全文获取类型
收费全文 | 386篇 |
免费 | 18篇 |
国内免费 | 1篇 |
专业分类
化学 | 282篇 |
晶体学 | 8篇 |
力学 | 22篇 |
数学 | 35篇 |
物理学 | 58篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 11篇 |
2021年 | 17篇 |
2020年 | 7篇 |
2019年 | 6篇 |
2018年 | 13篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 15篇 |
2013年 | 17篇 |
2012年 | 26篇 |
2011年 | 22篇 |
2010年 | 11篇 |
2009年 | 21篇 |
2008年 | 18篇 |
2007年 | 28篇 |
2006年 | 21篇 |
2005年 | 27篇 |
2004年 | 21篇 |
2003年 | 19篇 |
2002年 | 21篇 |
2001年 | 6篇 |
2000年 | 13篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 9篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1979年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有405条查询结果,搜索用时 171 毫秒
121.
P-Stereogenic oxazaphosphorine compounds of the form 4 have not previously been reported as asymmetric ligands for metal-catalyzed reactions. In an effort to explore the behavior of such oxazaphosphorine ligands, monomeric oxazaphosphorine borane 9 and dimeric oxazaphosphorine boranes 25 and 26 were synthesized as catalyst precursors. The absolute configuration of the phosphorus center contained in the oxazaphosphorines was determined by X-ray crystallography. Rhodium-catalyzed hydrogenation of methyl 2-acetamidoacrylate using a dimerized spiro oxazaphosphorine ligand was performed with up to 15% ee. The extreme sensitivity of the oxazaphosphorine ligands toward oxidation prevented further optimization of the enantioselectivity. 相似文献
122.
Umar Farooq Rizvi Hamid Latif Siddiqui Saeed Ahmad Matloob Ahmad Masood Parvez 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(10):o547-o549
Molecules of (E)‐3‐(2‐chloro‐6‐methylquinolin‐3‐yl)‐1‐(5‐iodo‐2‐thienyl)prop‐2‐en‐1‐one, C17H11ClINOS, (I), and (E)‐3‐(2‐chloro‐6‐methylquinolin‐3‐yl)‐1‐(5‐methyl‐2‐furyl)prop‐2‐en‐1‐one, C18H14ClNO2, (II), adopt conformations slightly twisted from coplanarity. Both structures are devoid of classical hydrogen bonds. However, nonclassical C—H...O/N interactions [with C...O = 3.146 (5) Å and C...N = 3.487 (3) Å] link the molecules into chains extended along the b axis in (I) and form dimers with an R22(8) motif in (II). The structural analysis of these compounds provides an insight into the correlation between molecular structures and intermolecular interactions in compounds for drug development. 相似文献
123.
124.
J. Michael Robinson Masood Ahmed Nicky J. Alaniz Timothy R. Boyles Chris D. Brasher Kimberly A. Floyd Preston L. Holland Laura D. Maruffo Terry L. Mcmahan Stan Middleton Kevin D. O'Hara Marcia J. Pack Brandon D. Reynolds Romelia R. Rodriquez Dennis E. Sawyer Elena Sharp Sharai L. Simpson Clint L. Vanlandingham Rebecca S. Velasquez Brian M. Welch C. David Wright 《Journal of heterocyclic chemistry》1998,35(1):65-69
Alternative complimentary syntheses of penta-substituted pyridine rings with full regiochemical control of substituents were studied as a method for the synthesis of Streptonigrin ( 1 ). Various α-substituted acetophenones 2 were reacted with enones 3 in acetic acid/ammonium acetate and air to afford penta-substituted pyridines 4 . α-Substituents that could provide a source of exocyclic nitrogen at position 3 of these Steptonigrin ring-C models proved to be the limiting factor. However, an inverse “3+2+1” cyclocondensation of α-cyanochalcone 5c with 2-furyl ethyl ketone ( 6b ) afforded the desired model 6-(2-furyl)-5-methyl2,4-diphenyl-3-pyridinecarbonitrile ( 4g ) in 75% yield. 相似文献
125.
