全文获取类型
收费全文 | 992篇 |
免费 | 17篇 |
国内免费 | 4篇 |
专业分类
化学 | 588篇 |
晶体学 | 4篇 |
力学 | 22篇 |
数学 | 146篇 |
物理学 | 253篇 |
出版年
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 13篇 |
2014年 | 11篇 |
2013年 | 59篇 |
2012年 | 41篇 |
2011年 | 53篇 |
2010年 | 20篇 |
2009年 | 24篇 |
2008年 | 21篇 |
2007年 | 33篇 |
2006年 | 33篇 |
2005年 | 28篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 37篇 |
2001年 | 16篇 |
2000年 | 29篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 17篇 |
1995年 | 22篇 |
1994年 | 19篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 6篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 8篇 |
1986年 | 21篇 |
1985年 | 22篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 22篇 |
1974年 | 20篇 |
1973年 | 21篇 |
1971年 | 8篇 |
1968年 | 6篇 |
排序方式: 共有1013条查询结果,搜索用时 203 毫秒
851.
Hadley Wickham Michael Lawrence Dianne Cook Andreas Buja Heike Hofmann Deborah F. Swayne 《Computational Statistics》2009,24(2):207-215
What is a pipeline, and why do we need one for interactive graphics? This conceptual paper attempts to answer these questions, building on previous work. A pipeline controls the transformation from data to graphical objects on our screens, and we argue that the pipeline must be present, in some form, in all graphics software. The pipeline is made explicit in descendants of DataViewer. 相似文献
852.
The syntheses and reaction chemistry of two electron mixed-valence diphosphazane-bridged dirhodium and diiridium complexes M(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2) [M = Rh (1), Ir (2); tfepma = MeN[P(OCH(2)CF(3))(2)](2), CN(t)Bu = tert-butyl isocyanide] are described. 1 and 2 undergo addition and two-electron oxidation and reduction chemistries. In the presence of CN(t)Bu, the addition product with the stoichiometry M(2)(0,II)(tfepma)(2)(CN(t)Bu)(3)Cl(2) [M = Rh (3), Ir (3)] is generated; in the presence of 1 equiv of CN(t)Bu and 2 equiv of bis(pentamethyl-cyclopentadienyl)cobalt(II), 1 and 2 are reduced to furnish M(2)(0,0)(tfepma)(2)(CN(t)Bu)(3) [M = Rh (5), Ir (6)], which feature both four- and five-coordinate M(0) centers. Complexes 1, 2, 5, and 6 all possess coordinatively unsaturated square planar M(0) centers that are reactive: (1) 2 reacts with PhICl(2) to produce Ir(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(4) (7); (2) protonation of 2 with HX yields Ir(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2)HX [X = Cl(-) (8), OTs(-) (9)]; (3) protonation of 5 with HOTs produces [Rh(2)(I,I)(tfepma)(2)(CN(t)Bu)(3)(μ-H)](OTs); and (4) the reversible hydrogenation of 2 proceeds smoothly, furnishing the cis-dihydride complex Ir(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)(H)(2)Cl(2) (11). Substitution of tfepma in 2 with bis(diphenylphsophino)methane (dppm) yields the orthometalated complex Ir(2)(II,II)(dppm)(PPh(o-C(6)H(4))CH(2)PPh(2))(CN(t)Bu)(2)Cl(2)H (12). The X-ray crystal structures of 11 compounds are presented and discussed, and spectroscopic characterization by multinuclear and variable temperature NMR provides details about solution structures and in some cases the formation of isomeric products. The electronic spectra of the new complexes are also described briefly, with absorption and emission features derived from the bimetallic core. 相似文献
853.
Ni(H)(X)(IMes)(2) was prepared by the addition of HX to Ni(IMes)(2) (X = Cl, Br; IMes = 1,3-dimesitylimidazol-2-ylidene). Ni(H)(Cl)(IMes)(2) (1) was isolated from the reaction mixture of Ni(IMes)(2) and 2 equiv of 2,6-lutidine·HCl. Ni(H)(Br)(IMes)(2) was prepared in a similar way. Although treatment of Ni(IMes)(2) with a HCl·dioxane solution gives rise to a mixture of 1, NiCl(2)(IMes)(2), and NiCl(IMes)(2), 1 was not isolable from the mixture. All three complexes cocrystallized. Photolysis of these nickel hydrides activates their Ni-H bonds by populating Ni-H σ* orbitals, which results in the formation of H(2). Treatment of 1 with HCl·dioxane gives rise to H(2) and NiCl(2)(IMes)(2). 相似文献
854.
