全文获取类型
收费全文 | 62628篇 |
免费 | 8863篇 |
国内免费 | 1809篇 |
专业分类
化学 | 57663篇 |
晶体学 | 613篇 |
力学 | 1458篇 |
综合类 | 16篇 |
数学 | 4350篇 |
物理学 | 9200篇 |
出版年
2023年 | 184篇 |
2022年 | 470篇 |
2021年 | 725篇 |
2020年 | 1683篇 |
2019年 | 2978篇 |
2018年 | 1370篇 |
2017年 | 986篇 |
2016年 | 3981篇 |
2015年 | 4044篇 |
2014年 | 4125篇 |
2013年 | 5431篇 |
2012年 | 4863篇 |
2011年 | 4242篇 |
2010年 | 4039篇 |
2009年 | 3884篇 |
2008年 | 4048篇 |
2007年 | 3288篇 |
2006年 | 2906篇 |
2005年 | 3001篇 |
2004年 | 2611篇 |
2003年 | 2259篇 |
2002年 | 2861篇 |
2001年 | 1992篇 |
2000年 | 1788篇 |
1999年 | 704篇 |
1998年 | 318篇 |
1997年 | 335篇 |
1996年 | 360篇 |
1995年 | 282篇 |
1994年 | 301篇 |
1993年 | 291篇 |
1992年 | 285篇 |
1991年 | 223篇 |
1990年 | 169篇 |
1989年 | 151篇 |
1988年 | 145篇 |
1987年 | 130篇 |
1986年 | 102篇 |
1985年 | 179篇 |
1984年 | 120篇 |
1983年 | 102篇 |
1982年 | 127篇 |
1981年 | 91篇 |
1980年 | 104篇 |
1978年 | 103篇 |
1977年 | 133篇 |
1976年 | 112篇 |
1975年 | 119篇 |
1974年 | 90篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Kinetic studies of the addition of benzylamines to a noncyclic dicarbonyl group activated olefin, ethyl alpha-acetyl-beta-phenylacrylate (EAP), in acetonitrile at 25.0 degrees C are reported. The rates are lower than those for the cyclic dicarbonyl group activated olefins. The addition occurs in a single step with concurrent formation of the Calpha-N and Cbeta-H bonds through a four-center hydrogen bonded transition state.The kinetic isotope effects (kH/kD > 1.0) measured with deuterated benzylamines (XC6H4CH2ND2) increase with a stronger electron acceptor substituent (deltasigmaX > 0) which is the same trend as those found for other dicarbonyl group activated series (1-4), but is in contrast to those for other (noncarbonyl) group activated series (5-9). For the dicarbonyl series, the reactivity-selectivity principle (RSP) holds, but for others the anti-RSP applies. These are interpreted to indicate an insignificant imbalance for the former, but substantial lag in the resonance delocalization in the transition state for the latter series. 相似文献
992.
Lee SW Lee K Seomoon D Kim S Kim H Kim H Shim E Lee M Lee S Kim M Lee PH 《The Journal of organic chemistry》2004,69(14):4852-4855
Tetraorganoindates, which were prepared easily from the reaction of 1 equiv of InCl(3) with 4 equiv of organometallics, could be employed as effective nucleophilic cross-coupling partners in Pd-catalyzed carbonylative cross-coupling reactions with a variety of organic electrophiles. The present method gave unsymmetrical ketones and 1,4-diacylbenzenes in good yields with highly efficient transfer of almost all the organic groups attached to the indium under a carbon monoxide atmosphere in THF at 60 degrees C. 相似文献
993.
A convergent formal total synthesis of OF 4949 III is described. Arene-ruthenium chemistry was used in the construction of the diaryl ether linkage in high yield, and cycloamidation under high dilution conditions (0.005 M) was achieved using DPPA as coupling reagent. SmI(2) was used to reductively remove the 2-iodoethyl ester protecting group in the presence of DMPU or HMPA. 相似文献
994.
Yeung LK Lee CT Johnston KP Crooks RM 《Chemical communications (Cambridge, England)》2001,(21):2290-2291
Dendrimer-encapsulated nanoparticles are shown to be versatile catalysts for both the hydrogenation of styrene and Heck heterocoupling of iodobenzene and methacrylate in supercritical CO2 (scCO2). 相似文献
995.
Houda Marouani Mohamed Rzaigui Muriel Bagieu‐Beucher 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(3):356-357
In hexakis(m‐toluidinium) cyclohexaphosphate, 6C7H10N+·P6O186?, the atomic arrangement is typical of a layer structure. Layers including the centrosymmetric P6O18 ring anions develop around the (100) planes at x = . The hydrogen‐bond distribution is described. 相似文献
996.
