全文获取类型
收费全文 | 8170篇 |
免费 | 918篇 |
国内免费 | 389篇 |
专业分类
化学 | 6024篇 |
晶体学 | 136篇 |
力学 | 993篇 |
综合类 | 18篇 |
数学 | 664篇 |
物理学 | 1642篇 |
出版年
2024年 | 10篇 |
2023年 | 49篇 |
2022年 | 107篇 |
2021年 | 159篇 |
2020年 | 320篇 |
2019年 | 200篇 |
2018年 | 160篇 |
2017年 | 218篇 |
2016年 | 333篇 |
2015年 | 319篇 |
2014年 | 344篇 |
2013年 | 545篇 |
2012年 | 549篇 |
2011年 | 434篇 |
2010年 | 405篇 |
2009年 | 540篇 |
2008年 | 579篇 |
2007年 | 618篇 |
2006年 | 517篇 |
2005年 | 407篇 |
2004年 | 434篇 |
2003年 | 344篇 |
2002年 | 293篇 |
2001年 | 202篇 |
2000年 | 192篇 |
1999年 | 222篇 |
1998年 | 187篇 |
1997年 | 136篇 |
1996年 | 93篇 |
1995年 | 112篇 |
1994年 | 71篇 |
1993年 | 70篇 |
1992年 | 55篇 |
1991年 | 48篇 |
1990年 | 27篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 14篇 |
1984年 | 24篇 |
1983年 | 5篇 |
1982年 | 14篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有9477条查询结果,搜索用时 31 毫秒
251.
Crystal Structures of MgCrO4-type Li2VCl4 and Spinel-type Li2MgCl4 and Li2CdCl4 The crystal structures of the ternary lithium chlorides Li2MCl4 (M = Mg, V, Cd) have been determined firstly by X-ray single-crystal experiments. Li2MgCl4 and Li2CdCl4 crystallize in an inverse spinel structure (space group Fd3 m, Z = 8, a = 1 040.1(2) and 1 062.06(9) pm, structural parameters u = 0.25699(2) and 0.2550(1), R = 1.7 and 3.7% for 218 and 211 unique reflections). The Li? Cl distances of the tetrahedrally coordinated Li+ ions are significantly greater than calculated with Shannon's crystal radii ( > 238 ± 1 instead of 233 pm). Contrary to the results of X-ray powder data reported in the literature, Li2VCl4 crystallizes in the distorted spinel structure of MgCr2O4 type (space group F4 3m, Z = 8, a = 1 037.49(2) pm, R = 5.9% for 217 unique reflections). The decrease of the site symmetry of the octahedrally coordinated ions (V2+, Li+) from 3 m to 3m resulting in contracted and widened tetrahedral M4 entities of the spinel structure is obviously caused by V? V metal—metal bonds (shortest V? V distance 366.2(7) pm). 相似文献
252.
Mahendra Madegowda Doreswamy Beeranahally H. Sridhar Mandayam A. Prasad Javaregowda S. Khanum Shaukath A. Shashikanth Sheena Sudha Belagur S. 《Structural chemistry》2004,15(3):211-214
Benzophenone analog 3 has been synthesized and characterized by the X-ray diffraction (XRD) method. The compound crystallizes in a monoclinic space group P21/c with cell parameters a = 7.701(8) Å, b = 7.151(5) Å, c= 28.323(3) Å, = 104.639(4)°, Z = 4. The structure exhibits intra- and intermolecular hydrogen bonding of the type N–HO, C–HO, and N–HN. The molecules are interlinked through hydrogen bonds forming an infinite chain. This polymeric-like structure may play an important role in biological activity. 相似文献
253.
Daniel Canseco-González 《Journal of organometallic chemistry》2003,679(1):101-109
The complexes cis-[M(Ph2PC6H4-2-S)2] M=Ni, Pd, Pt were stereoselectively synthesized by transmetallation reactions of [M(Cl)2(NCC6H5)2] M=Pd, Pt or NiCl2·6H2O with [Sn(R)2(Ph2PC6H4-2-S)2] R=Ph, nBu or tBu. The conformation of the Pd and Pt derivatives being unequivocally confirmed by single crystal X-ray diffraction studies showing both metal centers to be into a slightly distorted square planar environment, the main distortion being due to the steric hindrance caused by the aromatic rings in the phosphine moiety. 相似文献
254.
DNA tile based self-assembly provides an attractive route to create nanoarchitectures of programmable patterns. It also offers excellent scaffolds for directed self-assembly of nanometer-scale materials, ranging from nanoparticles to proteins, with potential applications in constructing nanoelectronic/nanophotonic devices and protein/ligand nanoarrays. This Review first summarizes the currently available DNA tile toolboxes and further emphasizes recent developments toward self-assembling DNA nanostructures with increasing complexity. Exciting progress using DNA tiles for directed self-assembly of other nanometer scale components is also discussed. 相似文献
255.