Henri‐Pierre Jacquot de Rouville Romain Garbage Rita E. Cook Adeline R. Pujol Agnès M. Sirven Prof. Dr. Gwénaël Rapenne 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(10):3023-3031
Two new nanovehicles that have extended aromatic platforms as the cargo zones have been obtained. Two strategies were considered for the formation of the perylene core from two naphthalene precursors. The first was based on a Scholl‐type reaction involving an oxidant, and the second used a brominated derivative to perform a homocoupling reaction. The first strategy failed under diverse coupling conditions in the presence of several strong oxidants. Nevertheless, the use of CoF3 in trifluoroacetic acid triggered a dimerization reaction between two ester groups of one molecule and the naphthalene unit of another, thereby surprisingly yielding a ten‐membered carbon macrocycle. The second strategy encountered a lack of reactivity of the substrate under several homocoupling conditions. The dimerization was not easily performed but Ullmann‐type conditions ultimately gave the expected product. The low yield and low solubility of the product encouraged us to modify our initial design. The synthesis of a new chassis that incorporated additional tert‐butyl groups improved the solubility of the molecules and also prevented overcyclization of the aromatic platform by blocking these positions. Some p‐phenylene spacers were also intercalated between the iodine and perylene centers to increase the reactivity of the halide towards coupling reactions. Two new chassis were obtained by Scholl‐type oxidative coupling using FeCl3 as the oxidant. The introduction of four triptycene wheels allowed the formation of the two corresponding nanovehicles. 相似文献
855.
856.
857.
The reduction of a palladium enolate prior to β-hydride elimination provides a unique reaction for the synthesis of the hydroazulene ring system. When combined with a transannular epoxide rearrangement cascade, the reductive-Heck reaction allows rapid entry to the oxo-bridged guaiane core of the englerins. 相似文献
858.
859.
This paper shows that ideodynamics is a differential equation model capable ofpredicting public opinions and behaviors from persuasive information. Statistics are also developed for the model. The methodology is applied to predict the time trend of public opinion about the economy as quantified by the Index of Consumer Sentiment compiled by the University of Michigan. The explanatory variables are derived from news coverage of the economy in positive and negative terms. 相似文献
860.
D.R. Haylett D.F. Davidson R.D. Cook Z. Hong W. Ren S.H. Pyun R.K. Hanson 《Proceedings of the Combustion Institute》2013,34(1):369-376
Species concentration time-histories were measured during oxidation for the large, normal-alkane, diesel-surrogate component n-hexadecane. Measurements were performed behind reflected shock waves in an aerosol shock tube, which allowed for high fuel loading without pre-test heating and possible decomposition and oxidation. Experiments were conducted using near-stoichiometric mixtures of n-hexadecane and 4% oxygen in argon at temperatures of 1165–1352 K and pressures near 2 atm. Concentration time-histories were recorded for five species: C2H4, CH4, OH, CO2, and H2O. Methane was monitored using DFG laser absorption near 3.4 μm; OH was monitored using UV laser absorption at 306.5 nm; C2H4 was monitored using a CO2 gas laser at 10.5 μm; and CO2 and H2O were monitored using tunable DFB diode laser absorption at 2.7 and 2.5 μm, respectively. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Comparisons were made with the predictions of two diesel-surrogate reaction mechanisms (Westbrook et al. [1]; Ranzi et al. [9]) that include n-hexadecane, and areas of needed improvement in the mechanisms were identified. Comparisons of the intermediate product yields of ethylene for n-hexadecane with those found for other smaller n-alkanes, show that an n-hexadecane mechanism derived from a simple hierarchical extrapolation from a smaller n-alkane mechanism does not properly simulate the experimental measurements. 相似文献