全文获取类型
收费全文 | 1595篇 |
免费 | 142篇 |
国内免费 | 406篇 |
专业分类
化学 | 2070篇 |
晶体学 | 30篇 |
综合类 | 8篇 |
物理学 | 35篇 |
出版年
2024年 | 1篇 |
2023年 | 16篇 |
2022年 | 41篇 |
2021年 | 47篇 |
2020年 | 83篇 |
2019年 | 71篇 |
2018年 | 57篇 |
2017年 | 47篇 |
2016年 | 70篇 |
2015年 | 70篇 |
2014年 | 88篇 |
2013年 | 238篇 |
2012年 | 111篇 |
2011年 | 121篇 |
2010年 | 91篇 |
2009年 | 79篇 |
2008年 | 75篇 |
2007年 | 83篇 |
2006年 | 85篇 |
2005年 | 95篇 |
2004年 | 82篇 |
2003年 | 76篇 |
2002年 | 46篇 |
2001年 | 49篇 |
2000年 | 56篇 |
1999年 | 28篇 |
1998年 | 34篇 |
1997年 | 42篇 |
1996年 | 46篇 |
1995年 | 18篇 |
1994年 | 13篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 3篇 |
1990年 | 16篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有2143条查询结果,搜索用时 0 毫秒
71.
Mekhman S. Yusubov Elena N. Tveryakova Elena A. Krasnokutskaya Irina A. Perederyna Viktor V. Zhdankin 《合成通讯》2013,43(8):1259-1265
A simple and environmentally safe general method of iodination of aromatic substrates under sovent‐free conditions using the I2/AgNO3 combination in a solid state is reported. Both activated and deactivated aromatic compounds afford the respective aryl iodides in generally high yields (80–90%). 相似文献
72.
Gowri Sridhar 《合成通讯》2013,43(1):21-29
The 1,3‐dipolar cycloaddition of an azomethine ylide, generated from isatin and sarcosine by a decarboxylative route with various p‐substituted 3,5 bis(aryl methylidene)N‐methyl‐4‐piperidinones in refluxing methanol, proceeded regioselectively to give novel dispiroheterocycles. The product on subsequent annulation with hydrazine hydrate afforded 1‐N‐methyl‐spiro[2.3′]oxindole‐spiro[3.7″](3″‐aryl)‐5″‐methyl‐3″,3a″,4″,5″,6″,7″‐hexahydro‐2H‐pyrazolo[4,3‐c]pyridine‐4‐aryl‐pyrrolidines in good yield. 相似文献
73.
A new and efficient method to synthesize a 3,3,6,6,9‐aryl‐1,8‐dioxo‐octahydroxanthene derivative using diammonium hydrogen phosphate as catalyst was performed in water at room temperature in a short periods. 相似文献
74.
A transition-metal-free method for the synthesis of indolines has been developed. In the presence of K2CO3, the cyclization reaction of N-(ortho-chloromethyl)aryl amides and iodonium ylides proceeded smoothly at room temperature in moderate to good yields. 相似文献
75.
Nina K. Gusarova Pavel A. Volkov Nina I. Ivanova Boris G. Sukhov Ludmila I. Larina Olga N. Kazheva Grigorii G. Alexandrov Oleg A. D’yachenko Boris A. Trofimov 《Tetrahedron letters》2013
The oxidative cross-coupling between 4-hydroxy-6-methyl-2-pyrone or 3-hydroxy-2-methyl-4-pyrone and secondary phosphine chalcogenides proceeds in CCl4/Et3N under mild conditions (20–52 °С, 0.75–10 h) through the hydroxyl group to give O-(6-methyl-2-oxo-2H-pyran-4-yl) diorganylphosphinochalcogenoates or O-(2-methyl-4-oxo-4H-pyran-3-yl) diorganylphosphinochalcogenoates, in high yields. 相似文献
76.
