全文获取类型
收费全文 | 1271篇 |
免费 | 36篇 |
专业分类
化学 | 1108篇 |
晶体学 | 11篇 |
力学 | 6篇 |
数学 | 64篇 |
物理学 | 118篇 |
出版年
2024年 | 2篇 |
2023年 | 19篇 |
2022年 | 116篇 |
2021年 | 108篇 |
2020年 | 35篇 |
2019年 | 31篇 |
2018年 | 37篇 |
2017年 | 36篇 |
2016年 | 78篇 |
2015年 | 44篇 |
2014年 | 56篇 |
2013年 | 92篇 |
2012年 | 80篇 |
2011年 | 93篇 |
2010年 | 64篇 |
2009年 | 48篇 |
2008年 | 71篇 |
2007年 | 69篇 |
2006年 | 41篇 |
2005年 | 47篇 |
2004年 | 37篇 |
2003年 | 26篇 |
2002年 | 24篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1307条查询结果,搜索用时 0 毫秒
61.
Efficient Solvent‐Free Synthesis of Benzothiazine‐Fused Pyrrolo[3,4‐c]coumarins: Cycloaddition Reactions between Coumarin‐Based Dihydrobenzothiazoles and Isocyanides 下载免费PDF全文
Mehdi Khoobi Ali Ramazani Mohammad Mahdavi Alireza Foroumadi Saeed Emami Sang Woo Joo Katarzyna Ślepokura Tadeusz Lis Abbas Shafiee 《Helvetica chimica acta》2014,97(6):847-853
A new and unusual synthesis of benzothiazine‐fused pyrrolo[3,4‐c]coumarins, involving the ring‐opening of coumarin‐based dihydrobenzothiazoles and subsequent [4+1] cycloaddition reaction with isocyanides, was described. Thus, simple heating of various 3‐(2,3‐dihydro‐2‐methylbenzo[d]thiazol‐2‐yl)coumarins with isocyanides produced the title compounds in good yields under solvent‐free conditions. 相似文献
62.
Synthesis of Novel α‐(Acyloxy)‐α‐(quinolin‐4‐yl)acetamides by a Three‐Component Reaction between an Isocyanide,Quinoline‐4‐carbaldehyde,and Arenecarboxylic Acids 下载免费PDF全文
Jafar Taran Ali Ramazani Sang Woo Joo Katarzyna Ślepokura Tadeusz Lis 《Helvetica chimica acta》2014,97(8):1088-1096
Novel α‐(acyloxy)‐α‐(quinolin‐4‐yl)acetamides were synthesized by the Passerini three‐component reaction between an isocyanide, quinoline‐4‐carbaldehyde, and arenecarboxylic acids in H2O. The reactions were carried out in one pot at room temperature with quantitative yields. 相似文献
63.
Janina Lulek Maurycy Opielewicz Katarzyna Szyrwińska Bart?omiej Milanowski 《Analytica chimica acta》2005,540(1):25-31
The database of the relative retention times (RRTs) of polychlorinated biphenyls (PCBs) reported in literature was used to calculate the retention indices (RIs) of all 209 PCB congeners on temperature programmed capillary column Rtx-5. Calculation of retention indices was based on reference series of seven congeners (PCB IUPAC Nos. 18, 52, 101, 143, 185, 203 and 206) that exhibit linear relative retention time behaviour as a function of chlorine number. The calculated indices were compared to those determined in our laboratory as well as to those obtained by other authors. The proposed indices system was applied for identification individual congeners in mixture of Aroclors 1242:1254:1260, using only reference series of PCBs. 相似文献
64.
The Diels-Alder reaction between cyclopentadiene and dienophiles in deanol derivatives containing bis(trifluoromethylsulfonyl)imide
anion as media have been studied. The effect of the substituents attached to the cation on the endo:exo selectivity as well
as the reaction yield have been evaluated in the absence and presence of Lewis acid catalyst - Y(OTf)3. Catalytic activity of metal triflates and the recycling of chosen catalytic systems have also been investigated
相似文献
65.
