全文获取类型
收费全文 | 7332篇 |
免费 | 346篇 |
国内免费 | 64篇 |
专业分类
化学 | 5197篇 |
晶体学 | 23篇 |
力学 | 183篇 |
数学 | 1270篇 |
物理学 | 1069篇 |
出版年
2023年 | 47篇 |
2022年 | 58篇 |
2021年 | 225篇 |
2020年 | 154篇 |
2019年 | 157篇 |
2018年 | 147篇 |
2017年 | 119篇 |
2016年 | 303篇 |
2015年 | 265篇 |
2014年 | 293篇 |
2013年 | 470篇 |
2012年 | 525篇 |
2011年 | 611篇 |
2010年 | 376篇 |
2009年 | 285篇 |
2008年 | 522篇 |
2007年 | 470篇 |
2006年 | 462篇 |
2005年 | 397篇 |
2004年 | 347篇 |
2003年 | 259篇 |
2002年 | 245篇 |
2001年 | 73篇 |
2000年 | 79篇 |
1999年 | 51篇 |
1998年 | 78篇 |
1997年 | 71篇 |
1996年 | 58篇 |
1995年 | 40篇 |
1994年 | 38篇 |
1993年 | 40篇 |
1992年 | 41篇 |
1991年 | 38篇 |
1990年 | 26篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 27篇 |
1985年 | 32篇 |
1984年 | 36篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 26篇 |
1980年 | 25篇 |
1979年 | 18篇 |
1978年 | 17篇 |
1977年 | 19篇 |
1976年 | 7篇 |
1974年 | 6篇 |
1973年 | 10篇 |
排序方式: 共有7742条查询结果,搜索用时 31 毫秒
81.
Coronado E Galán-Mascarós JR Martí-Gastaldo C Palomares E Durrant JR Vilar R Gratzel M Nazeeruddin MK 《Journal of the American Chemical Society》2005,127(35):12351-12356
The selectivity and sensitivity of two colorimetric sensors based on the ruthenium complexes N719 [bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) bis(tetrabutylammonium) bis(thiocyanate)] and N749 [(2,2':6',2' '-terpyridine-4,4',4' '-tricarboxylate)ruthenium(II) tris(tetrabutylammonium) tris(isothiocyanate)] are described. It was found that mercury ions coordinate reversibly to the sulfur atom of the dyes' NCS groups. This interaction induces a color change in the dyes at submicromolar concentrations of mercury. Furthermore, the color change of these dyes is selective for mercury(II) when compared with other ions such as lead(II), cadmium(II), zinc(II), or iron(II). The detection limit for mercury(II) ions--using UV-vis spectroscopy--in homogeneous aqueous solutions is estimated to be approximately 20 ppb for N719 and approximately 150 ppb for N749. Moreover, the sensor molecules can be adsorbed onto high-surface-area mesoporous metal oxide films, allowing reversible heterogeneous sensing of mercury ions in aqueous solution. The results shown herein have important implications in the development of new reversible colorimetric sensors for the fast, easy, and selective detection and monitoring of mercuric ions in aqueous solutions. 相似文献
82.
Optimization of solid-phase clean-up prior to liquid chromatographic analysis of ochratoxin A in roasted coffee 总被引:2,自引:0,他引:2
An extraction and clean-up method for ochratoxin A (OA) in roasted coffee has been developed and the HPLC method optimized. An interfering compound with a similar retention time as OA was adsorbed by the aminopropyl (NH2) material at < or = 5% NaHCO3. Residual OA on the column was recovered by washing with the extraction solution followed with methanol. Fractions were mixed together for further clean-up with Ochratest immunoaffinity columns (IACs). Analysis by HPLC resulted in a well resolved OA peak and reduction in matrix interferences. Recoveries ranged from 72 to 84% and the detection limit was 1 ng/g. 相似文献
83.