Bacillus cellulyticus K-12 Avicelase (Avicelase I; EC 3.2.1.4) gene (ace A) has been cloned in Escherichia coli by using the vector pT7T3U19 and HindIII-HindIII libraries of the chromosomal inserts. The libraries were screened for the expression of avicelase by monitoring the immunoreaction
of the antiavicelase (immunoscreening). Positive clones (Ac-3, Ac-5, and Ac-7) contained the identical 3.5-kb HindIII fragment as determined by restriction mapping and Southern hybridization, and expressed avicelase efficiently and constitutively
using its own promoter in the heterologous host. From the immunoblotting analysis, a polypeptide that showed a carboxymethylcellulase
(CMCase) activity with an M
r
, of 64,000 was detected. The recombinant endo 1,4-β-
d
-glucanase I was purified to homogeneity from an intracellular fraction of E. coli by DEAE-Toyopearl M650, Phenyl Toyoperal M650, and TSK gel HW50S chromatography. The enzyme had a monomeric structure, its
relative molecular mass being 65 kDa by gel filtration and 64 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.
The pI was 5.3 and the optimal pH was 4.6, and the enzyme was stable at pH 4.0–10.5. The enzyme had a temperature optimum of 50°C
and was stable at 55°C for 48 h, and retained approx 20% of its activity after 30 min at 70°C. It showed high activity toward
carboxymethylcellulose (CMC) as well as p-nitrophenyl-β-d-cellobioside, 4-methylumbelliferyl cellobioside, Avicel, filter paper, and some cellooligosaccharides. K
m
values for CMC and Avicel were 7.6 and 85.2 mg/mL, respectively, whereas V
max values were 201 and 9.2 μmol · min−1 · mg−1, respectively. Cellotetraose (G4) was preferentially cleaved into cellobiose (G2) and cellopentaose (G5) was cleaved into
G2 + cellotriose (G3), whereas cellohexaose (G6) was cleaved into G4 + G2 and, to a lesser extent, into G3 + G3. G3 was not
cleaved at all. G2 was the main product of Avicel hydrolysis. G2 inhibited whereas Mg++ stimulated the activity of CMCase and Avicelase. Hydrolysis of CMC took place with a rapid decrease in viscosity but a slow
liberation of reducing sugars. Based on these results, it appeared that the cellulase should be regarded as endo type, although
it hydrolyzed Avicel. 相似文献
997.
Zachary M. Schulte Yeon Hye Kwon Yi Han Chong Liu Lin Li Yahui Yang Austin Gamble Jarvi Sunil Saxena Gtz Veser J. Karl Johnson Nathaniel L. Rosi 《Chemical science》2020,11(47):12807
Metal–organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn–Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H2/CO2 separation ability of bMOF-200. We examine the H2/CO2 separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H2/CO2 separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H2 and CO2 and a significantly improved H2/CO2 separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H2/CO2 separations.Tailorable multicomponent MOFs and MOF membranes for efficient H2/CO2 separation. 相似文献
998.
Huang HC Lee JF Chao HP Yeh PW Yang YF Liao WL 《Journal of colloid and interface science》2005,286(1):127-133
The influence of natural organic matter (NOM) constituents on contaminant distribution coefficients was evaluated by determining the Koc values of aromatic and aliphatic organic compounds (solutes) with clays modified with both aromatic- and aliphatic-rich organic constituents. The studied compounds consisted of naphthalene, phenanthrene, n-pentane, and 2,3,4-trimethylmethane; the solid samples comprised two clays with little organic content, kaolinite and Ca-montmorillonite. Two aliphatic surfactants and three aromatic dyes, sorbed to the clays, served as reference NOM constituents. For solutes of comparable water solubilities, the organic-carbon normalized distribution coefficients (Koc) of the aliphatic solutes between sorbed aliphatic organic matter and aqueous solution slightly exceed those of the aromatic solutes. By contrast, the aromatic solutes exhibited higher Koc values than did the aliphatic compounds with sorbed aromatic-rich organic matter. The difference in Koc values could be attributed to either comparable solubility parameters or the difference in the chemical structure between nonionic organic solutes and specific components of the simulated NOM. The much higher Koc values observed for the aromatic solutes indicate that the NOM composition is a major factor determining the NOC environmental distribution. 相似文献
999.
1000.