全文获取类型
收费全文 | 236篇 |
免费 | 9篇 |
国内免费 | 5篇 |
专业分类
化学 | 150篇 |
晶体学 | 9篇 |
力学 | 4篇 |
数学 | 39篇 |
物理学 | 48篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 1篇 |
2018年 | 7篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 9篇 |
2012年 | 14篇 |
2011年 | 18篇 |
2010年 | 13篇 |
2009年 | 9篇 |
2008年 | 14篇 |
2007年 | 9篇 |
2006年 | 11篇 |
2005年 | 16篇 |
2004年 | 11篇 |
2003年 | 11篇 |
2002年 | 14篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 1篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有250条查询结果,搜索用时 859 毫秒
51.
Claudio Parra Emilio Soto Gloria León Cristian O. Salas Michael Heinrich Carlos Echiburú-Chau 《Natural product research》2018,32(6):719-722
Continuing with our study characterising Senecio nutans Sch. Bip., we have isolated and identified a simple coumarin, scopoletin, that could be relevant for the biological properties of the species related with the ancestral medical uses. This is the first report of scopoletin from S. nutans. In addition, the extract was analysed for its antioxidant activity using the ABTS and FRAP method as well as providing the first nutritional analyses of this plant from northern Chile highlands. 相似文献
52.
Zoltan T Benzo Z Murillo M Marcano E Gómez C Salas J Quintal M 《Analytical and bioanalytical chemistry》2005,382(6):1419-1430
A new nebulizer system is described that extends the analytical capability of the inductively coupled plasma technique to include the simultaneous determination of two elements Sb and Sn (hydride-forming), with two conventional elements, V and Zn. The main advantage of this system is its simultaneous determination of elements that form volatile hydrides and elements that do not, without any instrumental changes. Optimization of reaction and instrumental conditions was performed to characterize the new system. The performance of the new nebulizer system was evaluated by studying the effect of some transition metals (Ni, Cu, Co, and Fe, 1–1000 mg L–1) on the Sb, Sn, V, and Zn emission signals (1 mg L–1). Interferences from transition metal ions were found to be insignificant for determination of the four elements in presence of L-cysteine. Long-term and short-term stability was also evaluated. The precision, expressed as RSD for 15 replicate measurements was 0.7% for Sb, 1.7% for Sn, 2.5% for V, and 2.3% for Zn at 200 g L–1 of each analyte. The detection limits obtained were 0.52, 1.3, 3.2, and 4.7 g L–1 for Sb, Sn, V, and Zn, respectively. Spike and recovery experiments were performed on the NIST 1643c trace metals in water standard reference material and results were in agreement with the certified values. 相似文献
53.
Synthesis of 1,2,3,4-tetrahydroquinazoline derivatives as potential alpha-adrenergic blocking agents
The synthesis of N,N′-bis[6-(1,2,3,4-tetrahydro-3-quinazolidyl)hexyl]cystamine (I) and 3-(6-aminohexyl)-1,2,3,4-tetrahydroquinazoline (II) are described. Compound I is obtained by condensation of o-nitrobenzoyl chloride with 3-(6-aminohexyl)-1,3-thiazolidine (III) followed by dimerization, reduction and formation of tetrahydroquinazoline ring. A similar method was used for preparation of compound II. These compounds and some synthesis intermediates are potential alpha-adrenergic blockers. 相似文献
54.
The kinetics of piperidine replacement by pyridine at the pentacyano(piperidine)-ferrate(II) complex ion was studied under pseudo-first order conditions in binary aqueous mixtures of methanol, t-butanol, p-dioxane, and glycerol, from a mole fraction of co-solvent from 0 to about 0.15. The observed variations can be explained considering the degree of hydrophobic interaction between released ligand and water molecules which changes according to the structure-forming or struucture-breaking effect of added co-solvent on water. 相似文献
55.
T. Lopez M.E. Llanos J. Navarrete I. Schifter P. Salas R. Gomez 《Journal of Sol-Gel Science and Technology》1997,8(1-3):321-325
Mg−Si binary oxides were prepared by the sol-gel method, using magnesium diethoxide and tetraethoxysilane as starting materials.