Reactions of W(CO)(6) with formamidines contrast with those of Mo(CO)(6) and Cr(CO)(6) in that the former do not yield quadruply bonded dimetal species. From the reaction of W(CO)(6) with HDAniF (HDAniF = N,N'-di-p-anisylformamidine), several new ditungsten carbonyl compounds (W(2)(mu-CO)(2)(mu-DAniF)(2)(eta(2)-DAniF)(2) (1), W(2)(mu-CO)(2)(mu-DAniF)(2)(eta(2)-DAniF)(eta(2)-CH(2)DAniF) (2), and W(2)(mu-CO)(mu-CNC(6)H(4)OCH(3))(mu-DAniF)(2)(eta(2)-DAniF)(2) (3)) have been isolated and fully characterized. In 2, CH(2)DAniF represents a DAniF ligand in which a methylene group has been added to one of the nitrogen atoms. This ligand binds to the tungsten atom using a nitrogen and a carbon atom. Compound 1 has a tungsten-tungsten bond distance of 2.476(1) A and a planar W(2)(mu-CO)(2) core structure which has C(2)(h)() symmetry with short and long W-C bond distances (1.99(1) and 2.28(1) A, respectively). DFT calculations on a model of 1 indicate that (a) the C(2)(h)() instead of D(2)(h)() symmetry of the ditungsten core may be attributed to W --> CO pi back-bonding interactions and (b) the bond between the tungsten atoms may be formulated as a double bond. The new tetragonal paddlewheel compound W(2)(DAniF)(4) (4) and the edge-sharing bioctahedron W(2)(mu-O)(mu-NC(6)H(3)Cl(2))(mu-D(Cl)PhF)(2)(eta(2)-D(Cl)PhF)(2) (5) (D(Cl)PhF = N,N'-di-(3,5-dichlorophenyl)formamidinate) have also been prepared. 相似文献
84.
Alvaro M Formentín P García H Palomares E Sabater MJ 《The Journal of organic chemistry》2002,67(15):5184-5189
Three N-alkylpyridinium photosensitizers having chiral alkyl groups have been prepared by reacting 2,4,6-triphenylpyrylium tetrafluoroborate ion with (1R,2S)-(-)-norephedrine, (S)-(+)-2-(aminomethyl)pyrrolidine, and (R)-(-)-1-cyclohexylethylamine. Laser flash photolysis allows detection of the corresponding triplet excited states that are quenched by hydrogen atom donors and electron donors. Asymmetric quenching of the chiral triplet excited state was observed using enantiomerically pure 1,2-diamino cyclohexane as quencher. Low enantiomeric excess values (up to 7%) were measured for the photochemical cyclization of 5-methyl-4-hexenoic acid to its corresponding gamma-lactone using these chiral N-alkylpyridinium as photosensitizers. 相似文献
85.
[reaction: see text] Direct asymmetric catalytic Michael reactions have been performed using chiral-amine/acid bifunctional catalysts. Performed with 0.3 equiv of (S)-(+)-1-(2-pyrrolidinylmethyl)pyrrolidine and 0.3 equiv of trifluoroacetic acid as the catalyst, the reaction of alpha,alpha-dialkylaldehydes with (E)-beta-nitrostyrene provided the alpha,alpha-dialkyl Michael products in up to 96% yield with up to 91% ee. With respect to enantioselectivity, l-proline was a poor catalyst of this class of Michael reactions. 相似文献
86.
Carlos Díaz 《Polyhedron》1985,4(7):1269-1270
The reaction of copper(II) perchlorate hexahydrate with a number of sulphur(II) compounds in acetonitrile as solvent were studied. It is found that SCl2, N,N′-thiobisamines S(NR2)2, ethoxythiomorpholineC2H5O-S-Nand pentacarbonyl (thiobismorpholine) chromium(O)[Cr(CO)5·S(NO)2]reduce Cu(II) with formation of the complex Cu[(CH3CN)4]CIO4 and SO2. 相似文献
87.
Michel Spiesser Robert P. Gruska S.N. Subbarao Carlos A. Castro Aaron Wold 《Journal of solid state chemistry》1978,26(2):111-114
Crystals of antimony-doped In2Se3 were grown by the Bridgeman method. This compound, whose composition is In1.8Sb0.2Se3, appears to be isostructural with In1.9As0.1Se3. The refined unit cell parameters are a = 3.97(1), c = 18.87(1) Å. Orthorhombic crystals of InSbSe3 were grown from an isothermal melt. The refined unit cell parameters are a = 9.43(1), b = 14.02(5), and c = 3.96(1) Å. These parameters agree with those determined for α-InSbSe3 by other studies. The observed densities measured by a hydrostatic technique are 5.98(3) g/cm3 for In1.8Sb0.2Se3 and 6.07(2) g/cm3 for InSbSe3. The room temperature dc resistivity for In1.8Sb0.2Se3 has been found to be 4.4 × 104 Ω-cm, whereas that of InSbSe3 has been found to be 15.2(1) Ω-cm. A resistivity versus temperature study has beenn carried out for InSbSe3 between 230 and 400°K. Optical studies indicate that In1.8Sb0.2Se3 is an n-type semiconductor with a band gap of 1.1 eV and InSbSe3 is a p-type semiconductor with a band gap of 0.92 eV. 相似文献
88.
