全文获取类型
收费全文 | 3094篇 |
免费 | 91篇 |
国内免费 | 391篇 |
专业分类
化学 | 3097篇 |
晶体学 | 20篇 |
力学 | 4篇 |
综合类 | 14篇 |
数学 | 106篇 |
物理学 | 335篇 |
出版年
2024年 | 1篇 |
2023年 | 80篇 |
2022年 | 75篇 |
2021年 | 96篇 |
2020年 | 107篇 |
2019年 | 84篇 |
2018年 | 97篇 |
2017年 | 142篇 |
2016年 | 139篇 |
2015年 | 90篇 |
2014年 | 158篇 |
2013年 | 480篇 |
2012年 | 256篇 |
2011年 | 170篇 |
2010年 | 136篇 |
2009年 | 150篇 |
2008年 | 159篇 |
2007年 | 195篇 |
2006年 | 176篇 |
2005年 | 152篇 |
2004年 | 146篇 |
2003年 | 92篇 |
2002年 | 69篇 |
2001年 | 26篇 |
2000年 | 30篇 |
1999年 | 40篇 |
1998年 | 30篇 |
1997年 | 28篇 |
1996年 | 19篇 |
1995年 | 27篇 |
1994年 | 26篇 |
1993年 | 15篇 |
1992年 | 18篇 |
1991年 | 19篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有3576条查询结果,搜索用时 15 毫秒
71.
制备1┐氰基┐2,3,5┐三┐O┐苯甲酰基┐β┐D┐呋喃核糖的简便方法张万轩*雷策南黄锦霞陈家威(湖北大学化学系武汉430062)关键词鸟苷,制备,氰基三苯甲酰基-β-D-呋喃核糖,三甲基氯硅烷1996-08-12收稿,1997-03-05修回1-氰... 相似文献
72.
Sanjeev K. Garg 《Tetrahedron letters》2005,46(10):1721-1724
Copper(II) tetrafluoroborate has been found to be a new and highly efficient catalyst for Michael addition of thiols to α,β-unsaturated carbonyl compounds under solvent-free conditions and in H2O at room temperature. The reactions are very fast and are completed in 2 min to 1 h affording high yields. The rate of thiol addition was dependent on the steric hindrance at the β-carbon of the α,β-unsaturated carbonyl substrate. In the case of chalcones, the reactions are best carried out in MeOH as solvent. 相似文献
73.
Christopher H. Evans Morgan Partyka Jan Van Stam 《Journal of inclusion phenomena and macrocyclic chemistry》2000,38(1-4):381-396
Naphthalene forms 1 : 1 complexes with -cyclodextrin (-CD)in water. The binding constant is 377 ± 35 M-1. Addition of linear or branched alcohols causes a reduction in the apparent strength of naphthalene binding (Kapp) compared to the value in the absence of additives. For example, 1% 1-pentanol reduces Kapp to 184 ± 31 M-1. Branching does not alter Kapp much for a given number of carbon atoms, e.g., it is 113 ± 9 M-1for 2-pentanol and 116 ± 8 M-1for 3-pentanol. The exception to this is tert-butanol for which Kapp is 577 ± 40 M-1. The variation in Kapp as a function of [1-pentanol] yields values for the individual equilibrium constants contributing to Kapp. This reveals that a ternary complex forms involving naphthalene, the CD and 1-pentanol. The constant for formation of the ternary complex is 99 ± 29 M-2. NaI quenching of naphthalene fluorescence indicates that the CD cavity partially protects the naphthalene excited state fromthis water phase quencher. Interestingly, the Stern–Volmer constant is lower in the presence of 1-pentanol than in its absence, although there should be more unbound (and therefore more NaI accessible) naphthalene in the former system than in the latter. These apparently contradictory results are discussed in terms of ternary complex formation. 相似文献
74.
R. D. Sinisterra R. Najjar O. L. Alves P. S. Santos C. A. Alves De Carvalho A. L. Conde Da Silva 《Journal of inclusion phenomena and macrocyclic chemistry》1995,22(2):91-98
The preparation and characterization of the 1:1 inclusion compound of rhodium(II) -methyl cinnamate in -cyclodextrin is reported. Evidence of inclusion was obtained from X-ray powder diffraction results, Raman, IR and UV-Vis spectroscopic studies and thermal analysis. Given the potential antitumor activity of the rhodium(II) carboxylate and its virtual insolubility in water, its inclusion in -cyclodextrin opens the possibility for its transference to the aqueous phase. 相似文献
75.
