全文获取类型
收费全文 | 5802篇 |
免费 | 125篇 |
国内免费 | 19篇 |
专业分类
化学 | 3692篇 |
晶体学 | 77篇 |
力学 | 231篇 |
数学 | 510篇 |
物理学 | 1436篇 |
出版年
2021年 | 34篇 |
2020年 | 43篇 |
2019年 | 46篇 |
2018年 | 47篇 |
2017年 | 27篇 |
2016年 | 89篇 |
2015年 | 99篇 |
2014年 | 77篇 |
2013年 | 276篇 |
2012年 | 224篇 |
2011年 | 258篇 |
2010年 | 149篇 |
2009年 | 123篇 |
2008年 | 221篇 |
2007年 | 234篇 |
2006年 | 251篇 |
2005年 | 224篇 |
2004年 | 190篇 |
2003年 | 171篇 |
2002年 | 189篇 |
2001年 | 132篇 |
2000年 | 124篇 |
1999年 | 89篇 |
1998年 | 84篇 |
1997年 | 126篇 |
1996年 | 125篇 |
1995年 | 96篇 |
1994年 | 113篇 |
1993年 | 135篇 |
1992年 | 121篇 |
1991年 | 72篇 |
1990年 | 66篇 |
1989年 | 98篇 |
1988年 | 85篇 |
1987年 | 79篇 |
1986年 | 78篇 |
1985年 | 93篇 |
1984年 | 102篇 |
1983年 | 78篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 70篇 |
1979年 | 72篇 |
1978年 | 69篇 |
1977年 | 69篇 |
1976年 | 57篇 |
1975年 | 79篇 |
1974年 | 72篇 |
1973年 | 59篇 |
1972年 | 31篇 |
排序方式: 共有5946条查询结果,搜索用时 140 毫秒
991.
Structures of Ionic Di(arenesulfonyl)amides. 6. Limits to the Formation of Lamellar Metal Di(arenesulfonyl)amides: Three Lithium Complexes and One Cadmium Complex According to low‐temperature X‐ray studies, the new compounds LiN(SO2C6H4‐4‐X)2 · 2 H2O, where X = COOH ( 1 ) or COOMe ( 2 ), LiN(SO2C6H4‐4‐CONH2)2 · 4 H2O ( 3 ), and Cd[N(SO2C6H4‐4‐COOH)2]2 · 8 H2O ( 4 ) crystallize in the triclinic space group P1 ( 1 – 3 : Z′ = 1; 4 : Z′ = 1/2, Cd2+ on an inversion centre) and display almost perfectly folded anions approximating to mirror symmetry. The lithium ions in 1 – 3 have distorted tetrahedral environments respectively set up by two O=S groups drawn from different anions and two water molecules, two O=S groups of a chelating anion and two water molecules, or one O=C group and three water ligands, whereas the cation of 4 is fully hydrated to form an octahedral [Cd(H2O)6]2+ complex. The structure refinements for 3 and 4 were marred by positional disorder of the non‐coordinating N(SO2)2 moieties. Compounds 1 and 4 extend a previously described series of lamellar metal di(arenesulfonyl)amides where the two‐dimensional inorganic component is comprised of cations, N(SO2)2 groups and water molecules and the outer regions are formed by the 4‐substituted phenyl rings. Both crystal packings are governed by self‐assembly of parallel layers through exhaustive hydrogen bonding between carboxylic groups, and there is good evidence that the labile inorganic networks, generated via Li–O and hydrogen bonds in 1 or solely hydrogen bonds in 4 , are efficiently stabilized by the strong cyclic (COOH)2 motifs within the interlayer regions. In the absence of these, the lamellar architecture is seen to collapse in 2 and 3 , where the carboxyl groups are replaced by methoxycarbonyl or carbamoyl functions and the inorganic components are segregated in parallel tunnels pervading the anion lattices. 相似文献
992.
Metal Salts of Benzene‐1,2‐di(sulfonyl)amine. 4. Hydrophobically Wrapped Two‐Dimensional Polymers: Crystal Structures of the Isostructural Metal Complexes [M{C6H4(SO2)2N}(H2O)] (M = K, Rb) and of the Structurally Related Ammonium Salt [(NH4){C6H4(SO2)2N}(H2O)] The previously unreported compounds KZ · H2O ( 1 ), RbZ · H2O ( 2 ) and NH4Z · H2O ( 3 ), where Z– is Ndeprotonated ortho‐benzenedisulfonimide, are examples of layered inorgano‐organic solids, in which the inorganic component is comprised of metal or ammonium cations, N(SO2)2 groups and water molecules and the outer regions are formed by the planar benzo rings of the anions. The metal complexes 1 and 2 were found to be strictly isostructural, whereas 3 is structurally related to them by a non‐crystallographic mirror plane ( 1 – 3 : monoclinic, space group P21/c, Z = 4; single crystal X‐ray diffraction at low temperatures). In each structure, the five‐membered 1,3,2‐dithiazolide heterocycle possesses an envelope conformation, the N atom lying about 40 pm outside the mean plane of the S–C–C–S moiety. The metal complexes feature two‐dimensional coordination networks interwoven with O–H…O hydrogen bonds originating from the water molecules. The metal centres adopt an irregular nonacoordination formed by five sulfonyl O atoms, two N atoms and two μ2‐bridging water molecules; each M+ is connected to four different anions. When NH4+ is substituted for M+, the metal–ligand bonds are replaced by N+–H…O hydrogen bonds, but the general topology of the lamella is not affected. In the three structures, the lipophilic benzo groups protrude obliquely from the surfaces of the polar lamellae and display marked interlocking between adjacent layers. 相似文献
993.
994.