Pimenta AM Rates B Bloch C Gomes PC Santoro MM de Lima ME Richardson M Cordeiro Mdo N 《Rapid communications in mass spectrometry : RCM》2005,19(1):31-37
Previous studies of the fractionated venom of the Brazilian armed spider Phoneutria nigriventer, obtained by gel filtration, have demonstrated the presence of a fraction PhM, a pool of small peptides (up to 2000 Da) that provoke contractions in smooth muscle of guinea pig ileum. Initial attempts to sequence these peptides were largely unsuccessful because of the low purification yield and the fact that the majority seemed to be blocked at their N-termini. In the present work, analysis of this venom fraction by mass spectrometry has revealed the existence of a highly complex mixture of peptides with molecular weights corresponding to those observed for the muscle-active peptides previously described (800-1800 Da). These peptides appear to be a family of isoforms with some particular features. The amino acid sequences of 15 isoforms have been determined by tandem mass spectrometry (MS/MS) using both electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q/ToFMS) and matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-ToF/ToFMS). These molecules contain post-translational modifications such as proteolysis and C-terminal amidation, which combine to generate additional isoforms. All the isoforms sequenced in this study possess an N-terminal pyroglutamic acid residue. A search for sequence similarities with other peptides in databanks revealed that these peptides are structurally related to the tachykinins, a family of neuro-hormone peptides. The data obtained in this study will be essential for the subsequent steps of this research, the synthesis of these peptides and pharmacological characterization of their biological activity. 相似文献
89.
Xin-Liang Li Lars G. Ljungdahl Eduardo A. Ximenes Huizhong Chen Carlos R. Felix Michael A. Cotta Bruce S. Dien 《Applied biochemistry and biotechnology》2004,113(1-3):233-250
A β-glucosidase (BglA, EC 3.2.1.21) gene from the polycentric anaerobic fungus Orpinomyces PC-2 was cloned and sequenced. The enzyme containing 657 amino acid residues was homologous to certain animal, plant, and
bacterial β-glucosidases but lacked significant similarity to those from aerobic fungi. Neither cellulose- nor protein-binding
domains were found in BglA. When expressed in Saccharomyces cerevisiae, the enzyme was secreted in two forms with masses of about 110 kDa and also found in two forms associated with the yeast
cells. K
m
and V
max
values of the secreted BglA were 0.762 mM and 8.20 μmol/(min·mg), respectively, with p-nitrophenyl-β-d-glucopyranoside (pNPG) as the substrate and 0.310 mM and 6.45 μmol/(min·mg), respectively, for the hydrolysis of cellobiose. Glucose competitively inhibited the hydrolysis of
pNPG with a K
i
of 3.6 mM. β-Glucosidase significantly enhanced the conversion of cellulosic materials into glucose by Trichoderma reesei cellulase preparations, demonstrating its potential for use in biofuel and feedstock chemical production.
Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard
of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may be
suitable. 相似文献
90.
We analyzed the data from a replica exchange molecular dynamics simulation using the weighted histogram analysis method to combine data from all of the temperature replicas (T-WHAM) to obtain the room-temperature potential of mean force of the G-peptide (the C-terminal beta-hairpin of the B1 domain of protein G) in regions of conformational space not sampled at room temperature. We were able to determine the potential of mean force in the transition region between a minor alpha-helical population and the major beta-hairpin population and identify a possible transition path between them along which the peptide retains a significant amount of secondary structure. This observation provides new insights into a possible mechanism of formation of beta-sheet secondary structures in proteins. We developed a novel Bayesian statistical uncertainty estimation method for any quantity derived from WHAM and used it to validate the calculated potential of mean force. The feasibility of estimating regions of the potential of mean force with unfavorable free energy at room temperature by T-WHAM analysis of replica exchange simulations was further tested on a system that can be solved analytically and presented some of the same challenges found in more complex chemical systems. 相似文献