全文获取类型
收费全文 | 3876篇 |
免费 | 114篇 |
国内免费 | 9篇 |
专业分类
化学 | 2282篇 |
晶体学 | 97篇 |
力学 | 120篇 |
数学 | 437篇 |
物理学 | 1063篇 |
出版年
2021年 | 28篇 |
2020年 | 58篇 |
2019年 | 40篇 |
2018年 | 27篇 |
2017年 | 27篇 |
2016年 | 77篇 |
2015年 | 79篇 |
2014年 | 80篇 |
2013年 | 127篇 |
2012年 | 141篇 |
2011年 | 198篇 |
2010年 | 114篇 |
2009年 | 114篇 |
2008年 | 148篇 |
2007年 | 136篇 |
2006年 | 152篇 |
2005年 | 136篇 |
2004年 | 126篇 |
2003年 | 115篇 |
2002年 | 97篇 |
2001年 | 75篇 |
2000年 | 92篇 |
1999年 | 58篇 |
1998年 | 46篇 |
1997年 | 50篇 |
1996年 | 64篇 |
1995年 | 55篇 |
1994年 | 51篇 |
1993年 | 56篇 |
1992年 | 54篇 |
1991年 | 47篇 |
1990年 | 48篇 |
1989年 | 53篇 |
1988年 | 55篇 |
1987年 | 54篇 |
1986年 | 47篇 |
1985年 | 41篇 |
1984年 | 27篇 |
1983年 | 46篇 |
1982年 | 34篇 |
1981年 | 58篇 |
1980年 | 41篇 |
1979年 | 55篇 |
1978年 | 60篇 |
1977年 | 43篇 |
1976年 | 44篇 |
1975年 | 30篇 |
1974年 | 33篇 |
1973年 | 37篇 |
1957年 | 28篇 |
排序方式: 共有3999条查询结果,搜索用时 31 毫秒
121.
Erich Widmer Milan Soukup Reinhard Zell Emil Broger Hans Peter Wagner Marquard Imfeld 《Helvetica chimica acta》1990,73(4):861-867
Starting from the readily available, optically active (4R)-4-hydroxy-2,2,6-trimethylcyclohexanone ( 1 ), a new technical synthesis of (3R,3′R)-zeaxanthin is described. According to a 2(C9 + C6) + C10 = C40 construction scheme, the ketone 1 was first transformed with (E)-3-methylpent-2-en-4-yn-1-ol ( 5 ) into a C15-intermediate which, by a three-step sequence, could be converted into the known olefinic C15-Wittig salt 4 . Optimized conditions for the final Wittig reaction of 4 with the C10-dialdehyde 3 are discussed. Based on 1 , the overall yield of the entire technical process is ca. 40%. 相似文献
122.
Pawe Wagner Krzysztof
wierczek Maciej Kubicki 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(8):o445-o447
Infinite chains connected by N—H...N hydrogen bonding form the primary packing motif in two closely related 4‐nitroimidazole derivatives, viz. 5‐bromo‐2‐methyl‐4‐nitro‐1H‐imidazole, C4H4BrN3O2, (I), and 2‐methyl‐4‐nitro‐1H‐imidazole‐5‐carbonitrile, C5H4N4O2, (II). These chains are almost identical, even though in (II) there are two symmetry‐independent molecules in the asymmetric unit. The differences appear in the interactions between the chains; in (I), there are strong C—Br...O halogen bonds, which connect the chains into a two‐dimensional grid, while in (II), the cyano group does not participate in specific interactions and the chains are only loosely connected into a three‐dimensional structure. 相似文献
123.
124.
The structure of Ph[PhC(O)NH]2P=NC(O)Ph (2) in solid state and in solution is discussed on the basis of a crystal structure analysis and IR and Raman spectra. In crystalline state 2 forms dimers which are associated via two pairs of bifurcated (N–H)2O=C hydrogen bonds. The spectroscopic data are in good agreement with the crystallographic results with respect to the hydrogen bonding and they suggest that 2 also in solution is associated. 相似文献
125.
New Metal Oxides with Doubles of Tetrahedra as Building Units: Rb6[Tl2O6] and Cs6[In2O6] We prepared the hitherto unknown Rb6[Tl2O6] and Cs6[In2O6] by heating mixtures of Tl2O3 and RbO0.60 (Rb:Tl = 3.5:1) as well as In2O3 and CsO0.53 (Cs:In = 3.5:1) as single crystals [closed Ag-cylinder, 650°C, 14 d]. The single crystals of Rb6[Tl2O6] are yellow, those of Cs6[In2O6] pale yellow, all transparent and rude. The new type of structure was elucidated by 4-circle-diffractometer (PW 1100) data. Rb6[Tl2O6]: P21/a; a = 1145,7(3), b = 713,3(1), c = 783,9(2) pm, β = 93,73° (2), Z = 2; Ag–Kα, 2100 out of 2531 I0(hkl), R = 9,6% and Rw = 8,9%. Cs6[In2O6]: P21/a; a = 1178,5(4), b = 730,7(2), c = 816,3(2) pm, β = 95,38° (3), Z = 2; Mo–Kα, 1584 out of 2032 I0(hkl), R = 9,25%, and Rw = 8,44%. The Madelung Part of Lattice Energy, MAPLE, is calculated and discussed. 相似文献
126.
