全文获取类型
收费全文 | 2370篇 |
免费 | 113篇 |
国内免费 | 108篇 |
专业分类
化学 | 501篇 |
晶体学 | 6篇 |
力学 | 374篇 |
综合类 | 12篇 |
数学 | 1146篇 |
物理学 | 552篇 |
出版年
2024年 | 4篇 |
2023年 | 20篇 |
2022年 | 26篇 |
2021年 | 33篇 |
2020年 | 47篇 |
2019年 | 100篇 |
2018年 | 95篇 |
2017年 | 61篇 |
2016年 | 68篇 |
2015年 | 77篇 |
2014年 | 86篇 |
2013年 | 156篇 |
2012年 | 58篇 |
2011年 | 88篇 |
2010年 | 92篇 |
2009年 | 153篇 |
2008年 | 152篇 |
2007年 | 117篇 |
2006年 | 157篇 |
2005年 | 150篇 |
2004年 | 142篇 |
2003年 | 129篇 |
2002年 | 134篇 |
2001年 | 50篇 |
2000年 | 43篇 |
1999年 | 50篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 9篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1986年 | 8篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1982年 | 6篇 |
1981年 | 18篇 |
1980年 | 17篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有2591条查询结果,搜索用时 484 毫秒
61.
Kiyokawa T Kanaori K Tajima K Kawaguchi M Mizuno T Oku J Tanaka T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(14):3548-3554
The alpha-helical coiled coils have a representative amino acid sequence of (abcdefg)(n) heptad repeats. We previously reported that two peptides named IZ-2A and IZ-2W formed an (IZ-2A)(2)/IZ-2W heterotrimer with an Ala-Ala-Trp interaction in the hydrophobic core. In this paper, we describe the selective formation of AAB- and ABC-type heterotrimers. To increase the selectivity of the AAB-type heterotrimeric formation, Lys residues at the f position were mutated to either an Ala or a Gln residue to form IZ-2A(fA) or IZ-2W(fQ). Separately, both IZ-2A(fA) and IZ-2W(fQ) have a random structure at pH 7 and 20 degrees C. However, together IZ-2A(fA) and IZ-2W(fQ) form a 2:1 complex with a thermal transition midpoint (Tm) of 48 degrees C. This procedure was applied to prepare the ABC-type heterotrimer, in which two sets of Ala-Ala-Trp interactions were designed in the hydrophobic core. Interhelical interaction between the e and g positions and the alpha-helical propensity of the amino acid at the f position were also considered in the design. The resultant three peptides selectively formed the ABC-type heterotrimer with a Tm of 51 degrees C. Other peptide combinations had random coil properties. 相似文献
62.
Alphonse Ekouya Raymond Calas Jacques Dunogues Claude Biran Norbert Duffaut 《Journal of organometallic chemistry》1979,177(1):137-144
Aliphatic saturated amides treated with Me3SiCl/Li/THF were found to react in two ways. Either alkoxysilanes, mono- or di-silylated at the functional carbon, or C-silylated amines were obtained dependent of the original amide structure and the experimental conditions. (Me3Si)2CHN(SiMe3)2 exhibited Physicochemical properties that are particular for hindered rotation about the CN bond. A general mechanism is proposed to explain these results. 相似文献
63.
In an effort to develop an efficient synthetic method of highly diastereoselective (2′R)- and (2′S)-2′-deoxy[2′-2H]guanosines, chemoenzymatic conversion was investigated. The synthesis of (2′R > 98% de)-2′-deoxy[2′-2H]guanosine was achieved by biological transdeoxyribosylation using (2′R > 98% de)-2′-deoxy[2′-2H]uridine, 2,6-diaminopurine, and Enterobacter aerogenes AJ-11125, followed by treatment with adenosine deaminase. (2′S > 98% de)-2′-Deoxy[2′-2H]guanosine was synthesized from (2′S > 98% de)-2′-deoxy[2′-2H]uridine and 2,6-diaminopurine using thymidine phosphorylase and purine nucleoside phosphorylase instead of E. aerogenes AJ-11125. 相似文献
64.
