全文获取类型
收费全文 | 1366篇 |
免费 | 32篇 |
国内免费 | 10篇 |
专业分类
化学 | 898篇 |
晶体学 | 85篇 |
力学 | 43篇 |
数学 | 215篇 |
物理学 | 167篇 |
出版年
2021年 | 8篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2016年 | 24篇 |
2015年 | 28篇 |
2014年 | 13篇 |
2013年 | 60篇 |
2012年 | 57篇 |
2011年 | 72篇 |
2010年 | 43篇 |
2009年 | 41篇 |
2008年 | 55篇 |
2007年 | 71篇 |
2006年 | 63篇 |
2005年 | 70篇 |
2004年 | 56篇 |
2003年 | 39篇 |
2002年 | 55篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 27篇 |
1998年 | 17篇 |
1997年 | 19篇 |
1996年 | 25篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 20篇 |
1992年 | 22篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 18篇 |
1988年 | 12篇 |
1987年 | 14篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 24篇 |
1983年 | 20篇 |
1982年 | 22篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 18篇 |
1977年 | 13篇 |
1976年 | 23篇 |
1975年 | 20篇 |
1974年 | 10篇 |
1973年 | 15篇 |
1972年 | 15篇 |
1971年 | 7篇 |
1970年 | 10篇 |
排序方式: 共有1408条查询结果,搜索用时 125 毫秒
11.
A novel two-dimensional coordination polymer containing infinite, coherently pitched single and triple helical motifs is formed by the self-assembly of Cd2+, succinate, water and a bipyridyl ligand. 相似文献
12.
13.
Deuterium labeling in the acetate derivatives of cholic acid demonstrates that electron impact induced consecutive loss of HOAc from the molecular ion occurs in the following order of decreasing preference: 12α-OAc > 7α-OAc > 3α-OAc. Numerous examples are presented showing that consecutive loss of HOAc from a common ion generally increases progressively with relative intensity. In the acetate pregnan-20-one derivatives of cholic acid, loss of 43 mass units from the molecular ion comes from the 12α-OAc and not the 17β-acetyl sidechain. Ketene is ejected from an acetoxy group located in an α position to a carbonyl on which an ion-radical charge is localized. 相似文献
14.
Jerry Ray Dias 《Journal of mathematical chemistry》1990,4(1):17-29
The genesis of the aufbau and excised internal structure concepts is traced. These concepts were pivotal to the first published enumerations of the strictly peri-condensed, non-Kekuléan, and essentially strain-free total resonant sextet benzenoid groups. Aperiodic table set is defined as a partially ordered set forming a two-dimensional array which complies with thetriad principle where a central element has a metric property that is the arithmetic mean of two adjacent surrounding member elements. 相似文献
15.
Jerry Ray Dias 《Journal of mass spectrometry : JMS》1976,11(4):333-346
Mass spectra of various functionalized lanostanes and their deuterated analogs are compared. It is proposed that the transfer of a hydrogen from the 18-methyl group to the 11-oxo group via a McLafferty rearrangement is made geometrically possible by prior ionization of the 13,14 bond which allows an optimum interatomic distance between the oxygen and hydrogen to be acquired; the 8,9 double bond is a requisite for electronic induction of this specific process. Transfer of the hydrogen from the 7β-hydroxy group to position 14 is implicated in the mass spectrum of 3β-acetoxy-7β-hydroxy-5α-lanostan-11-one. 相似文献
16.
Jerry Ray Dias 《Journal of mathematical chemistry》1990,4(1):127-142
The molecular orbital parameters for tricarbonyl(tetrahapto-unsaturated-hydrocarbon)iron complexes are computed using graph-theoretical methods. The results are in agreement with their experimental properties. 相似文献
17.
18.
Simon G. Bott Matthew Clark Joseph S. Thrasher Jerry L. Atwood 《Journal of chemical crystallography》1987,17(2):187-196
The X-ray structure of S-methyl(pentafluorosulfanyl)thiocarbamate, SF5NHC(O)SCH3, has been determined from three-dimensional diffractometer data and refined by full-matrix least-squares techniques. The crystals are monoclinic:P21/m,a=5.008 (5),b=7.811 (2),c=9.348 (4) Å, =99.08 (7)°, andZ=2; finalR=0.046 for 517 observed reflections. The structure comprises hydrogen-bonded (NHO) chains with the component monomers in thecis, cis-conformation, i.e., the arrangement of the nonhydrogen and nonfluorine atoms is nearly planar, the SF5 group and the O are in acis position with respect to the C-NH bond, and the O and the CH3 group are in acis position with respect to the C(O)-S bond. Theoretical methods including MNDO and molecular modeling were used to study the relative stabilities of the four possible conformations of SF5NHC(O)SCH3. 相似文献
19.
Simon G. Bott Ulrich Kynast Jerry L. Atwood 《Journal of inclusion phenomena and macrocyclic chemistry》1986,4(3):241-246
18-crown-6 reacts with TiCl3 in CH2Cl2 to form a complex in which the crown ether functions as a tridentate ligand. Addition of moist hexane affords a molecular complex in which the crown ether functions as a bidentate ligand. A water molecule is bonded directly to the titanium atom and is further hydrogen bonded to three of the oxygen atoms of the crown. The deep blue crystals of the CH2Cl2 adduct belong to the monoclinic space groupP21/n witha=13.481(8),b=8.021(5),c=21.425(9) Å, =97.32(5)°, and
calc = 1.51 g cm–3 forZ=4. Refinement led to a conventionalR value of 0.040 based on 873 observed reflections. The Ti–O bond distances for the crown oxygen atoms are 2.123(8) and 2.154(9) Å, while the oxygen atom of the water molecule is bonded at 2.072(8) Å. The octahedral coordination sphere of the titanium atom is completed by the three chlorine atoms at distances of 2.340(5), 2.352(4), and 2.373(4) Å.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82034 (10 pages). 相似文献
20.
Conductance measurements for fifteen uni-univalent salts in 2-methylpyridine-N-oxide (2-MPO) were made over the concentration range of 2–120×10–4 mol-dm–3 at 55°C. All conductance data were evaluated by the Lee-Wheaton equation. The results indicate that all of the salts studied are slightly associated in 2-MPO, which is, nevertheless, a reasonably good ionizing solvent compared to other solvents having a similar value for the dielectric constant. The tetraalkylammonium halides are weaker electrolytes than the tetraphenylborate salts in 2-MPO. Ionic limiting molar conductances were obtained by using as a reference electrolyte tris(iso-pentyl)butylammonium tetraphenylborate. Anions appear to be poorly solvated in this solvent but certain cations, particularly Na+ and K+, show evidence of specific ion-solvent interaction.This study was presented, in part, at the 6th Central Regional Meeting of the American Chemical Society, Detroit, Michigan, April 1974. 相似文献