全文获取类型
收费全文 | 986篇 |
免费 | 40篇 |
国内免费 | 2篇 |
专业分类
化学 | 684篇 |
晶体学 | 2篇 |
力学 | 21篇 |
数学 | 195篇 |
物理学 | 126篇 |
出版年
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 25篇 |
2019年 | 25篇 |
2018年 | 9篇 |
2017年 | 12篇 |
2016年 | 34篇 |
2015年 | 30篇 |
2014年 | 29篇 |
2013年 | 39篇 |
2012年 | 53篇 |
2011年 | 53篇 |
2010年 | 37篇 |
2009年 | 37篇 |
2008年 | 64篇 |
2007年 | 73篇 |
2006年 | 54篇 |
2005年 | 62篇 |
2004年 | 43篇 |
2003年 | 50篇 |
2002年 | 33篇 |
2001年 | 24篇 |
2000年 | 30篇 |
1999年 | 19篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1987年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1964年 | 3篇 |
1962年 | 3篇 |
1960年 | 2篇 |
1920年 | 2篇 |
排序方式: 共有1028条查询结果,搜索用时 31 毫秒
11.
The synthesis of two new aromatic cyclic enediynes 1a and 1b using a double Wittig condensation, followed by bromination and di-dehydrobromination, is reported. The structures were determined using X-ray crystallography. The observed C(1)-C(6) distances of the enediyne moiety in the oxygen- and sulfur-containing eleven-membered rings are quite short with 3.44 and 3.50 A, respectively, indicating the highly strained nature of these compounds. Isodesmic calculations at the B3LYP/6-31G level of theory indicate a strain energy of 8.2 and 5.8 kcal/mol, respectively. Despite the highly strained ring system, the compounds were found to be stable under thermal, photochemical, and electron-transfer conditions. Using BLYP/6-311+G/BLYP/6-31G calculations, the origin of this unusual stability was traced to the high energy of the 1,4 benzyne diradicals formed via Bergman cyclization. For the radical cation, both the C(1)-C(6) and the C(1)-C(5) cyclization mode previously reported for acyclic analogues of 1 were found to be energetically prohibitive. 相似文献
12.
Inderwildi OR Lebiedz D Deutschmann O Warnatz J 《The Journal of chemical physics》2005,122(15):154702
In this study, density functional theory calculations were performed to investigate the influence of oxygen preoccupation on the nitrogen oxide decomposition on rhodium. Besides gauging the coverage dependence of the adsorption energy of NO on the (111) rhodium facet, the influence of the initial oxygen coverage on the kinetics and thermodynamics of the nitrogen oxide decomposition reaction was also studied. The results are discussed with respect to a novel NOx decomposition catalyst. Furthermore, the influence of spin effects on the adsorption geometry as well as the adsorption energy is examined. It will be addressed why spin effects only have a minor influence on the behavior of NO on a rhodium surface. 相似文献
13.
Mg(AlH(4))(2) was produced as a nanocrystalline powder by metathesis of NaAlH(4) and MgCl(2). Starting with a structure estimation which was developed from an evaluation of FTIR data and comparison of structural properties of two solvent adducts, quantum chemical calculations were performed on the density functional theory (DFT) level. The calculated atomic positions were used to simulate an X-ray powder diffraction pattern, based on a trigonal unit cell. The simulated pattern was congruent to experimental data. Thus, magnesium alanate exhibits a CdI(2) layer structure, the layers being formed by Mg atoms occupying the Cd sites and AlH(4) tedrahedra occupying the sites of the iodine atoms in CdI(2). 相似文献
14.
Armin Presser Olaf Kunert Irmgard P?tschger 《Monatshefte für Chemie / Chemical Monthly》2006,26(1):365-374
Several improved approaches for the preparation of tetra-O-benzyl-α-D-glucopyranosyl bromide and tetra-O-pivaloyl-α-D-glucopyranosyl bromide are discussed. The importance of these compounds, which are useful glycosyl donors, was demonstrated
by successful preparation of cholesteryl glucopyranosides in an almost neutral medium without the formation of orthoesters.
In addition, accurate 1H and 13C NMR resonance assignments of the synthesized cholesteryl glycosides were performed by 2D NMR spectroscopy. 相似文献
15.
V. Mshvildadze O. Kunert G. Dekanosidze E. Kemertelidze E. Haslinger 《Chemistry of Natural Compounds》2004,40(6):561-564
Five triterpenoid saponins were isolated from the stems of Hedera colchica K. Koch, Araliaceae. Two of them are new natural substances. HCSt-A (1): 3-O--D-arabinopyranoside; 28-O--L-rhamnopyranosyl-(1 4)-O--D-glucopyranosyl-(16)-O--D-glucopyranosyl-arjunolic acid. HCSt-B (2): 3-O--D-xylopyranoside; 28-O--L-rhamnopyranosyl-(14)-O--D-glucopyranosyl-(16)-O--D-glucopyranosyl-hederagenin. The derivative of arjunolic acid is described for the first time in Araliaceae family. The chemical structures of isolated compounds were established on the base of chemical and 1D and 2D NMR experiments.Published in Khimiya Prirodnykh Soedinenii, No. 6, pp. 461–463, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date. 相似文献
16.