Preparation, Raman Spectra, and Crystal Structures of V2O3(SO4)2, K[VO(SO4)2], and NH4[VO(SO4)2] The oxo-sulfato-vanadates(V) V2O3(SO4)2, K[VO(SO4)2], and NH4[VO(SO4)2] have been prepared as crystals suitable for X-ray structure determination. In all structures sulfate acts as an unidentate ligand only toward a single vanadium atom. The structure of V2O3(SO4)2 consists of a threedimensional network of pairs of cornershared VO6 octahedra with one terminal oxygen atom each, and SO4 tetrahedra. All oxygen atoms of the sulfate ions are coordinated. NH4[VO(SO4)2] and K[VO(SO4)2] are isostructural. VO6 octahedra with one terminal oxygen atom and pairs of sulfate tetrahedra form infinite chains by corner sharing. The chains are weakly interlinked to layers. The sulfate ions are distorted towards planar SO3 molecules and single oxygen atoms attached to vanadium. This structural detail gives an explanation for the mechanism of the reversible reaction K[VO(SO4)2] ? K[VO2(SO4)] + SO3 at 400°C. Raman spectra of the compounds have been recorded and interpreted with respect to their structures. Crystal data: V2O3(SO4)2, monoclinic, space group P21/a, a = 947.2(4), b = 891.3(3), c? 989.1(4) pm, β = 104.56(3)°, Z = 4, 878 unique data, R(Rw) = 0.039(0,033); K[VO(SO4)2], orthorhombic, space group P212121, a = 495.3(2), b = 869.6(9), c = 1 627(1)pm, Z = 4, 642 unique data, R(Rw) = 0,11(0,10); NH4[VO(SO4)2], orthorhombic, space group P212121, a = 495.3(1), b = 870.0(2), c = 1 676.7(4)pm, Z = 4, 768 unique data, R(Rw) = 0.088(0.083). 相似文献
256.
The solid state structures of three nitroformate (NF) salts were determined using single crystal X‐ray crystallography. The NF anion was found to be a non‐planar moiety which adopts either the commonly observed C2v conformation or distorted propeller conformation (D3) in the case of the silver salts, or, a C2 conformation in the case of the potassium salt. This latter C2 conformation has been uniquely observed for potassium nitroformate. All structures exhibit cation‐anion interactions that influence the structure of the anion. The 13C and 14N NMR spectra of the NF anion show broad singlets, which indicates the equivalence of the nitro groups in solution within the NMR time‐scale. In addition, the vibrational and mass spectra of potassium nitroformate and silver nitroformate monohydrate were recorded. Furthermore, the gaseous decomposition products of potassium nitroformate at 25 °C were detected using IR spectroscopy and mass spectrometry. 相似文献
257.
Victor Wee Lin Ng 《Journal of organometallic chemistry》2005,690(9):2323-2332
The reaction of [CpCr(CO)3]2 (Cp = η5-C5H5) (1) with 1 mol equivalent of 2,5-dimercapto-1,3,4-thiadiazole (DMcTH2) at ambient temperature led to the isolation of a reddish-brown crystalline solid of CpCr(CO)3(η1-DMcTH) (5) and a green solid of CpCr(CO)3H (2) in yields of ca. 28% and 30%, respectively, along with some [CpCr(CO)2]2 (3) and [CpCr(CO)2]2S (4). The reaction of 1 with 1 mol equivalent of vinylene trithiocarbonate (SCS(CH)2S) (VTTC) at 90 °C led to the isolation of a red crystalline solid of CpCr(CO)2(η2-SCHSC2H2) (6) in ca. 15% yield while the reaction of 1 with isopropylxanthic disulfide ((CH3)2CHOCS2)2 resulted in the formation of CpCr(CO)2(η2-S2COCH(CH3)2) (8) in ca. 80% yield. The complexes 5, 6 and 8 have been determined by single crystal X-ray diffraction analysis. 相似文献
258.