Gerard G. Henn Colin Birkinshaw Martin Buggy Eric Jones 《Macromolecular bioscience》2001,1(6):219-222
Poly(D ,L ‐lactide) has been evaluated as a material for the manufacture of intra‐medullary plugs to be used in total hip arthoplasty. Plugs were manufactured by compression moulding and subjected to in‐vitro and in‐vivo degradation. In‐vitro hydrolysis was carried out by immersion in phosphate buffered saline (Ringer's solution) at 37°C and rates of degradation were relatively rapid with molecular weight halving after 30 days. In‐vivo degradation was assessed by implantation into dogs followed by retrieval at intervals up to 24 months. Molecular weight was found to reduce to half the original value in about 190 days. It is thought that this difference in degradation rate is because of diffusional control of the overall process. Histology showed that the implanted plugs were resorbed over 24 months. 相似文献
995.
996.
Structures of Ionic Di(arenesulfonyl)amides. 3. Four Sodium Di(arenesulfonyl)amides: Lamellar Layers Exhibiting Short C–H…O(nitro), C–H…F–C, or C–I…I–C Interlayer Contacts Low‐temperature X‐ray crystal structures are reported for NaN(SO2C6H4‐4‐X)2 · n H2O, where X = NO2 and n = 3 ( 1 , monoclinic, space group P21, Z = 2), X = F and n = 3 ( 2 , monoclinic, P21/c, Z = 4), X = F and n = 1 ( 3 , orthorhombic, Pccn, Z = 8), or X = I and n = 1 ( 4 , monoclinic, P21/c, Z = 4). The four compounds are examples of layered inorgano‐organic solids where the inorganic component is comprised of metal cations, N(SO2)2 groups and H2O molecules and the outer regions are formed by the 4‐substituted phenyl rings of the folded anions. In the two‐dimensional coordination networks, the cations adopt either an octahedral [Na(O–S)2(OH2)4] ( 1 , 2 ) or a distorted monocapped octahedral [NaN(O–S)4(OH2)2] ( 3 , 4 ) environment. Taking into account the differing crystal symmetries within the two pairs of compounds, it is remarkable that the trihydrates 1 / 2 and the monohydrates 3 / 4 each display chemically identical and nearly isometric Na–O or Na–O/N networks. In the crystal packings, parallel layers are connected through weak hydrogen bonds C–H…O(nitro) ( 1 ) or C–H…F ( 2 , 3 ), or through short “type I” I…I contacts ( 4 ). There is good evidence that the strikingly distinct crystal symmetries in the halogenated homologues 3 / 4 are determined by the specific complementarity requirements of the interlayer binding centres. 相似文献
997.
998.
Songlan Sun Gaia De Angelis Dr. Stefania Bertella Marie J. Jones Dr. Graham R. Dick Prof. Esther Amstad Prof. Jeremy S. Luterbacher 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(5):e202312823
Concerns over the sustainability and end-of-life properties of fossil-derived surfactants have driven interest in bio-based alternatives. Lignocellulosic biomass with its polar functional groups is an obvious feedstock for surfactant production but its use is limited by process complexity and low yield. Here, we present a simple two-step approach to prepare bio-based amphiphiles directly from hemicellulose and lignin at high yields (29 % w/w based on the total raw biomass and >80 % w/w of these two fractions). Acetal functionalization of xylan and lignin with fatty aldehydes during fractionation introduced hydrophobic segments and subsequent defunctionalization by hydrogenolysis of the xylose derivatives or acidic hydrolysis of the lignin derivatives produced amphiphiles. The resulting biodegradable xylose acetals and/or ethers, and lignin-based amphiphilic polymers both largely retained their original natural structures, but exhibited competitive or superior surface activity in water/oil systems compared to common bio-based surfactants. 相似文献
999.
E. D. T. Atkins M. J. Hill N. A. Jones S. J. Cooper 《Journal of Polymer Science.Polymer Physics》1998,36(13):2401-2412
Chain-folded lamellar crystals of nylon 2 4 have been prepared from dilute solution by addition of poor solvent. Two crystal structures are observed at room temperature: a monoclinic form I, precipitated at elevated temperature, and a less-defined, orthorhombic form II, precipitated at room temperature. The unit cell parameters for both forms are similar to those reported for its isomer, nylon 3. Nylon 2 4 form II is a liquid–crystal-like or disordered phase, consisting of hydrogen-bonded sheets in poor register in the hydrogen bond direction. Form I crystals have two characteristic interchain spacings of 0.41 nm and 0.39 nm at room temperature and on heating, exhibit a structural transformation and a Brill temperature (250°C) characteristic of many other even–even nylons. Nylon 2 4 is a member of the nylon 2 Y and nylon 2N 2(N+1) families, and the form I crystals show behavior commensurate with both. We propose they contain a proportion of intersheet hydrogen bonds at room temperature, similar to that for the nylon 2 Y family, and the short dimethylene alkane segments mean that the structure consists of hydrogen-bonded a-sheets, with an amide unit in each fold, similar to that of nylon 4 6. The fold geometry and sheet structure is compared with chain-folded apβ-sheet polypeptides and nylon 3. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2401–2412, 1998 相似文献
1000.
M. Carmen Blanco Eduardo J. Fernndez Peter G. Jones Antonio Laguna Jos M. Lpez-de-Luzuriaga M. Elena Olmos 《Angewandte Chemie (International ed. in English)》1998,37(21):3042-3043
The covalent radius of Au I is about 0.07 Å smaller than that of AgI. This was determined from the crystal structures of the isostructural complexes [N(PPh3)][{Au(C6F5)3(μ-PPh2)}2M] (M=Au (structure shown in the picture), Ag). These mixed AuIII–M phosphides were synthesized from [Au(C6F5)3(PPh2H)], the first gold complex to contain a secondary phosphane. 相似文献