全文获取类型
收费全文 | 760篇 |
免费 | 19篇 |
专业分类
化学 | 572篇 |
晶体学 | 9篇 |
力学 | 4篇 |
数学 | 47篇 |
物理学 | 147篇 |
出版年
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 8篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 7篇 |
2016年 | 16篇 |
2015年 | 23篇 |
2014年 | 17篇 |
2013年 | 26篇 |
2012年 | 36篇 |
2011年 | 38篇 |
2010年 | 17篇 |
2009年 | 23篇 |
2008年 | 37篇 |
2007年 | 16篇 |
2006年 | 31篇 |
2005年 | 32篇 |
2004年 | 23篇 |
2003年 | 22篇 |
2002年 | 22篇 |
2001年 | 14篇 |
2000年 | 11篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 6篇 |
1996年 | 17篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1971年 | 4篇 |
1964年 | 4篇 |
1962年 | 4篇 |
排序方式: 共有779条查询结果,搜索用时 15 毫秒
11.
A sensitive and selective bioanalytical liquid chromatographic method for diclofenac is described. The drug was detected as a flourescent derivative, which was demonstrated by 1H NMR and mass spectrometric studies to be carbazole acetic acid. Diclofenac was derivatized by UV irradiation of the substance performed as a post-column photoreaction. The reactor was a PTFE capillary wound around a 254-nm UV lamp. Diclofenac was isolated from the plasma samples by precipitation of the proteins with acetonitrile. A 50-microliters volume of the supernatant was injected onto a Nucleosil C18 column. The mobile phase was 32% acetonitrile in pH 6.6 buffer. Carbazole acetic acid was detected by a fluorescence detector using an excitation wavelength of 288 nm and an emission wavelength of 360 nm. The recovery was 92%, the standard curve was linear in the range 10-5500 ng diclofenac per ml plasma, and the relative standard deviation at 10 and 5000 ng of diclofenac per ml plasma was 9.0% and 3.3%, respectively. The limit of detection was 6 ng/ml at an injection volume of 50 microliters. Chromatograms of human and rat plasma containing diclofenac are shown. 相似文献
12.
Zhdankin VV Kuehl CJ Krasutsky AP Bolz JT Simonsen AJ 《The Journal of organic chemistry》1996,61(19):6547-6551
Organosulfonyloxy derivatives of 1,2-benziodoxol-3(1H)-one (3a-c) and 3,3-bis(trifluoromethyl)-3(1H)-1,2-benziodoxole (5a-c) can be prepared in high yield by the reaction of 1-hydroxybenziodoxoles 1 or 4 and the corresponding sulfonic acids or Me(3)SiOTf in the form of stable, but moderately hygroscopic, microcrystalline solids. Reaction of the triflate derivatives 3a and 5a with alkynyltrimethylsilanes affords either alkynyliodonium triflates 6, or (E)-beta-(trifluoromethanesulfonyloxy)alkenyliodonium triflates 7, while the same reaction in the presence of pyridine selectively gives the respective 1-alkynylbenziodoxoles 8 and 9 in 82-90% yield. 相似文献
13.
Holzmueller J Genson KL Park Y Yoo YS Park MH Lee M Tsukruk V 《Langmuir : the ACS journal of surfaces and colloids》2005,21(14):6392-6398
Amphiphilic dendron-rod molecules with three hydrophilic poly(ethylene oxide) (PEO) branches attached to a hydrophobic octa-p-phenylene rod stem were investigated for their ability to form two-dimensional micellar structures on a solid surface. A treelike shape of the molecules was reported to be a major factor in the formation of nonplanar micellar structures in solution and in the bulk state (cylindrical and spherical). We observed that in these treelike amphiphilic molecules the hydrophilic terminated dendron branches assemble themselves in surface monolayers with the formation of two-dimensional layered or circular micellar structures. We suggested the formation of the planar ribbon-like structures with interdigitated layering within the loosely packed monolayers and circular, ringlike structures (2D circular aggregates) in the precollapsed state. 相似文献
14.
15.
With the help of a dilatometer which permits the continuous determination of volume changes occuring in chemical reactions in aqueous solutions, the volume change during the precipitation of BaSO4 was followed and applied to the determination of the endpoint of the precipitation. In the case of precipitations of 1 mM BaSO4 the end-point determination can be reproduced with a standard deviation of less than 0.2%. A strict linearity between the amount of Na2SO4 applied and the position of the end-point does not exist. However, a correction factor could be established, so that the absolute determination of sulphate is possible (standard deviation 0.2%). The time required to carry out a single measurement is about 1.5 min. The acid concentration does not influence the composition. On the other hand, the composition was shown to be considerably dependent on Na+ and NH4 + ions. The presence of the cation N(CH3)4 + does not alter the composition of the precipitate. It is obviously too big to substitute the Ba2+ ion. 相似文献
16.
