PO4]: A Chloride‐Rich Chloride Silicate of Cerium as Compared to the Phosphate By reacting CeCl3 with CeO2, cerium and SiO2, or P2O5, respectively, in molar ratios of 5 : 3 : 1 : 3 or 8 : 3 : 1 : 2, respectively, in sealed evacuated silica tubes (7 d, 850 °C) colorless, rod‐shaped single crystals of Ce3Cl5[SiO4] (orthorhombic, Pnma; a = 1619.7(2), b = 415.26(4), 1423.6(1) pm; Z = 4) and Ce3Cl6[PO4] (hexagonal, P63/m; a = 1246.36(9), c = 406.93(4) pm; Z = 2) are obtained as products insensitive to air and water. Excess cerium trichloride as flux promotes crystal growth and can be rinsed off again with water after the reaction. The crystal structures are determined by discrete [SiO4]4– or [PO4]3– tetrahedra as isolated units. Both, the chloride silicate Ce3Cl5[SiO4] and the chloride phosphate Ce3Cl6[PO4], exhibit structural similarities to CeCl3 (UCl3 type), when four or three Cl– anions are each substituted formally by one [SiO4]4– or [PO4]3– unit, respectively, in the tripled formula (Ce3Cl9). The coordination number for Ce3+ is thus raised from nine in CeCl3 to ten in Ce3Cl5[SiO4] and Ce3Cl6[PO4], along with a drastic reduction of the molar volume with the transition from Ce3Cl9 (Vm = 186.17 cm3/mol) to Ce3Cl5[SiO4] (Vm = 144.15 cm3/mol) and Ce3Cl6[PO4] (Vm = 164.84 cm3/mol). The polyhedra of coordination around Ce3+ can be described as quadruple‐capped trigonal prisms, which in addition to seven Cl– anions each also show another three oxygen atoms of two ortho‐silicate or ortho‐phosphate tetrahedra, respectively. 相似文献
35.
Pr4S3[Si2O7] and Pr3Cl3[Si2O7]: Derivatives of Praseodymium Disilicate Modified by Soft Foreign Anions For synthesizing both the disilicate derivatives Pr4S3[Si2O7] and Pr3Cl3[Si2O7], Pr, Pr6O11 and SiO2 are brought to reaction with S and PrCl3, respectively, in suitable molar ratios (850 °C, 7 d) in evacuated silica tubes. By using NaCl as a flux, Pr4S3[Si2O7] crystallizes as pale green, transparent single crystals (tetragonal, I41/amd, a = 1201.6(1), c = 1412.0(2) pm, Z = 8) with the appearance of slightly compressed octahedra. On the other hand, Pr3Cl3[Si2O7] emerges as pale green, transparent platelets and crystallizes monoclinically (space group: P21, a = 530.96(6), b = 1200.2(1), c = 783.11(8) pm, β = 109.07(1)°, Z = 2). In both crystal structures ecliptically conformed [Si2O7]6– units of two corner‐linked [SiO4] tetrahedra with Si–O–Si bridging angles of 131° in the sulfide and 148° in the chloride disilicate are present. In Pr4S3[Si2O7] both crystallographically independent Pr3+ cations show coordination numbers of 8 + 1 (5 S2– and 3 + 1 O2–) and 9 (3 S2– and 6 O2–). For Pr1, Pr2 and Pr3 in Pr3Cl3[Si2O7] coordination numbers of 10 (5 Cl– and 5 O2–) and 9 (2 ×; 4 Cl– and 5 O2– or 3 Cl– and 6 O2–, respectively) occur. 相似文献
36.
Catherine Pale-Grosdemange Corinna Feil Michel Rohmer Karl Poralla 《Angewandte Chemie (International ed. in English)》1998,37(16):2237-2240
Tetracyclic triterpenes (for example, 3 ) are formed as minor products of the cyclization of squalene ( 1 ) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2 ) in the cyclization process. 相似文献
37.
