全文获取类型
收费全文 | 638篇 |
免费 | 15篇 |
国内免费 | 6篇 |
专业分类
化学 | 352篇 |
力学 | 57篇 |
数学 | 114篇 |
物理学 | 136篇 |
出版年
2023年 | 5篇 |
2022年 | 22篇 |
2021年 | 22篇 |
2020年 | 17篇 |
2019年 | 17篇 |
2018年 | 13篇 |
2017年 | 10篇 |
2016年 | 33篇 |
2015年 | 20篇 |
2014年 | 16篇 |
2013年 | 34篇 |
2012年 | 41篇 |
2011年 | 48篇 |
2010年 | 23篇 |
2009年 | 21篇 |
2008年 | 43篇 |
2007年 | 22篇 |
2006年 | 32篇 |
2005年 | 21篇 |
2004年 | 27篇 |
2003年 | 15篇 |
2002年 | 11篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1973年 | 6篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1967年 | 2篇 |
1966年 | 2篇 |
1958年 | 2篇 |
排序方式: 共有659条查询结果,搜索用时 15 毫秒
181.
Castillo-Tejas J Alvarado JF González-Alatorre G Luna-Bárcenas G Sanchez IC Macias-Salinas R Manero O 《The Journal of chemical physics》2005,123(5):054907
Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edward's [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls. 相似文献
182.
Marvelli L Mantovani N Marchi A Rossi R Brugnati M Peruzzini M Barbaro P de los Rios I Bertolasi V 《Dalton transactions (Cambridge, England : 2003)》2004,(5):713-722
A family of new Fischer-type rhenium(III) benzoyldiazenido-2-oxacyclocarbenes of formula [(ReCl2[eta1-N2C(O)Ph][=C(CH2)nCH(R)O](PPh3)2][n = 2, R = H (2), R = Me (3); n = 3, R = H (4), R = Me (5)] have been prepared by reaction of [ReCl2[eta2-N2C(Ph)O](PPh3)2] (1) with omega-alkynols, such as 3-butyn-1-ol, 4-pentyn-1-ol, 4-pentyn-2-ol, 5-hexyn-2-ol in refluxing THF. The correct formulation of the carbene derivatives 2-5 has been unambiguously determined in solution by NMR analysis and confirmed for compounds 2-4 by X-ray diffraction methods in the solid state. All complexes are octahedral with the benzoyldiazenido ligand, Re[N2C(O)Ph], adopting a "single bent" conformation. The coordination basal plane is completed by an oxacyclocarbene ligand and two chlorine atoms. Two triphenylphosphines in trans positions with respect to each other complete the octahedral geometry around rhenium. The reactivity of 1 towards different alkynes and alkenes including propargyl- and allylamine has been also studied. With propargyl amine, monosubstituted or bisubstituted complexes, [(ReCl2[eta1-N2C(O)Ph][eta1-NH2CH2C triple bond CH]n(PPh3)(3-n)][n= 1 (6); n = 2 (7)], have been isolated depending on the reaction conditions. In contrast, the reaction with allylamine gave only the disubstituted complex [(ReCl2[eta1-N2C(O)Ph][eta1-NH2CH2CH=CH2]2(PPh3)] (8). The molecular structure of the monosubstituted adduct has been confirmed by X-ray analysis in the solid state. 相似文献
183.
Isaac B. Bersuker 《Journal of computational chemistry》1997,18(2):260-267
It is shown that the claims that density functional theory (DFT) can handle orbitally degenerate states are ungrounded. The constraint search formulation of DFT allows one to determine a set of densities and eigenvalues for the degenerate term that, however, are neither observables, nor can they be used to solve the system of coupled equations for the nuclear motions to obtain observables, as in the wave function presentation. A striking example of the failure of the existing versions of DFT to describe degenerate states is provided by the Berry phase problem: the strong dependence of the results on the phase properties of the electronic wave function that are smeared out in the density formulation. The solution of the Jahn-Teller E-e problem illustrates these statements. For nondegenerate states with the full wave function taken in the adiabatic approximation as a product of the electronic and nuclear parts, the formulation of DFT is rigorous if and only if the dependence of the electronic wave function on nuclear coordinates is ignored. This lowers the accuracy of the results, in general, and may lead to erroneous presentation as in the case of molecular systems in strong magnetic fields. © 1997 by John Wiley & Sons, Inc. 相似文献
184.
