全文获取类型
收费全文 | 2868篇 |
免费 | 54篇 |
国内免费 | 294篇 |
专业分类
化学 | 2789篇 |
晶体学 | 30篇 |
力学 | 3篇 |
综合类 | 1篇 |
物理学 | 393篇 |
出版年
2024年 | 4篇 |
2023年 | 70篇 |
2022年 | 32篇 |
2021年 | 31篇 |
2020年 | 71篇 |
2019年 | 107篇 |
2018年 | 45篇 |
2017年 | 96篇 |
2016年 | 68篇 |
2015年 | 64篇 |
2014年 | 71篇 |
2013年 | 91篇 |
2012年 | 309篇 |
2011年 | 170篇 |
2010年 | 126篇 |
2009年 | 185篇 |
2008年 | 177篇 |
2007年 | 205篇 |
2006年 | 152篇 |
2005年 | 142篇 |
2004年 | 136篇 |
2003年 | 103篇 |
2002年 | 86篇 |
2001年 | 93篇 |
2000年 | 70篇 |
1999年 | 51篇 |
1998年 | 78篇 |
1997年 | 55篇 |
1996年 | 56篇 |
1995年 | 34篇 |
1994年 | 25篇 |
1993年 | 29篇 |
1992年 | 23篇 |
1991年 | 35篇 |
1990年 | 24篇 |
1989年 | 20篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1984年 | 4篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1966年 | 2篇 |
排序方式: 共有3216条查询结果,搜索用时 15 毫秒
91.
Copper(II) complex with
-piperidine-3-carboxylic acid (
-Hpipe-3):[Cu(
-pipe-3)2(H2O)] and cobalt(II) and nickel(II) complexes with piperidine-4-carboxylic acid (Hpipe-4):[M(Hpipe-4)2(H2O)4]Cl2 (M: Co, Ni) have been prepared and characterized by means of IR and powder diffuse reflection spectra, thermal analysis, and magnetic susceptibility. The crystal structures of these complexes have been determined by X-ray diffraction. The crystal of [Cu(
-pipe-3)2(H2O)] is orthorhombic with the space group Pbcn. The copper atom is in a square pyramidal geometry, ligated by two carboxylato oxygen atoms, two nitrogen atoms, and a water molecule. One molecule of this complex consists of either
-piperidine-3-carboxylic acid or
-piperidine-3-carboxylic acid. The crystals of [M(Hpipe-4)2(H2O)4]Cl2 are monoclinic with space group P21/n. In these complexes the metal atom is in an octahedral geometry ligated by two carboxylato oxygen atoms and four water molecules. 相似文献
92.
The protonation constants of two nitro-Schiff bases SB1, SB2 and three asymmetric tetradentate diimines H2L1, H2L2 and H2L3 and the stability constants of their ML type binuclear Ni(II) and Fe(III) complexes have been determined potentiometrically. The asymmetric diimines are (2OH) RCHNC6H4CHNR′ (2OH) type compounds [where R = R′ = phenyl for H2L1; R = naphthyl, R′ = phenyl for H2L2 and R = R′ = naphthyl for H2L3]. The effect of tautomeric forms on the acid-base properties of the diimines has been investigated and discussed. In addition, dimeric and binuclear Ni(II) and Fe(III) complexes of the diimines have been synthesized and characterized by physical and spectroscopic techniques. Also, in vitro antimicrobial activities of the diimines and the complexes have been evaluated against three bacteria: Micrococcus luteus (NRLL B-4375), Bacillus cereus (RSKK 863), Escherichia coli (ATCC 11230) and the fungus: Candida albicans (ATCC 10239). 相似文献
93.
Maurí cio Braga de Araú jo Nelson Moreira Pinhal Ney Vernon Vugman 《Radiation Physics and Chemistry》2002,64(5-6):389-393
The kinetics of oxidized and reduced Ni2+ complexes produced by X-ray irradiation on single crystals of NaCl doped with [Ni(CN)4]2− is studied by Electron Paramagnetic Resonance at room temperature. The interdependent generation of these two complexes is attributed to migration of the charge compensating vacancy from the reduced to the oxidized complex in a reversible reaction. At higher X-ray doses, there is a predominant formation of the reduced complex. 相似文献
94.
NiTiO3 shows an order-disorder transition from an ordered ilmenite structure to a corundum structure at high temperatures. The transition is followed by a strong increase of the specific electrical conductivity. The conductivity was investigated as a function of temperature and oxygen activity. An order parameter according to common phase transition theories can be used to describe the behaviour of the conductivity in the transition region and vice versa. A model for the defect structure of NiTiO3 is presented. 相似文献
95.