Otto Funk Lothar Schwabe Karl-Heinz Frömming 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(4):299-314
The purpose of the study was to examine the formation of inclusion compounds in the freeze-dried products obtained from aqueous solutions of nicotinic acid and -cyclodextrin (-CD), or heptakis (2,6-0-dimethyl)--cyclodextrin (DIMEB). The molar ratios used were 1:1 and 2:1. In addition two freezing temperatures (–40 and –196°C) and different secondary drying temperatures (+50 and +80°C) were used. Freeze-dried products with -CD obtained after low temperature freezing are of the same crystallographic structure as seen in a pure inclusion compound prepared by coprecipitation. Amorphous products were formed after fast freezing. The molar ratios of included nicotinic acid in the freeze-dried products vary — dependent on the preparation conditions — between 0.75:1 and 1:1. A factorial design proves that the included drug amount can be increased by enhancement of the amount of nicotinic acid used, by faster freezing, and by the combination of these two factors. The proof of inclusion formation was given by a combination of X-ray diffractography, differential scanning calorimetry, thermogravimetry and thermofractography.The freeze-dried preparations obtained with DIMEB were amorphous mixtures of the two components. No proof for inclusion of the nicotinic acid in the cyclodextrin cavity could be given. Higher (–40°C) or lower (–196°C) freezing temperatures and the running of the secondary drying process could not influence these results. The very low stability constant of the complex and steric reasons are responsible for this behavior. 相似文献
76.
Ramakrishna P. Bhat 《Tetrahedron letters》2005,46(28):4801-4803
β-Acetylamino ketones have been obtained in a one-pot coupling of an aldehyde with an enolisable ketone, acetyl chloride and acetonitrile in the presence of zeolite Hβ as catalyst at room temperature (26-28 °C). The procedure has the advantages of mild workup, circumvention of high temperature and inert atmosphere. The catalyst was found to be recyclable. 相似文献
77.
The separation of chlorthalidone enantiomers in capillary electrochromatography on an achiral stationary phase when adding a chiral selector, hydroxypropyl-β-cyclodextrin, to the mobile phase, was optimised. The goal was to investigate the feasibility of modelling retention times and resolution when during the optimisation procedure regular replacement of columns is required due to their fragility. Therefore, it is essential that the packing procedure delivers reproducible columns. The optimisation of an existing chlorthalidone separation was chosen as case study. The influence of two factors, chiral selector concentration and organic modifier content, on the responses was modelled. The experiments performed prior to modelling were defined by a central composite design. Results on different columns, obtained under identical experimental conditions, were found comparable and thus modelling was possible in situations where several columns were required to complete a design. A second-order polynomial model was built for both responses. Optimal separations were also predicted using Derringer’s desirability functions. The optimum was found at 33 mM cyclodextrin and 16% (v/v) acetonitrile on two types of columns (with different packing times) leading to a strong reduction in analysis time for an equally good separation compared to the initial conditions. Measured and predicted responses were found comparable, indicating that acceptable models were obtained. 相似文献
78.
The hydrolysis reaction of
,
and
,
-dipalmitoylphosphatidylcholine (DPPC) catalized by bee venom phospholipase A2 was studied in spreading monolayer at the water/air interface. DPPC and the hydrolysis products, palmitic acid and
-lysophosphatidylcholine, palmitoyl were characterized at the interface by means of surface pressure, surface potential and ellipsometric measurements. Furthermore, mixed monolayers of reagents and products were investigated to ascertain their miscibility. The results show that the hydrolysis reaction can be followed by the decrease of surface pressure with time on subphases containing β-cyclodextrin, a well-known complexing agent of many amphiphilic compounds. The order of the reaction, the kinetic constant and other kinetic parameters are deduced. 相似文献
79.
Application of mono (6A-N-ethylenediamine-6A-deoxy) perphenylcarbamoylated β-cyclodextrin (β-CD) bonded stationary phase (CSP) in micro-high performance liquid chromatography (micro-HPLC) and pressurized capillary electrochromatography (p-CEC) was firstly presented. A series of racemic α-amidophosphonates were resolved in reversed- and normal-phase modes on this CSP. The investigated chromatographic parameters include retention factor (k′), separation factor (α) and resolution (Rs) of solutes. In addition, the structural variation of the solutes and the experimental factors affecting chiral separations have been examined, including the percentage of alcohol modifier, the linear velocity (u) of the mobile phase, electrical field strength, etc. Baseline separation was achieved for most of the entities. Hydrophobic interaction, steric effect and π-π interaction contribute to the possible mechanism. Comparative results indicate that higher Rs value up to 3.1 was found in micro-HPLC, higher efficiency up to 29,970 in p-CEC. 相似文献
80.
A brief treatment of 2-thiopyrimidine nucleosides (s2U∗) with trans-2-phenylsulfonyl-3-phenyloxaziridine (PSO) results in efficient substrate desulfurization leading to the corresponding 4-pyrimidinone analogues (H2U∗). The key transformation proceeds through oxidation of the 2-thiocarbonyl group to a sulfur oxyacid derivative and subsequent elimination of sulfur dioxide. 4-Pyrimidinone 1-β-d-riboside (H2U) has been transformed into the respective phosphoramidite, a ready-to-use monomer for the introduction of a modified nucleoside into an oligonucleotide chain. Moreover, the effective desulfurization of the 2-thiouridine nucleotide could be achieved directly at the oligonucleotide level, by treatment of the TdA(s2U)dGdC oligonucleotide with PSO, as verified by MALDI-TOF mass spectrometry. 相似文献