全文获取类型
收费全文 | 4363篇 |
免费 | 191篇 |
国内免费 | 3篇 |
专业分类
化学 | 2856篇 |
晶体学 | 22篇 |
力学 | 78篇 |
数学 | 890篇 |
物理学 | 711篇 |
出版年
2023年 | 35篇 |
2022年 | 29篇 |
2021年 | 50篇 |
2020年 | 67篇 |
2019年 | 73篇 |
2018年 | 96篇 |
2017年 | 84篇 |
2016年 | 128篇 |
2015年 | 130篇 |
2014年 | 151篇 |
2013年 | 307篇 |
2012年 | 325篇 |
2011年 | 340篇 |
2010年 | 190篇 |
2009年 | 159篇 |
2008年 | 269篇 |
2007年 | 260篇 |
2006年 | 222篇 |
2005年 | 232篇 |
2004年 | 196篇 |
2003年 | 153篇 |
2002年 | 147篇 |
2001年 | 53篇 |
2000年 | 41篇 |
1999年 | 43篇 |
1998年 | 30篇 |
1997年 | 32篇 |
1996年 | 42篇 |
1995年 | 26篇 |
1994年 | 17篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 23篇 |
1989年 | 12篇 |
1988年 | 17篇 |
1987年 | 19篇 |
1985年 | 30篇 |
1984年 | 29篇 |
1983年 | 19篇 |
1982年 | 34篇 |
1981年 | 27篇 |
1980年 | 42篇 |
1979年 | 30篇 |
1978年 | 30篇 |
1977年 | 30篇 |
1976年 | 29篇 |
1975年 | 28篇 |
1974年 | 16篇 |
1973年 | 17篇 |
1964年 | 13篇 |
排序方式: 共有4557条查询结果,搜索用时 250 毫秒
991.
Multiple experiments (17O NMR, 1H NMR, and EPR) have been performed in the past to understand the microscopic parameters that control the magnetic relaxation rate enhancement induced by paramagnetic molecules on neighboring water protons, the so-called relaxivity. The generally accepted theories of the electron spin relaxation of S = 7/2 ions such as Gd3+ (Solomon-Bloembergen-Morgan or simplified Hudson-Lewis) are unsatisfactory for a simultaneous analysis. Recently, an improved theory, where the electron spin relaxation is due to the combination of a static (thus explicitly linked to the molecular structure) and a dynamic zero field splitting, has been developed and tested on experimental EPR data. The model has also been extended beyond the electronic Redfield limit using Monte Carlo simulations. Using the aqua ion [Gd(H2O)8]3+ as a test case, we present here the first simultaneous analysis of 17O NMR, 1H NMR, and EPR relaxation data using this rigorous approach of the electron spin relaxation. We discuss the physical meaning of the calculated parameters. The consequences on future experiments are also considered, especially regarding the analysis of nuclear magnetic relaxation dispersion (NMRD) profiles in the study of Gd3+ complexes. 相似文献
992.
Preto MA Melo A Maia HL Mavromoustakos T Ramos MJ 《The journal of physical chemistry. B》2005,109(37):17743-17751
Angiotensin II (Ang II) is an octapeptidic hormone, which plays an important role in the mechanisms of blood pressure control. In this work, extensive molecular dynamics (MD) simulations have been carried out on this peptide, both in aqueous and in dimethyl sulfoxide (DMSO) environments. Experimentally proposed models for the structure of angiotensin II in both environments are not consensual and the results obtained have provided some further insight about the structural properties of this hormone. In these simulations, the N-terminus of Ang II in the aqueous environment has been associated with a considerable larger flexibility than the correspondent C-terminus, but this was not found in the case of the DMSO environment. This is consistent with the assumption that the biological activity of Ang II is associated with its C-terminal residues embedded in a hydrophobic environment of its AT1 receptor. Other features detected in DMSO environment were an H(His6 imidazole)-O(Phe8 carboxylate) hydrogen bond and a salt-bridge structure involving the Asp1 and Arg2 side chains. An additional important conformational feature is the spatial proximity between Tyr4 and His6 in both water and DMSO environments. This molecular feature may trigger the interest for the synthetic chemists to apply rational design for the synthesis of novel AT1 antagonists. 相似文献
993.
Homogeneous precipitation of mullite precursors 总被引:1,自引:0,他引:1
The synthesis of mullite powders or gels from an aqueous precursor solution has been studied. The starting solution is made by hydrolyzing TEOS into an aqueous solution of aluminium nitrate. When this solution is sprayed into a solution of ammonia in isopropanol, a nearly monophasic mullite precursor is obtained, whereas if this precipitation is made into an aqueous solution of ammonia, or ammonium carbonate, a diphasic colloidal precipitate is formed. By a slow and homogeneous precipitation in aqueous solution, induced by in-situ generation of ammonia by thermal hydrolysis of urea, a monophasic gel is also obtained. The samples have been studied by DTA, DSC, TGA, XRD and dilatometry. 相似文献
994.