Olaf von Grudzinski 《manuscripta mathematica》1977,20(3):263-276
Fundamental solutions in a space D(Mp) of Roumieu ultradistributions are constructed for convolutors f whose Fourier transform
is slowly decreasing. The solutions are of exponential growth if
satisfies a stronger condition. These results include a constructive proof of the known existence theorem of Chou. For families of convolutors our method yields solutions which depend continuously on parameters. 相似文献
17.
Olaf Heitzsch Karsten Gloe Alfréd Sabela Jiří Koryta Edwin Weber 《Journal of inclusion phenomena and macrocyclic chemistry》1992,13(4):311-319
The complex forming properties with alkali metal and ammonium ions of a series of oligo benzo-condensed 18-crown-6 ethers1–8 having a different gradation of lipophilicity and of molecular rigidity are investigated by voltammetry at the interface of two immiscible electrolyte solutions (ITIES) and by a liquid-liquid extraction technique. The experimental results obtained in the two phase system H2O/nitrobenzene are discussed in relation to the structure of the crown and the cation type. The stability constants for the 1 : 1 complexes of Na+, K+, Rb+, Cs+ and NH
4
+
in nitrobenzene have been determined and compared with the extraction constants for the 1: 1 complexes of Na+ and K+ and for the 1 : 1 and 1 : 2 complexes of Cs+, showing the effect of oligo benzo condensation for the 18-crown-6 system. 相似文献
18.
In this paper the mineralization of CaCO(3) in various hydrogel matrices is presented. Sulfonic acid based hydrogels were prepared by introduction of sulfonate-containing monomers into a polyacrylamide network. The sulfonate content of polyacrylamide-co-vinylsulfonate and polyacrylamide-co-allylsulfonate decreases during elution of the copolymers in demineralized water, indicating insufficient linking of the sulfonate-bearing monomers within the hydrogel. In contrast to this, acrylamidomethylpropanesulfonate (AMPS) effectively copolymerizes with acrylamide (AAm) monomers. To study the influence of spatial arrangement of ionic functional groups within hydrogel networks on the mineralization of CaCO(3), AMPS copolymers with different degrees of AMPS cross-linking were synthesized. For the mineralization experiments the copolymers were placed into a double-diffusion arrangement. Calcite as the thermodynamically stable modification of CaCO(3) was obtained with a particular morphology. The pseudocubic habitus resembles aggregates obtained by mineralization in pure polyacrylamide. However, closer examination of the aggregates by scanning electron microscopy (SEM) shows that the crystal growth in the AMPS copolymers is different from that observed in polyacrylamide. Whereas the morphology of the calcite aggregates could be fine-tuned by using copolymers with different sulfonate content, the spatial distribution of the ionic functional groups alters the course of crystallization. Calcium ions are locally accumulated due to the heterogeneous distribution of functional sulfonate groups within the copolymer network. Thereby the nucleation of calcite is triggered, resulting in enhanced mineralization. 相似文献
19.
Density functional theory (DFT) calculations are employed to compare the mechanism of the *OH attacks at all carbon atoms in quinoline. The computational analysis of the energy surface for the reaction of *OH with quinoline reveals that the formation of OH adducts proceeds through exothermic formation of pi-complexes/H-bonded complexes. The gas-phase reactions have activation energies ranging from <1.3 kcal/mol for the attack at positions C3 through C8 to 8.6 kcal/mol for the attack at the C2 position. Solvation, as described by the CPCM cavity model, lowers these activation barriers so that the attack at all carbon atoms except C2 is effectively barrierless. The *OH attack at C2 in solution is significantly different than at all other quinoline positions because it involves the only transition structure with energy higher than that of the starting materials and with an energetic barrier of 5.1 kcal/mol. The specific solvation approach also corroborates this finding because the attack at C2 was shown to have an energy barrier of 2.3 kcal/mol compared to the barrierless attack at C5. These results are in agreement with our recent experimental studies but differ from literature reports on the degradation of quinoline using the photo-Fenton reaction. 相似文献
20.
Synthesis, Crystal Structure and Spectroscopic Characterization of [Au12(PPh)2(P2Ph2)2(dppm)4Cl2]Cl2 The reaction of [(AuCl)2dppm] (dppm = Ph2PCH2PPh2) with P(Ph)(SiMe3)2 in CHCl3 results in the formation of [Au12(PPh)2(P2Ph2)2(dppm)4Cl2]Cl2 ( 1 ), the crystal structure of which was determined by single crystal X‐ray analysis (space group P21/c, a = 1425.3(3) pm, b = 2803.7(6) pm, c = 2255.0(5) pm, β = 95.00(3)°, V = 8977(3)·106 pm3, Z = 2). The dication in 1 consists of two Au6P3 units built by highly distorted Au3P and Au2P2 heterotetrahedra, connected via four bidentate phosphine ligands. Additionally, the compound was characterized by IR‐, UV‐ and NMR spectroscopy. The 31P{1H} NMR spectrum is discussed in detail. 相似文献