A. Abel Lozano M. Dolores Santana Gabriel García J. Elaine Barclay Sin C. Davies David J. Evans 《无机化学与普通化学杂志》2005,631(11):2062-2066
A series of heteronuclear nickel‐iron complexes [Fe2(CO)6(μ‐SH)(μ3‐S){NiCl(PPh3)2}] ( 1 ), [Fe2(CO)6(μ‐SH)(μ3‐S){NiCl(dppe)}] ( 2 ), [Fe2(CO)6(μ3‐S)2{Ni(PPh3)2}] ( 3 ), [Fe2(CO)6(μ3‐S)2{Ni(dppe)}] ( 4 ) and [Fe2(CO)6(μ‐SPh)(μ3‐S){NiCl(dppe)}] ( 5 ) have been prepared. The structure of 4 has been determined by X‐ray crystallography. The central metal‐sulfur core of 4 has a trigonal bipyramidal shape with a NiFe2 base plane with two axial sulfur atoms. Each iron atom is 5‐coordinate forming a distorted square pyramid; the nickel is square planar coordinated by two sulfur atoms and two phosphorus atoms. 相似文献
96.
Alternative Ligands. XXXII [1]. Novel Tetraphosphane Nickel Complexes with Tripod-Ligands of the Type XM′(OCH2PMe2)n(CH2CH2PR2)3 – n (M′ = Si, Ge; n = 0 – 3) Tripod Ligands of the type XM′(OCH2PMe2)n(CH2CH2PMe23 – n (M′ = Si, Ge; n = 0 – 3) ( 1 – 6 , Table 1) have been used together with PPh3 or PMe3 for the preparation of novel tetraphosphane complexes of Nickel. The representatives LNiPPh3 ( 7 – 12 ) are obtained by reaction of Ni(COD)2 (COD = 1,5-cyclooctadiene) with the corresponding ligands and PPh3 in toluene in moderate yields. The synthesis of the derivatives LNiPMe3 ( 13 – 18 ) is partly ( 16 – 18 ) accomplished in analogy to the Ph3P-complexes; compounds 13 – 16 are obtained in higher yields by reaction of Ni(PMe3)4 with the respective ligand. As a rule, 13 – 18 cannot be separated from by-products. The trinuclear complex FSi(CH2CH2PMe2)3[Ni(PMe2CH2CH2)3SiF]3 ( 19 ) is formed together with 18 in the reaction of Ni(COD)2 with 6 and PMe3. The new compounds have been characterized (if possible) by analytical (C, H), but in general by spectroscopic investigations (IR; 1H-, 13C-, 19F-, 31P-NMR; MS). A weak, but significant Ni → Si interaction through the cage is indicated by the following results: (i) Large low-field shifts δδF of 35.2 ppm ( 12 ), 38.3 ppm ( 18 ) and 37.7 ppm ( 19 ); (ii) 6J(PF) coupling constants [or 3J(PNiSiF) through the cage] of 6.0 Hz ( 12 ) and 7.6 Hz ( 18 ) together with a low-field shift δδSi of 12.8 ppm ( 12 ); (iii) NiSi distances of 3.95 Å in 7 and 3.92 Å in 12 , accompanied by a compression of the cage along the Ni ··· Si axis. An additional release from the high charge density on Ni results from π-backbonding to the phosphane ligands. 相似文献
97.
[Ni(H2O)6][Cu3Cl8(H2O)2] · (15-crown-5)2 · 2H2O can be conveniently prepared by the interaction of NiCl2 · 6H2O, CuCl2 · 2H2O and 15-crown-5 in water. The X-ray crystal structure reveals an ionic complex involved in a hydrogen-bonded two dimensional network with the [Ni(H2O)6]2+ and [Cu3Cl8(H2O)2]2− ions sandwiched between the 15-crown-5 macrocycles. The magnetic susceptibility data (4–300 K) and magnetisation isotherms (2–5.5 K; 0–5 T) are best interpreted in terms of intra-trimer ferromagnetic coupling within the [Cu3Cl8(H2O)2]2− moieties, with J ∼ 6 cm−1, and antiferromagnetic coupling between the trimers, the latter mediated by H-bonding pathways. Comparisons are made to other reported quaternary ammonium salts of [Cu3Cl8]2− and [Cu3Cl12]6−, most of which display structures that involve close stacking of such Cu(II) trimers, rather than being of the present isolated, albeit H-bonded, types. 相似文献
98.