Mg−Si mixed oxides were prepared at pH3 and 9 at nominal concentrations of 1.3, 4.0, and 6.7 mole% MgO. Density and strength
of the acid sites were determined by pyridine thermal programmed desorption. Comparable Lewis acidity was found in pH3 and
9 preparations. However, when the samples were sulfated with sulfuric acid, strong Bronsted acidity was developed, reaching
a maximum at 6.7 mole% MgO. The strong acidity of sulfated samples was corroborated using the isopropanol decomposition reaction
as catalytic test. Similar conversions are obtained at 100°C on sulfated samples, whereas 260°C are required for non-sulfated
ones. 相似文献
56.
Barea E Romero MA Navarro JA Salas JM Masciocchi N Galli S Sironi A 《Inorganic chemistry》2005,44(5):1472-1481
In searching for coordination polymers containing the highly polarized 5-nitro-pyrimidin-2-olate ligand (NP), a number of species containing 3d transition metals have been prepared and characterized, namely Co(NP)2(H2O)4, [Co(NP)2]n, Ni(NP)2(H2O)4, [Ni(NP)2]x, and [Zn(NP)2]n. Their structures have been determined by X-ray powder diffraction methods. The hydrated compounds contain mononuclear M(NP)2(H2O)4 units interconnected by means of a three-dimensional (3D) network of hydrogen bonds. The homoleptic species, at variance from the already known metal(II) pyrimidin-2-olate ones, crystallize as two-dimensional (2D) slabs, where the metal coordination is of the MN3O kind. The electron-withdrawing nitro group, never bound to the metal ion, is likely to influence the observed stereochemistry through steric and dipolar effects within the crystal lattice. The thermal, spectroscopic, and magnetic properties of these species are presented. The M(NP)2(H2O)4/[M(NP)2]x,n systems interconvert reversibly upon dehydration/rehydration processes. 相似文献
57.
58.
Bethany Lowe Alejandro L. Cardona Juana Salas Andras Bodi Maxi A. Burgos Paci Paul M. Mayer 《Journal of mass spectrometry : JMS》2022,57(6):e4868
The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH3OC(O)H, were elucidated to be CH3OH + CO, 2 CH2O and CH4 + CO2 as in part distinct from the dissociation of the radical cation (CH3OH+• + CO and CH2OH+ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH2O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol. 相似文献
59.
Brandstätter A Salas A Niederstätter H Gassner C Carracedo A Parson W 《Electrophoresis》2006,27(13):2541-2550
Haplogroup H (hg H) includes about 40-50% of the West Eurasian mitochondrial DNA (mtDNA) samples investigated so far. In order to enhance discrimination within this haplogroup we selected 45 coding region SNPs that allow to ascribe samples to the main phylogenetic branches of super hg HV (that embraces hg H) and, in particular, to H sublineages with a much finer resolution than previous studies. SNP selection was carried out using the most up-to-date available literature on population and forensic genetics and extended by means of phylogenetic analysis of complete or coding region genomes (<430) and control region sequences. A meticulous inspection of the H phylogeny led us to the observation of various but uncharacterized subclades of hg H. The selected SNPs were amplified in two PCR-multiplex reactions and subsequently targeted in three single-base extension multiplex reactions. A total of 2214 West Eurasian samples were screened for hg H specific loci 2706 and 7028, of which 859 fell in hg H and were further subjected to subhaplogroup typing. We observed 35 different subhaplogroups in total, 33 of which were found at frequencies below 5%. This assay can be used as a prescreening tool in forensic casework for rapid discrimination between divergent lineages (very effective for high-volume crime cases) or as discriminatory assay, when identical hg H haplotypes were obtained by control region sequencing. 相似文献
60.
Moss SJ Carletti I Olano C Sheridan RM Ward M Math V Nur-E-Alam M Braña AF Zhang MQ Leadlay PF Méndez C Salas JA Wilkinson B 《Chemical communications (Cambridge, England)》2006,(22):2341-2343
We report the directed biosynthesis of borrelidin analogues and their selective anti-proliferative activity against human cancer cell lines. 相似文献