全文获取类型
收费全文 | 3319篇 |
免费 | 199篇 |
国内免费 | 17篇 |
专业分类
化学 | 2273篇 |
晶体学 | 20篇 |
力学 | 85篇 |
数学 | 544篇 |
物理学 | 613篇 |
出版年
2023年 | 36篇 |
2022年 | 26篇 |
2021年 | 57篇 |
2020年 | 74篇 |
2019年 | 92篇 |
2018年 | 46篇 |
2017年 | 48篇 |
2016年 | 134篇 |
2015年 | 134篇 |
2014年 | 144篇 |
2013年 | 187篇 |
2012年 | 249篇 |
2011年 | 278篇 |
2010年 | 137篇 |
2009年 | 131篇 |
2008年 | 217篇 |
2007年 | 164篇 |
2006年 | 158篇 |
2005年 | 144篇 |
2004年 | 135篇 |
2003年 | 101篇 |
2002年 | 94篇 |
2001年 | 56篇 |
2000年 | 51篇 |
1999年 | 39篇 |
1998年 | 33篇 |
1997年 | 39篇 |
1996年 | 53篇 |
1995年 | 52篇 |
1994年 | 43篇 |
1993年 | 41篇 |
1992年 | 35篇 |
1991年 | 21篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 25篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1978年 | 12篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1973年 | 13篇 |
1972年 | 7篇 |
1971年 | 7篇 |
排序方式: 共有3535条查询结果,搜索用时 15 毫秒
81.
Feth Martin P. Weber Achim Merkle Rotraut Reinöhl Ulrich Bertagnolli Helmut 《Journal of Sol-Gel Science and Technology》2003,27(2):193-204
Two solid solutions of lead zirconium titanates PbZr
x
Ti1 – x
O3 (x = 0.1 and 0.35) as well as the reference compounds lead titanate and lead zirconate were prepared from zirconium and titanium n-propoxide, dissolved in 2-methoxyethanol, by sol-gel process. The amorphous products after pyrolysis of the dried gels and the crystalline phases were studied by EXAFS spectroscopy to monitor the structural changes from the amorphous oxide mixture to the crystalline ceramics after calcination. Additionally, the crystalline phases were identified by X-ray diffraction (XRD).It follows from the analysis of the EXAFS data that the local order of the amorphous phases seems to be completely different from that of the crystalline phase. There is no indication of a preformation of the local order of the perovskite structure. The analysis of our EXAFS spectra can be interpreted very consistently with the assumption that in the amorphous samples a segregation exists on molecular level and the low crystallisation temperatures are a consequence of very short diffusion paths. 相似文献
82.
83.
The determination of carbonyl compounds in gaseous samples is usually accomplished by enrichment methods, in which 2,4-dinitrophenyl-hydrazine (DNPH) as a derivatization reagent has become established to a large extent. However, the conventional methods of DNPH-impingers and of DNPH-cartridges are applicable to emission measurements in a limited way only, depending on the NO2-concentration in the exhaust gas. It could be proved that DNPH-derivatives, as well as DNPH, are also decomposed by NO2 at a different speed, in which the hydrazones of unsaturated carbonyl compounds are probably more sensitive than those of the saturated carbonyl compounds. In view of this fact, the collecting methods had to be modified to avoid losses with the enrichment. The analysis of the compounds is carried out by HPLC with an effective gradient-system which is able to separate and detect the carbonyl compounds in exhaust gas within 16 min. Furthermore, a simple working-up procedure is presented which facilitates a parallel analysis by GC. 相似文献
84.
The reaction of primary alkyl bromides or chlorides with diethylzinc in the presence of Ni(acac)(2) (5 mol %) furnishes the corresponding alkylzinc halides (X = Br, Cl) via a halogen-zinc exchange reaction. The treatment of terminal alkenes with diethylzinc (neat, 25-60 degrees C) in the presence of Ni(acac)(2) as a catalyst (1-5 mol %) and 1,5-cyclooctadiene (COD) affords the corresponding dialkylzincs via a hydrozincation reaction. Whereas the conversion for simple alkenes bearing a remote functionality reaches 40 to 63%, the hydrozincation of allylic, homoallylic alcohols and allylic amines proceeds very efficiently (85-95% conversion). All the zinc organometallics obtained react with various electrophiles (allylic halides, enones, acid chlorides, alkynyl halides, ethyl propiolate) after transmetalation with CuCN.2LiCl. In the presence of the chiral catalyst 12, the dialkylzincs prepared add to aldehydes with high enantioselectivity. 相似文献
85.