The electronic effects of sulfur in gamma-sulfenyl enones are not transmitted to the carbonyl carbon through the pi bond as reported previously. The diastereoselectivity is rather controlled by a combination of several other factors. The steric effects arising from the substituents on the sulfur atom and the gamma-carbon and the bulk of the nucleophile constitute the major control elements. 相似文献
126.
Srivatsan SG Parvez M Verma S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5184-5191
We have synthesized and characterized novel, copper-metalated, polymeric templates that contain adenine nucleobases. These promote hydrolysis of non-natural and natural phosphate ester substrates in a highly efficient and catalytic fashion. The crystal structure of the cooper-containing adenylated monomer reveals the formation of a polymeric array, through coordination to both N1 and N7 atoms. Possible implications of these studies for prebiotic catalysis, involving synergism between adenine and copper ions, are also discussed. 相似文献
127.
Saeed‐ur‐Rehman Muhammad Arshad Masood Ahmad Khalid Masud Abdul Ghani Nasima Arshad Ammad Hussain Qureshi Sohail Akhtar Ghauri 《中国化学会会志》2000,47(3):501-508
Some of the bis‐complexes of a derivative of phenhomazine (dibenzo[b,f][1:5]diazocine‐6:12‐dione; PHZD) with Ni(II), Cu(II), Co(II), Cd(II), Zn(II) and Hg(II) of the general formula M(PHZD)2X2 [where X = C1, Br and I], were prepared and identified. These complexes have been characterised on the basis of elemental analysis, and spectroscopic, magnetic and conductance data. The thermal mode of decomposition and thermal stability of these complexes was investigated on the basis of the respective thermal curves in a static air atmosphere. The thermoanalytical investigations indicate that these complexes undergo two‐step changes as temperature is raised, except for Cd(II) and Hg(II) complexes, with the formation of metal oxides as end product. The degradation mechanism of the complexes has also been proposed. 相似文献
128.
Chaudry Masood Khalique 《Applied mathematics and computation》2011,217(18):7400-7404
This paper studies the Biswas-Milovic equation by the aid of Lie symmetry analysis. Four types of nonlinearity are being studied for this equation. They are Kerr law, power law, parabolic law and the dual-power law. A closed form stationary solution is obtained for each case. 相似文献
129.
Isaiah Elvis Mhlanga Chaudry Masood Khalique 《Mediterranean Journal of Mathematics》2014,11(2):487-496
In this paper, we study two nonlinear evolution partial differential equations, namely, a modified Camassa–Holm–Degasperis–Procesi equation and the generalized Korteweg–de Vries equation with two power law nonlinearities. For the first time, the Lie symmetry method along with the simplest equation method is used to construct exact solutions for these two equations. 相似文献
130.
Shayessteh Dadfarnia Ali Mohammad Haji Shabani Masood Reza Shishebor Kolsom Hosseini Cisakht 《International journal of environmental analytical chemistry》2013,93(13):1320-1328
A sensitive and simple method for determination of chromium species after separation and preconcentration by solid phase extraction (SPE) has been developed. For the determination of the total concentration of chromium in solution, Cr(VI) was efficiently reduced to Cr(III) by addition of hydroxylamine and Cr(III) was preconcentrated on a column of immobilised ferron on alumina. The adsorbed analyte was then eluted with 5?mL of hydrochloric acid and was determined by flame atomic absorption spectrometery. The speciation of chromium was affected by first passing the solution through an acidic alumina column which retained Cr(VI) and then Cr(III) was preconcentrated by immobilised ferron column and determined by FAAS. The concentration of Cr(VI) was determined from the difference of concentration of total chromium and Cr(III). The effect of pH, concentration of eluent, flow rate of sample and eluent solution, and foreign ions on the sorption of chromium (III) by immobilised ferron column was investigated. Under the optimised conditions the calibration curve was linear over the range of 2–400?µg?L?1 for 1000?mL preconcentration volume. The detection limit was 0.32?µg?L?1, the preconcentration factor was 400, and the relative standard deviation (%RSD) was 1.9% (at 10?µg?L?1; n?=?7). The method was successfully applied to the determination of chromium species in water samples and total chromium in standard alloys. 相似文献