Bruno Ameduri Bernard Boutevin Fouad Malek 《Journal of polymer science. Part A, Polymer chemistry》1994,32(16):3161-3169
The synthesis of maleimides that have pyrazolic or bipyrazolic pendant groups is described. Their homopolymerization and their copolymerization with 2-chloroethyl vinyl ether (CEVE) is reported. The homopolymerizations of such maleimides were performed under various conditions and led to low molecular-weight polymers. However, alternating copolymers were obtained from CEVE as comonomers whatever the monomers feed compositions. A similar behavior was also observed for maleimides that do not exhibit any spacer, whereas for bulky vinyl ethers, random copolymers were produced. A comparison of the thermal behavior between these copolymers (glass transition temperatures, Tg, and decomposition temperatures) and other copolymers having different spacers between the nitrogenated cycles and the chain are related. Thus, an important decrease of Tg, was observed when C3H6CO2CH2 groups were used as the spacer instead of methylene groups. Moreover, the thermal weakness of these copolymers may come from the substituents of the vinyl ether and is discussed. © 1994 John Wiley & Sons, Inc. 相似文献
65.
The present study contains the analyses performed for pigment samples taken from the Princely church of Curtea de Arges, one of the oldest churches in Romania. The results of our investigations have shown the source of these samples, thus being identified the pigments: natural ultramarine, cinnabar, red earth, and calcium carbonate in the painting from the 14th century, the pigments: lead white, zinc white, and Prussian blue in the repainting from the 19th century and the pigments zinc white, titanium dioxide white, bone white, yellow ochre, red ochre, green earth, artificial ultramarine, and mars red in the interventions carried out in the 20th century. The analyses consisted of light microscopy (LM) and microchemical tests, as well as energy dispersive X-ray (EDX) analysis. This system of analyses allows one to precisely determine the authenticity of certain pigments, thus avoiding the dating errors for different interventions carried out on the original mural painting from the Saint Nicholas Princely church of Curtea de Arges. 相似文献
66.
M. S. Shvartsberg I. D. Ivanchikova S. F. Vasilevsky 《Russian Chemical Bulletin》1998,47(10):1971-1974
I-Acetylenyl-9,10-anthraquinones react with an excess of NH2NH2 at 80–115°C to give a mixture of substituted 7H-dibenzo[de,h]quinolin-7-ones and anthra[9,1-cd]-1,2-diazepin-8-ones. The latter compounds undergo reductive contraction of the sevenmembered ring to give the corresponding
7H-dibenzo[de,h]quinolin-7-ones. Bulky substituents in position 2 of the initial acetylenylanthraquinones prevent the formation of the sevenmembered
heterocycle. A scheme of the cyclocondensation was proposed.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2027–2030, October, 1998. 相似文献
67.
A very large diversity space of synthetically accessible compounds for use with drug design programs
Nikitin S Zaitseva N Demina O Solovieva V Mazin E Mikhalev S Smolov M Rubinov A Vlasov P Lepikhin D Khachko D Fokin V Queen C Zosimov V 《Journal of computer-aided molecular design》2005,19(1):47-63
We have constructed a very large virtual diversity space containing more than 1013 chemical compounds. The diversity space is built from about 400 combinatorial libraries, which have been expanded by choosing sizeable collections of suitable R-groups that can be attached to each link point of their scaffolds. These R-group collections have been created by selecting reagents that have drug-like properties from catalogs of available chemicals. As members of known combinatorial libraries, the compounds in the diversity space are in general synthetically accessible and useful as potential drug leads. Hence, the diversity space can be used as a vast source of compounds by a de novo drug design program. For example, we have used such a program to generate inhibitors of HIV integrase enzyme that exhibited activity in the micromolar range. 相似文献
68.
B. Gillot F. Bouton J.F. Ferriot F. Chassagneux A. Rousset 《Journal of solid state chemistry》1977,21(4):375-385
When Fe3+ ions are substituted by aluminum or chromium on magnetite octahedral sites, the ir spectrum shows the conversion of an inverse spinel to a normal spinel. Both broad bands of magnetite are gradually replaced by the four characteristic bands of normal spinels II–III. They are also observed for solid solutions, FeCr2O4FeAl2O4, with, however, a further band at 780 cm?1 which may be assigned to Al3+ ions on tetrahedral sites. Low-temperature (<400°C) oxidation of these compounds whose sizes are less than 2000 Å results in lacunar spinels III–III. The ir spectrum of these solids is characterized by two absorption bands (as for inverse spinels II–III) except for compounds close to pure γFe2O3 in which an order of vacancies could be put in evidence. 相似文献
69.
70.
A study on the synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La0.8Ag0.2MnO3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts. 相似文献