Samples of silica gel dried at different temperatures, silica gel modified with 3-aminopropylsilyl (APS) and silica gel modified with APS and further with a ferrocenyl phosphine derivative were investigated by DRIFT, transmission FTIR and MicroRaman spectroscopy. The reaction between 3-aminopropyltrimethoxysilane (APTMS) and silica gel was mainly identified by the diminishing or vanishing intensity of the stretching band of the free OH groups in the silica gel. Further chemical reaction of the APS groups with a ferrocenyl phosphine derivative (suitable as ligand in homogeneous catalysis) was identified in the IR spectra by the appearance of the CN stretching band of the formed Schiff base, and diminishing intensity of the δ(NH2) modes. According to the IR spectra the reaction of the ferrocenyl phosphine derivative with the APS-modified silica gel is almost quantitative. From the recorded IR and Raman spectra, conclusions concerning the substitution of APTMS methoxy groups during the chemisorption on silica gel were derived. Through deconvolution of the complex Raman band in the siloxy stretching region of the APS-modified silica gel, the newly formed siloxy bond was identified. 相似文献
77.
Chinmoy Mukherjee 《Journal of carbohydrate chemistry》2013,32(4):213-221
A general, odorless, one‐pot methodology has been developed for the preparation of 1,2‐trans‐thioglycosides and thio‐Michael addition products of carbohydrate derivatives through triphenyl phosphine‐mediated cleavage of disulfides and reaction of the thiolate formed in situ with glycosyl bromides and glycosyl conjugated alkenes. 相似文献
78.
Benjamin M. Gridley Dr. Graeme J. Moxey Dr. William Lewis Prof. Alexander J. Blake Dr. Deborah L. Kays 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(34):11446-11453
The synthesis and characterization of the first series of low‐coordinate bis(terphenyl) complexes of the Group 12 metals, [Zn(2,6‐Naph2C6H3)2] ( 1 ), [Cd(OEt2)(2,6‐Naph2C6H3)2] ( 2 ) and [Hg(OEt2)(2,6‐Naph2C6H3)2] ( 3 ) (Naph=1‐C10H7) are described. The naphthyl substituents of the terphenyl ligands confer considerable steric bulk, and as a result of limited flexibility introduce multiple conformations to these unusual systems. In the solid state, complex 1 features a two‐coordinate Zn centre with the ligands oriented in a syn/anti conformation, whereas the three‐coordinate distorted T‐shaped complexes 2 and 3 feature the ligands in the syn/syn configurations. The results of DFT calculations are in good agreement with the solid‐state configurations for these complexes and support the spectroscopic measurements, which indicate several conformers in solution. 相似文献
79.
An efficient and convenient method for the synthesis of aryl hydrazines has been developed via copper-catalyzed cross-coupling of aryl bromides and hydrazine with a readily accessible ligand and water as a solvent. The multigram scale procedure is applicable to aryl bromides bearing both moderately electron-donating and electron-withdrawing substituents in the aromatic nucleus. No column chromatography is required to obtain aryl hydrazine hydrochlorides in good yields. 相似文献
80.
The gas‐phase acidities (GA) of various aryl‐substituted fluoroalkanes, XC6H4CH(R1)R2, were calculated at the B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p). The acidity values of alkanes having a common substituent X varied significantly with the change of R1 and R2. Their changes in acidity of 1 and 2 having two strong electron‐withdrawing groups (CF3 or C2F5) at the deprotonation site and 8 , 9 , 10 , 11 having no fluorine atom at β‐position were linearly correlated with the corrected number of fluorine atoms contained in the fluorinated alkyl group (R2 > 0.999). On the other hand, the GA values of β‐fluorine substituted alkanes ( 3 , 4 , 5 , 6 , 7 ) deviated in a stronger acid direction from the line. The enhanced acidity was attributed to the additional stabilization of the conjugate anion caused by the β‐fluorine negative hyperconjugation. The magnitude of β‐fluorine negative hyperconjugation of the fluorinated alkyl group (ΔGoβ‐F) given by the deviations from the line decreased with increasing electron‐withdrawing ability of substituent X on the benzene ring, indicating that β‐fluorine negative hyperconjugation competes with the electronic effect of the substituent X. The GAel values obtained by subtraction ΔGoβ‐F from the apparent GA value were successfully correlated in terms of the Yukawa–Tsuno equation. The obtained ρel and r?el values were linearly related to the GAel value of the respective phenyl‐substituted fluoroalkanes, supporting our previous conclusion that the ρ and r? values for the substituent effect caused by the electronic effects of the substituent on the acidity are determined by the thermodynamic stability of the parent ion (ring substituent = H). Copyright © 2015 John Wiley & Sons, Ltd. 相似文献