Binuclear Nickel(0) Alkyne Coordination Compounds – Correlation between Ligand Periphery and Supramolecular Structure Reaction of Ni(cdt: 1,5,9-cyclododecatriene) with functionalized alkynes and subsequent reaction with ethylenediamines gives binuclear compounds of the type (diamine)Ni(μ-alkyne)Ni(alkyne). Compounds with alkyne-diols (N?N)Ni2(HOR1R2C? C?C? CR1R2OH)2 show supramolecular structures in which two identical intramolecular and one intermolecular hydrogen bonds are realized. 1 and 2 (chelate ligand in each case N,N,N′,N′-tetramethylethylenediamine, TMEDA, in 1 R1 = R2 = Me, in 2 R1 = R2 = Et) polymer-like chains are built up by connecting the binuclear units. Via two intermolecular hydrogen bonds per organometallic unit in 1 and via one intermoleculare hydrogen bond in 2 the chains are connected to give double chains. By substitution of one methyl group of TMEDA by hydrogen ( 3 : R1 = R2 = Me) a polymerlike network is produced by connecting the polymer-like chains. In compound 4 in which one of the methyl groups of TMEDA is substituted by CH2CH2NMe2 the polymer-like chains remain unconnected. In 5 (diamine = TMEDA, alkyne = (CH3)3C? C?C? CMe2OH) one intermolecular hydrogen bond per organometallic unit is observed forming again polymer-like chains that are independent of each other. 相似文献
99.
An artificial neural network (ANN) procedure was used in the development of a catalytic spectrophotometric method for the determination of Cu(II) and Ni(II) employing a stopped-flow injection system. The method is based on the catalytic action of these ions on the reduction of resazurin by sulfide. ANNs trained by back-propagation of errors allowed us to model the systems in a concentration range of 0.5-6 and 1-15 mg l−1 for Cu(II) and Ni(II), respectively, with a low relative error of prediction (REP) for each cation: REPCu(II) = 0.85% and REPNi(II) = 0.79%. The standard deviations of the repeatability (sr) and of the within-laboratory reproducibility (sw) were measured using standard solutions of Cu(II) and Ni(II) equal to 2.75 and 3.5 mg l−1, respectively: sr[Cu(II)] = 0.039 mg l−1, sr[Ni(II)] = 0.044 mg l−1, sw[Ni(II)] = 0.045 mg l−1 and sw[Ni(II)] = 0.050 mg l−1. The ANNs-kinetic method has been applied to the determination of Cu(II) and Ni(II) in electroplating solutions and provided satisfactory results as compared with flame atomic absorption spectrophotometry method. The effect of resazurin, NaOH and Na2S concentrations and the reaction temperature on the analytical sensitivity is discussed. 相似文献
100.
We have theoretically examined the reaction course of the butadiene insertion into the arylNiII bond in the [NiII(η5-Cp)(η1-phenyl)(η2-butadiene)] complex (1), by employing a gradient-corrected DFT method. Critical elementary processes have been scrutinized, viz. monomer insertion, rotational allylic isomerization and allylic η1-σ→η3-π rearrangement. The first mechanism suggested by Lehmkuhl et al. was refined and supplemented with important details. The critical factors that determine the generation of anti-η3- and syn-η3-allyl isomers of the [NiII(η5-Cp)(1-benzyl-allyl)] product have been elucidated. This let us to rationalize the experimentally observed, almost exclusive formation of the anti-η3-allyl isomer. Butadiene preferably inserts in η2-mode into the η1-phenylNiII bond, initially giving rise to the η1(C3)-allyl product species, 3σ. The direct formation of the η3-allyl product species, 3π, along the alternative path for η4-butadiene insertion, however, is found to be almost entirely disabled kinetically. The thermodynamically favorable η2-trans form of 1 is also shown to be more reactive in accomplishing CC bond formation. Species 3σ is indicated to be a metastable intermediate, occurring in an appreciable stationary concentration. Its respective anti and syn isomeric forms are likely to be in equilibrium, due to the facile rotational isomerization. The subsequent allylic rearrangement into the thermodynamically strongly favorable η3-allylNiII coordination mode is shown to be the crucial elementary step that discriminates which of the isomeric η3-allyl forms is preferably generated. The higher reactivity of the anti isomer in this process decisively determines the almost exclusive formation of the anti-η3-allyl product species under kinetic control. The requirement of elevated temperatures for the anti-η3-allyl→syn-η3-allyl isomerization to occur, as revealed from experiment, is attributed to the pronounced thermodynamic stability of the η3-allylNiII coordination. 相似文献