Leporatti S Sczech R Riegler H Bruzzano S Storsberg J Loth F Jaeger W Laschewsky A Eichhorn S Donath E 《Journal of colloid and interface science》2005,281(1):101-111
Colloidal probe microscopy was employed to study forces between cellulose surfaces upon addition of a series of cationic copolymers in aqueous solution, as model compounds for wet strength agents. The content of quaternary ammonium groups and primary amines was systematically varied in the cationic polymers, to distinguish between the importance of electrostatical and H-bonding effects. Cellulose microspheres were glued at the apex of tipless microfabricated cantilevers and used as colloidal probes. Ultra thin cellulose films and cellulose fibres were employed as model surfaces. The cellulose films of a thickness of about 5 nm were spin-coated from cellulose solution onto silicon substrates. The root-mean-square-roughness (RMS) was 0.3-0.8 nm. The cationic model polymers were compared to Servamine, a polymer employed as standard wet strength resin in papermaking industries. Force versus separation measurements showed a detailed picture of adhesion and contact breaking. Relatively strong adhesion of the order of 0.3 mJ/m(2) was observed with Servamine within a range of approximately 10 nm. At larger distances weak bond breaking and elastic chain pulling were identified. When approaching the surface one to two small jump-in's possibly related to strong binding of Servamine and subsequent attraction could be found in the case of Servamine. In contrast, all the model copolymers showed only a weak adhesion of 8-30 micro/m(2), i.e., an order of magnitude less than that of Servamine and subsequent elastic rupture domains. The contour length, persistence length and characteristic rupture distances were calculated by means of applying the WLC model. Measurements against cellulose fibres obtained from the production process proved the relevance of the model systems. 相似文献
995.
Mamdouh W Uji-I H Dulcey AE Percec V De Feyter S De Schryver FC 《Langmuir : the ACS journal of surfaces and colloids》2004,20(18):7678-7685
We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquid-solid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential "supramolecular containers" can be influenced. 相似文献
996.
J. Ignacio Andrés Rosario Herranz M. Teresa García López 《Journal of heterocyclic chemistry》1984,21(4):1221-1224
The synthesis of 6-methyl-7-(β-D-ribofuranosyl)imidazo[4,5-d]-v-triazin-4-one (8-methyl-2-azainosine ( 2) ) and 6-methyl-7-(β-D-glucopyranosyl)imidazo[4,5-d]-v-triazin-4-one ( 5 ) by diazotization of 5-amino-1-(β-D-ribofuranosyl)-2-methylimidazole-4-carboxamide ( 1 ) and diazotization of 5-amino-1-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)-2-methylimidazole-4-carboxamide ( 3 ), followed by deacetylation of the resulting compound 4 , is described. The preparation of 6-methyl-5-(β-D-ribofuranosyl)imidazo[4,5-d]-v-triazin-4-one ( 10 ) and 6-methyl-5-(β-D-glucopyranosyl)imidazo[4,5-d]-v-triazin-4-one ( 11 ) by glycosylation of 6-methylimidazo[4,5-d]-v-triazin-4-one (8-methyl-2-azahypoxanthine, ( 7) ) is also described. Structural assignments were made on basis of analytical and 1H-nmr and uv spectral data. 相似文献
997.
G. Wytze Meindersma Lara M. Galán Sánchez Antje R. Hansmeier André B. de Haan 《Monatshefte für Chemie / Chemical Monthly》2007,1(2):1125-1136
Ionic liquids offer tremendous opportunities to intensify reactions and separations in process technologies by tuning their
physical and chemical properties. Several ionic liquids are suitable for the separation of aromatic and aliphatic hydrocarbons.
CO2 absorption behavior was influenced by the functionalized chains appended to the room temperature ionic liquid (RTIL) cation.
Ionic liquids seem able to combine the chemical features of amine solutions with the characteristic advantages of the physical
solvents used for CO2 absorption. 相似文献
998.
Boezio AA Pytkowicz J Côté A Charette AB 《Journal of the American Chemical Society》2003,125(47):14260-14261
We have shown that Me-DuPHOS monoxide (BozPHOS) is a very effective ligand in the copper-catalyzed addition of dialkylzinc to N-phosphinoylimines providing access to alpha-chiral amines. The new ligand is particularly effective for the addition of the lesser reactive dimethylzinc. The major advantages of this process are high yields, broad substrate scope, and high enantioselectivities with low catalyst loading (3 mol %). 相似文献
999.
H. Nölle A. Horn J. Hüve B. Nölle J. Reinhardt J. Schmand H. J. Andrä E. W. Weber 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,30(2):227-229
Cooling and optical pumping by a circularly polarized two mode laser is applied to a Na atomic beam in transverse geometry. The low velocity components of the beam are transversly cooled to the Doppler limit temperature of 240 µK and are simultaneously spin polarized for more than 90%. 相似文献
1000.
Érika de C. Bastone Andréa D. Quintão Reinaldo O. Vianna José R. Mohallem 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,39(1):69-73
We report the development of new gaussian integral transforms one-electron functions. A straightforward recipe to generate gaussian contractions of these functions is introduced. The applications call into question the disseminated belief of the superiority of Slater type orbitals (STO) in molecular calculations. A first successful application of the Bessel K v (qr) function to a molecular system is presented. One of the integral transforms that yields the 0s function is used to generate contractions that have a better performance than the STO ones in cases that diffuse functions are needed. Applications are presented to some Li n clusters and a new conformation of the Li3 cluster is then proposed. 相似文献