Lukas Fischer Alexander K. Strzelczyk Nils Wedler Christian Kropf Stephan Schmidt Laura Hartmann 《Chemical science》2020,11(36):9919
Catechol and amine residues, both abundantly present in mussel adhesion proteins, are known to act cooperatively by displacing hydration barriers before binding to mineral surfaces. In spite of synthetic efforts toward mussel-inspired adhesives, the effect of positioning of the involved functional groups along a polymer chain is not well understood. By using sequence-defined oligomers grafted to soft hydrogel particles as adhesion probes, we study the effect of catechol–amine spacing, as well as positioning relative to the oligomer terminus. We demonstrate that the catechol–amine spacing has a significant effect on adhesion, while shifting their position has a small effect. Notably, combinations of non-charged amides and catechols can achieve similar cooperative effects on adhesion when compared to amine and catechol residues. Thus, these findings provide a blueprint for the design of next generation mussel-inspired adhesives.The catechol driven adhesion of precision macromolecules on glass surfaces is quantified by soft colloidal probe readout. Catechol moieties are shown to synergize with amine and amide residues depending on residue spacing and residue order. 相似文献
86.
Convenient high-yield syntheses of the primary and di-primary heteroaryl-phosphines R-PH2 and H2P-R'-PH2(with R = 2-thienyl, 2-furyl, and R'= 2,5-thiophenediyl, 2,5-furandiyl, respectively) are presented. The products and a set of precursor molecules have been characterized by analytical and spectral data, and the crystal structures of selected molecules have been determined: 2-C4H3O-PCl2, 2,5-(Cl2P)2C4H2O, 2,5-[(Et2N)2P]2C4H2E (with E = O, S). In the crystals, the two molecules with -PCl2 substituents adopt trans conformations, while the other two have the -P(NEt2)2 groups rotated into a twist conformation. The reaction of the thienyl compounds with tris[(tert-phosphine)gold]oxonium tetrafluoroborates gave almost quantitative yields of the tri- and hexanuclear gold complexes, respectively: {2-C4H3S-P[Au(PR3)]3}+BF4- and [2,5-{[(R3P)Au]3P}2C4H2S]2+(BF4(-)2, (R =tBu, Ph). The structures of the compounds with R3P =tBu3P ligands have been determined. In both cases the [2-C4H3/2S-P] units cap triangles of gold atoms in an array that can be described as three [Au(PR3)]+ cations bridged by a phosphido dianion (RP)2-. 相似文献
87.
Pedersen TM Hansen EL Kane J Rein T Helquist P Norrby PO Tanner D 《Journal of the American Chemical Society》2001,123(40):9738-9742
A new method for enantioconvergent synthesis has been developed. The strategy relies on the combination of an asymmetric Horner-Wadsworth-Emmons (HWE) reaction and a palladium-catalyzed allylic substitution. Different alpha-oxygen-substituted, racemic aldehydes were initially transformed by asymmetric HWE reactions into mixtures of two major alpha,beta-unsaturated esters, possessing opposite configurations at their allylic stereocenters as well as opposite alkene geometry. Subsequently, these isomeric mixtures of alkenes could be subjected to palladium-catalyzed allylic substitution reactions with carbon, nitrogen, and oxygen nucleophiles. In this latter step, the respective (E) and (Z) alkene substrate isomers were observed to react with opposite stereospecificity: the (E) alkene reacted with retention and the (Z) alkene with inversion of stereochemistry with respect to both the allylic stereocenter and the alkene geometry. Thus, a single gamma-substituted ester was obtained as the overall product, in high isomeric purity. The method was applied to a synthesis of a subunit of the iejimalides, a group of cytotoxic macrolides. 相似文献
88.
Stephan Kupfer Guillermo Pérez-Hernández Leticia González 《Theoretical chemistry accounts》2012,131(12):1-14
Peptidylarginine deiminase 4 (PAD4), also known as protein arginine deiminase 4, performs a post-translational deimination that converts arginine to citrulline. The dysregulation of PAD4 has been implicated in a number of diseases, including rheumatoid arthritis (RA) and cancer. This makes PAD4 an important therapeutic target. To develop small-molecule inhibitors as potential treatments, it is advantageous if the catalytic mechanism is well understood. The protonation states of the active site residues, which have long been under controversy, have a direct impact on the catalytic mechanism. Two competing mechanisms are under investigation in the current literature. The first is a reverse protonation mechanism that depends on the active site histidine and cysteine existing as an ion pair. The second is a substrate-assisted mechanism that depends on the active site histidine and cysteine being neutral. This study uses the semimicroscopic protein dipoles Langevin dipoles (PDLD/S) linear response approximation method in the MOLARIS software package to calculate the change in solvation energy of moving the residue from water to the protein interior, and then using that information to assess the protonation states of the active site residues of PAD4. Results from these calculations suggest that in the enzyme–substrate complex of PAD4, the cysteine and histidine are protonated and deprotonated, respectively, and are therefore both neutral, analogous to the proposed protonation states of the active site residues in the Michaelis complex in the substrate-assisted mechanism. 相似文献
89.
Inside Cover: Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5‐“Click”‐Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes (Chem. Eur. J. 50/2016) 下载免费PDF全文
90.