Kirsten Gesenberg Prashant P. Deshpande Annie Pullockaran Feng Xu Dedong Wu Qi Gao Charles Pathirana John Castoro Nachimuthu Soundararajan Andrew Staab 《Tetrahedron letters》2007,48(15):2675-2677
Transformation of 4,7-dimethoxy-6-azaindole into 4-hydroxy-7-methoxy-6-azaindole or 7-hydroxy-4-methoxy-6-azaindole can be readily controlled by careful selection of a reagent. Treatment with concentrated HCl results in hydrolysis at the 4-position exclusively, while TMS-I provides demethylation at the 7-position only. Products were unambiguously identified by single crystal X-ray crystallography. 相似文献
17.
Salvador Puig-Torres Gary E. Martin Steven B. Larson Stanley H. Simonsen 《Journal of heterocyclic chemistry》1984,21(4):995-1000
Synthesis of the 1,3-diazaphenoxathiin ring system and the confirmation of its structure by 13C-nmr spectroscopy and X-ray crystallography are reported. Implications of the 13C-nmr chemical shift of the C-10a resonance and its relationship to the molecular dihedral angle are presented. The molecule crystallizes in the Pbca space group and was found to have a dihedral angle of 165.5(9)°, the structure refining to a final R-factor of T = 0.0427. 相似文献
18.
Joseph H. Montoya Kirsten T. Winther Raul A. Flores Thomas Bligaard Jens S. Hummelshj Muratahan Aykol 《Chemical science》2020,11(32):8517
We present an end-to-end computational system for autonomous materials discovery. The system aims for cost-effective optimization in large, high-dimensional search spaces of materials by adopting a sequential, agent-based approach to deciding which experiments to carry out. In choosing next experiments, agents can make use of past knowledge, surrogate models, logic, thermodynamic or other physical constructs, heuristic rules, and different exploration–exploitation strategies. We show a series of examples for (i) how the discovery campaigns for finding materials satisfying a relative stability objective can be simulated to design new agents, and (ii) how those agents can be deployed in real discovery campaigns to control experiments run externally, such as the cloud-based density functional theory simulations in this work. In a sample set of 16 campaigns covering a range of binary and ternary chemistries including metal oxides, phosphides, sulfides and alloys, this autonomous platform found 383 new stable or nearly stable materials with no intervention by the researchers.We present an end-to-end computational system for autonomous materials discovery. 相似文献
19.
R. A. Loghry S. Kasina S. H. Simonsen Jay Nematollahi 《Journal of heterocyclic chemistry》1976,13(4):931-932
The structures of two of the three isomeric products of the reactions between cis- and trans-cyclohexanedicarboxylic anhydrides and methylhydrazine were determined by single-crystal X-ray methods to ascertain their tautomeric forms and stereochemistries. One product was found to be cis-N-methylamino-1,2-cyclohexanedicarboximide and the other trans-hexahydro-2-methylcyclohexa[d]pyridazine-1,2-dione. 相似文献
20.
John G. Stuart Michael J. Quast Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Milton L. Lee Raymond N. Castle Jerry L. Dallas Boban K. John LeRoy F. Johnson 《Journal of heterocyclic chemistry》1986,23(4):1215-1234
Syntheses of benzo[3,4]phenanthro[1,2-b]thiophene, benzo[3,4]phenanthro[2,1-b]thiophene and their 1-methyl analogs are reported as potential constituents of solvent refined coal liquids and for mutagenicity testing. The attempted synthesis of the 13-methyl analogs which gave the 11-methyl isomers is also described. Total assignments of the 1H- and 13C-nmr spectra based on long range optimized heteronuclear protoncarbon two-dimensional chemical shift correlation are reported. Carbon assignments obtained for benzo[3,4]-phenanthro[1,2-b]thiophene using this approach were confirmed with a 125 MHz 13C–13C INADEQUATE spectrum. X-Ray crystal structures were determined for benzo[3,4]phenanthro[1,2-b]thiophene and 1-methyl-benzo[3,4]phenanthro[2,1-b]thiophene. Both molecules were helically distorted from planarity. Close intramolecular contacts between the bay region H1–H13 and ClMe-H13 of 2.03 and 2.28 Å, respectively, were responsible for the distortions. There were no close intermolecular contacts of <3.5Å. both molecules refined to an R value of <0.05. 相似文献