Crystalline [Li{N(SiMe2OMe)C(tBu)C(H)(SiMe3)}]2 (5), [Li{N(SiMe2OMe)C(Ph)C(H)(SiMe3)}]2 (6), [C(C6H3Me2-2,5)C(H)(SiMe3)}(TMEDA)](7), [Li{N(SiMe(OMe)2)C(tBu)C(H)(SiMe3)}(THF)]2 (8), Li{N(SiMe(OMe)2)C(Ph)C(H)(SiMe3)}(TMEDA) (9) and [Li{N(SiMe2OMe)C(tBu)C(H)(SiMe2OMe)}]2 (10) were readily obtained at ambient temperature from (i) [Li{CH(SiMe3)(SiMe2OMe)}]8 (1) and an equivalent portion of RCN (R=tBu (5), Ph (6) or 2,5-Me2C6H3 (7)); (ii) [Li{CH(SiMe3)(SiMe(OMe)2)}]∞ (2) and an equivalent portion of tBuCN (8) or PhCN (9); and (iii) [Li{CH(SiMe2OMe)2}]∞ (3) and one equivalent of tBuCN (10). Reactions (i) and (ii) were regiospecific with SiMe3−n(OMe)n>SiMe3 in 1,3-migration from C (in 1 or 2)→N. The 1-azaallyl ligand was bound to the lithium atom as a terminally bound κ1-enamide (8 and 10), a bridging η3-1-azaallyl (6), or a bridging κ1-enamide (5). The stereochemistry about the CC bond was Z for 5, 8 and 10 and E for 7. X-ray data are provided for 5, 6, 7, 8 and 10 and multinuclear NMR spectra data in C6D6 or C6D5CD3 for each of 5-10. 相似文献
259.
A. A. Isaeva A. I. Baranov Th. Doert M. Ruck V. A. Kulbachinskii R. A. Lunin B. A. Popovkin 《Russian Chemical Bulletin》2007,56(9):1694-1700
New metal-rich mixed nickel-silicon and nickel-germanium chalcogenides, Ni5.68SiSe2, Ni5.46GeSe2, and Ni5.42GeTe2, were synthesized by high-temperature ceramic techniques. The X-ray diffraction study of single crystals grown from a molten
flux revealed that the compounds are isostructural and crystallize in the tetragonal system (space group I4/mmm, Z = 2). These compounds are the first members of the family of M7−δEX2-type (M = Ni or Pd; E = Sn or Sb; X is chalcogen) intergrowth structures containing “light” p elements E. Resistivity measurements on pressed textured pellets showed that both selenides are anisotropic metallic conductors
in the directions parallel and perpendicular to the heterometallic bond systems. The geometric criteria of stability of the
intergrowth structure type under consideration are discussed.
Dedicated to Academician G. A. Abakumov on the occasion of his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1632–1638, September, 2007. 相似文献
260.
G. A. Jeffrey 《Journal of inclusion phenomena and macrocyclic chemistry》1984,1(3):211-222
There are three general classes of hydrate inclusion compounds: the gas hydrates, the per-alkyl onium salt hydrates, and the alkylamine hydrates. The first are clathrates, the second are ionic inclusion compounds, the third are semi-clathrates. Crystallization occurs because the H2O molecules, like SiO2, can form three-dimensional four-connected nets. With water alone, these are the ices. In the inclusion hydrates, nets with larger voids are stabilized by including other guest molecules. Anions and hydrogen-bonding functional groups can replace water molecules in these nets, in which case the guest species are cations or hydrophobic moieties of organic molecules. The guest must satisfy two criteria. One is dimensional, to ensure a comfortable fit within the voids. The other is functional. The guest molecules cannot have either a single strong hydrogen-bonding group, such as an amide or a carboxylate, or a number of moderately strong hydrogen-bonding groups, as in a polyol or a carbohydrate.The common topological feature of these nets is the pentagonal dodecahedra: i.e., 512-hedron. These are combined with 51262-hedra, 51263-hedra, 51264-hedra and combinations of these polyhedra, to from five known nets. Two of these are the well-known 12 and 17 Å cubic gas hydrate structures,Pm3n, Fd3m; one is tetragonal,P4
2/mnm, and two are hexagonal,P6
3/mmc andP6/mmm. The clathrate hydrates provide examples of the two cubic and the tetragonal structures. The alkyl onium salt hydrates have distorted versions of thePm3n cubic, the tetragonal, and one of the hexagonal structures. The alkylamine hydrate structures hitherto determined provide examples of distorted versions of the two hexagonal structures.There are also three hydrate inclusion structures, represented by single examples, which do not involve the 512-hedra. These are 4(CH3)3CHNH2·39H2O which is a clathrate; HPF6·6H2O and (CH3)4NOH·5H2O which are ionic-water inclusion hydrates. In the monoclinic 6(CH3CH2CH2NH2)·105H2O and the orthorhombic 3(CH2CH2)2NH·26H2O, the water structure is more complex. The idealization of these nets in terms of the close-packing of semi-regular polyhedra becomes difficult and artificial. There is an approach towards the complexity of the water salt structures found in the crystals of proteins. 相似文献