Beata Kolesińska Katarzyna Kasperowicz Adam Mazur Zbigniew J. Kamiński 《Tetrahedron letters》2010,51(1):20-4513
The concept of a chiral coupling reagent for the enantioselective synthesis of peptides with a predictable configuration and enantiomeric purity from racemic substrates is presented. The reagent was prepared by treatment of strychninium tetrafluoroborate with 2-chloro-4,6-dimethoxy-1,3,5-triazine in the presence of sodium bicarbonate yielding N-(4,6-dimethoxy-1,3,5-triazin-2-yl)strychninium tetrafluoroborate in high yield, which is stable at room temperature, and in a broad range of solvents gave enriched Z-Ala-Phe-OMe (dr from 95/5 to 60/40) in high yield with d-configuration on the alanine residue starting from rac-Z-Ala-OH. 相似文献
66.
Magda Milewska Katarzyna Guzow Wies?aw Wiczk 《Central European Journal of Chemistry》2010,8(3):674-686
The ability of new chelate ligands, benzoxazol-5-yl-alanine derivatives substituted in position 2 by heteroaromatic substituent,
to form complexes with selected metal ions in acetonitrile are studied by means of absorption and steady-state and time-resolved
fluorescence spectroscopy. Among the ligands studied, only azaaromatic derivatives form stable complexes with transition metal
ions in the ground state. Their absorption bands are bathochromically shifted enabling to use those ligands as ratiometric
sensors. The fluorescence of each ligand is quenched by metal ions, however, in the presence of Cd(II) and Zn(II) ions a new
red shifted emission band is observed.
相似文献
67.
Katarzyna Gajda Bartosz Zarychta Zdzisaw Daszkiewicz Andrzej A. Domaski Krzysztof Ejsmont 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(6):575-579
The crystal and molecular structures of two para‐substituted azobenzenes with π‐electron‐donating –NEt2 and π‐electron‐withdrawing –COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the –NEt2 group in N,N,N′,N′‐tetraethyl‐4,4′‐(diazenediyl)dianiline, C20H28N4, (I), may be caused by steric hindrance and the π‐electron‐donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp2‐hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4′‐(diazenediyl)dibenzoate, C18H18N2O4, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N—C—C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II). 相似文献
68.
A sensitive method of Co(II) determination by adsorptive stripping voltammetry is presented. The method exploits the enhancement of cobalt peak current observed in the system Co(II)-nioxime-cetyltrimethylammonium bromide-piperazine-N,N′-bis(2-ethanesulfonic acid). The calibration plot for an accumulation time of 60 s is linear from 5 × 10−11 to 3 × 10−9 mol L−1. The relative standard deviation is 3.8% for Co(II) determination at concentration 1 × 10−9 mol L−1. The detection limit is 1.7 × 10−11 mol L−1. The validation of the method is performed by the analyses of certified reference materials and comparing the result of Co(II) determination in river water sample by the proposed method with those obtained by ET AAS. The main advantage of this new system is the micro-trace Co(II) determination by adsorptive stripping voltammetry, as compared to those described before, a low concentration of the supporting electrolyte used, and so commercially available reagents without additional purification can be used. 相似文献
69.
Ting Zhao Katarzyna Kurpiewska Justyna Kalinowska‐Tłuścik Eberhardt Herdtweck Prof. Dr. Alexander Dömling 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(9):3009-3018
The synthesis of all 20 common natural proteinogenic and 4 otherα‐amino acid‐isosteric α‐amino tetrazoles has been accomplished, whereby the carboxyl group is replaced by the isosteric 5‐tetrazolyl group. The short process involves the use of the key Ugi tetrazole reaction followed by deprotection chemistries. The tetrazole group is bioisosteric to the carboxylic acid and is widely used in medicinal chemistry and drug design. Surprisingly, several of the common α‐amino acid‐isosteric α‐amino tetrazoles are unknown up to now. Therefore a rapid synthetic access to this compound class and non‐natural derivatives is of high interest to advance the field. 相似文献
70.
The lipophilicity of the series of alpha-(4-phenylpiperazin-1-yl)-gamma-phthalimido-butyramides (1-8) has been investigated. Several methods, like reversed-phase thin-layer chromatography and high-performance liquid chromatography using reversed-phase RP18 and IAM.DD2 columns, were applied to determine RMO, log k0 and log k(0IAM) factors. The RP-TLC investigations were performed in mixtures of acetone-water and acetonitrile-water. For RP-HPLC method mixtures of acetonitrile, water and 0.01% TFA were used as the mobile phases while for IAM.DD2 investigations mixtures of acetonitrile and water were applied. The partition coefficients of compounds (1-8) were also calculated with the Pallas and CAChe programs. All the obtained data, both from experimental methods and computational calculations, were compared and a suitable conclusion was reached. 相似文献