Pr(BO2)3 and PrCl(BO2)2: Two Praseodymium meta‐Borates in Comparison Single‐crystalline PrCl(BO2)2 can be obtained by the reaction of praseodymium, Pr6O11 and PrCl3 with a small excess of B2O3 in evacuated silica tubes after seven days at 850 °C. If NaCl is additionally used as flux, single crystals of Pr(BO2)3 dominate the main product. Both praseodymium(III) meta‐borates are air and water stable. The crystals of PrCl(BO2)2 emerge as long, thin, pale green needles which tend to severe twinning due to their fibrous habit. The crystal structure (triclinic, P1¯; a = 420.56(4), b = 655.42(7), c = 808.34(8) pm, α = 82.361(8), β = 89.173(9), γ = 71.980(7)°, Z = 2) exhibits zigzag chains {[(B1)ot1/1Oe2/2(B2)Ot1/1Oe2/2]2−} (≡ {[BO2]−}) of corner‐linked [BO3]3− triangles with syndiotactic orientation of the terminal oxygen atoms which are running parallel to the [100] direction. The Pr3+ cations are surrounded by three Cl− and seven O2− anions with the shape of a tetracapped trigonal prism. The green, transparent crystals of Pr(BO2)3 (monoclinic, C2/c; a= 984.98(9), b = 809.57(8), c = 641.02(6) pm, β = 126.783(9)°, Z = 4) appear either lath‐shaped or rather spherical. In the crystal structure the B3+ cations reside both in trigonal planar as well as in tetrahedral coordination of oxygen atoms. Both types of borate polyhedra ([BO3]3− and [BO4]5−) are linked via corners to form chains of the composition {[(B2)‐Ot1/1Oe2/2(B1)Oe4/2(B2)Ot1/1Oe2/2]3−} (≡ {[BO2]−}) which run parallel [101]. The coordination sphere of the Pr3+ cations consists of ten oxide anions which build up a bicapped square antiprism. 相似文献
38.
Sm4S3[Si2O7] and NaSm9S2[SiO4]6: Two Sulfide Silicates with Trivalent Samarium The sulfide silicates Sm4S3[Si2O7] and NaSm9S2[SiO4]6 are obtained as light yellow transparent crystals by the reaction of Sm, Sm2O3, S, and SiO2 with fluxing SmCl3 or NaCl, respectively, in suitable molar ratios in fused evacuated silica tubes (850 °C, 7 d). Tetragonal crystals of Sm4S3[Si2O7] (I41/amd; Z = 8; a = 1186.4(1); c = 1387.0(2) pm) with ecliptically conformed [Si2O7]6–‐groups of corner sharing [SiO4]‐tetrahedra are formed. These double tetrahedra as well the sulfide anions (S2–) coordinate two crystallographically independent metal cations. They provide coordination numbers of 8 + 1 (5 S2– and 3 + 1 O2–) for Sm1 and 9 (3 S2– and 6 O2–) for Sm2. NaSm9S2[SiO4]6 crystallizes hexagonally (P63/m; Z = 1; a = 975.32(9); c = 676.46(7) pm) in a modified bromapatite‐type structure. The coordination spheres about the two crystallographically different Sm3+ cations are built up by oxygen atoms of the orthosilicate units ([SiO4]4–) and sulfide anions (S2–). As a result, Sm1 and Sm2 have coordination numbers of 9 and 8, respectively. Na+ and (Sm1)3+ occupy the position 4 f in a molar ratio of 1 : 3 whereas the lower coordinated (Sm2)3+ occupies the 6 h position. 相似文献
39.
Sm3Cl[SiO4]2: A Chlorine‐poor Chloride Orthosilicate of Samarium Pale yellow, plate‐like single crystals of Sm3Cl[SiO4]2 (orthorhombic, Pnma; a = 701.74(8), b = 1800.8(2), c = 626.63(7) pm; Z = 4) are obtained upon the reaction of SmCl3, Sm2O3 and SiO2 (”︁Kieselgel”︁”︁) in 1 : 4 : 6 molar ratios, most advantageously in the presence of substantial amounts of NaCl as fluxing agent, after seven days at 850 °C in evacuated silica ampoules. The B‐type crystal structure (isotypic with e. g. Yb3Cl[SiO4]2) contains discrete orthosilicate tetrahedra [SiO4]4– which form anionic double layers ({(Sm1)2[SiO4]2}2–) with (Sm1)3+. These are alternatingly sheethed along [010] with cationic monolayers ({(Sm2)Cl}2+) consisting of (Sm2)3+ and Cl–. Both crystallographically independent Sm3+ cations exhibit coordination numbers of eight (Sm1: 1 Cl + 7 O; Sm2: 2 Cl + 6 O) with respect to the involved electronegative particles. 相似文献
40.
In the paper methods for the coupling of different meshed local domains of a product model, which occur in an adaptivity process are compared. The properties of the applied coupling methods are demonstrated on finite element models from structural mechanics to point out the advantages and disadvantages of the methods in a model adaptivity process in the field of automotive engineering. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献