S. Schennach A. Müller O. Uwira J. Haselbauer W. Spies A. Frank M. Wagner R. Becker M. Kleinod E. Jennewein N. Angert P. H. Mokler N. R. Badnell M. S. Pindzola 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,30(4):291-306
Dielectronic recombination (DR) of Ar15+(1s 22s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3? (Δn=1) core excitations and even individual terms in the 1s 23?3?′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2 p 1/2? and 2 p 3/2? Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s 23?n?′) Rydberg series. 相似文献
127.
Martin Kirchner Walter Schnelle Frank R. Wagner Rüdiger Kniep Rainer Niewa 《无机化学与普通化学杂志》2005,631(8):1477-1486
Single phase powders of (A19N7)[In4]2 (A = Ca, Sr) and (Ca4N)[In2] were prepared by reaction of melt beads of the metallic components with nitrogen. The crystal structure of (Ca19N7)[In4]2 was refined based on neutron and X‐ray powder diffraction data. The crystal structure of (Sr19N7)[In4]2 was solved from the X‐ray powder pattern. The structure refinements in combination with results from chemical analyses ascertain the compositions. The compounds (A19N7)[In4]2 (A = Ca, Sr) are isotypes of (Ca19N7)[Ag4]2; (Ca19N7)[In4]2 is probably identical to the earlier reported (Ca18.5N7)[In4]2. The crystal structure of the isotypes (A19N7)[In4]2 (A = Ca, Sr; cubic, , Ca: a = 1471.65(3) pm; Sr: a = 1561.0(1) pm) contains isolated [In4] tetrahedra embedded in a framework of edge‐ and vertex‐sharing (A6N) octahedra. Six of these octahedra are condensed by edge‐sharing around one central A2+ ion to form “superoctahedra” (A19N6) which are connected three‐dimensionally via further octahedra by corner‐sharing. The crystal structure of (Ca4N)[In2] (tetragonal, I41/amd, a = 491.14(4) pm, c = 2907.7(3) pm) consists of alternating layers of perovskite type slabs of vertex‐sharing octahedra (Ca2Ca4/2N) and parallel arranged infinite zigzag chains equation/tex2gif-stack-1.gif[In2]. In the sense of Zintl‐type counting the compounds (A2+)19(N3?)7[(In2.125?)4]2 present an electron excess, (Ca2+)4(N3?)[(In2.5?)2] is electron deficient. Metallic properties are supported by electrical resistivity and magnetic susceptibility measurements. The analysis of the electronic structures gives evidence for the existence of homoatomic interactions In–In and significant heteroatomic metal–metal interactions Ca–In which favor the deviations of the title compounds from the (8 – N) rule. 相似文献
128.
Godoi AF Vilegas W Godoi RH Van Vaeck L Van Grieken R 《Journal of chromatography. A》2004,1027(1-2):127-130
Turnera diffusa Willd. var. afrodisiaca (Ward) Urb. (syn. T. aphrodisiaca) belongs to the family of Turneraceae and is an aromatic plant growing wild in the subtropical regions of America and Africa. It is widely used in the traditional medicine as e.g. anti-cough, diuretic, and aphrodisiac agent. This work presents a 3 min chromatographic analysis using low-pressure (LP) gas chromatography (GC)-ion-trap (IT) mass spectrometry (MS). The combination of a deactivated 0.6 m x 0.10 mm i.d., restrictor with a wide-bore CP-Wax 52 capillary column (10 m x 0.53 mm i.d., 1 microm) reduces the analysis time by a factor of 3-7 in comparison to the use of a conventional narrow bore column. Chromatographic conditions have been optimized to achieve the fastest separation with the highest signal/noise ratio in MS detection. These results allow fast and reliable quality control of the essential oil to be achieved. 相似文献
129.
A new DNA assay has been designed, prepared and applied for the chemical investigation of reductive electron transfer through the DNA. It consists of 5-(10-methyl-phenothiazin-3-yl)-2'-deoxyuridine (Ptz-dU, 1) as the photoexcitable electron injector and 5-bromo-2'-deoxyuridine (Br-dU) as the electron trap. The Ptz-dU-modified oligonucleotides were synthesised by means of a Suzuki-Miyaura cross-coupling protocol and subsequent automated phosphoramidite chemistry. Br-dU represents a kinetic electron trap, since it undergoes a chemical modification after its one-electron reduction that can be analysed by piperidine-induced strand cleavage. The quantification of the strand cleavage yields from irradiation experiments reveals important information about the electron-transfer efficiency. The performed DNA studies focused on the base sequence dependence of the electron-transfer efficiency with respect to the proposal that C*- and T*- act as intermediate electron carriers during electron hopping. From our observations it became evident that excess-electron transfer is highly sequence dependent and occurs more efficiently over T-A base pairs than over C-G base pairs. 相似文献
130.