Gregory J. O. Beran Isaac J. Sugden Chandler Greenwell David H. Bowskill Constantinos C. Pantelides Claire S. Adjiman 《Chemical science》2022,13(5):1288
With 12 crystal forms, 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecabonitrile (a.k.a. ROY) holds the current record for the largest number of fully characterized organic crystal polymorphs. Four of these polymorph structures have been reported since 2019, raising the question of how many more ROY polymorphs await future discovery. Employing crystal structure prediction and accurate energy rankings derived from conformational energy-corrected density functional theory, this study presents the first crystal energy landscape for ROY that agrees well with experiment. The lattice energies suggest that the seven most stable ROY polymorphs (and nine of the twelve lowest-energy forms) on the Z′ = 1 landscape have already been discovered experimentally. Discovering any new polymorphs at ambient pressure will likely require specialized crystallization techniques capable of trapping metastable forms. At pressures above 10 GPa, however, a new crystal form is predicted to become enthalpically more stable than all known polymorphs, suggesting that further high-pressure experiments on ROY may be warranted. This work highlights the value of high-accuracy crystal structure prediction for solid-form screening and demonstrates how pragmatic conformational energy corrections can overcome the limitations of conventional density functionals for conformational polymorphs.Crystal structure prediction suggests that the low-energy polymorphs of ROY have already been found, but a new high-pressure form is predicted. 相似文献
185.
Krzysztof Ejsmont Jacek Zaleski Isaac H. Joe Victor S. Jayakumar 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(10):o553-o555
The structure of the title compound, 2C4H9N2O3+·C2O42−, which has been determined by X‐ray diffraction, contains discrete glycylglycine (HGly–Gly)+ cations in general positions and oxalate anions which lie across centres of inversion. Although the geometry of the (HGly–Gly)+ cation is not significantly different compared with other structures containing this residue, a few changes in conformation are observed which indicate the presence of molecular interactions. The molecular network in the crystal consists of one nearly linear O—H⋯O, five N—H⋯O and two weak C—H⋯O hydrogen bonds. 相似文献
186.
A novel and efficient synthesis of both (±)-methyl epijasmonate and (±)-cis-3-(2-oxopropyl)-2-(pent-2Z-enyl)-cyclopentan-1-one is described. The key step to establish the cis-stereochemistry on the 5-membered ring is an ionic Diels–Alder reaction, which is high yielding and highly regioselective. Subsequent key steps include oxidative cleavage of the six-membered ring, Wittig coupling and for the synthesis of epijasmonate, the haloform reaction. 相似文献
187.
Genovese Dario Elishakoff Isaac 《Archive of Applied Mechanics (Ingenieur Archiv)》2019,89(10):1995-2003
Archive of Applied Mechanics - We discuss the role of the principle of virtual work, of the objectivity and of the energy balance in the formulation of planar static rod theories, in a large... 相似文献
188.
Alexandru D. Asandei Isaac W. Moran Gobinda Saha Yanhui Chen 《Journal of polymer science. Part A, Polymer chemistry》2006,44(6):2015-2026
The effects of solvents, additives, ligands, and solvent in situ drying agents as well as catalyst and initiator concentrations have been investigated in the Cp2TiCl‐catalyzed radical polymerization of styrene initiated by epoxide radical ring opening. On the basis of the solubilization of Cp2Ti(III)Cl and the polydispersity of the resulting polymer, the solvents rank as follows: dioxane ≥ tetrahydrofuran > diethylene glycol dimethyl ether > methoxybenzene > diphenyl ether ≥ bulk > toluene ? pyridine > dimethylformamide > 1‐methyl‐2‐pyrrolidinone > dimethylacetamide > ethylene carbonate, acetonitrile, and trioxane. Alkoxide additives such as aluminum triisopropoxide and titanium(IV) isopropoxide are involved in alkoxide ligand exchange with the epoxide‐derived titanium alkoxide and lead to broad molecular weight distributions, whereas similarly to strongly coordinating solvents, ligands such as bipyridyl block the titanium active site and prevent the polymerization. By contrast, softer ligands such as triphenylphosphine improve the polymerization in less polar solvents such as toluene. Although mixed hydrides such as lithium tri‐tert‐butoxyaluminum hydride, sodium borohydride, and lithium aluminum hydride react with bis(cyclopentadienyl)titanium dichloride to form mixed titanium hydride species ineffective in polymerization control, simple hydrides such as lithium hydride, sodium hydride, and especially calcium hydride are particularly effective as in situ trace water scavengers in this polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2015–2026, 2006 相似文献
189.
Probabilistic interval reliability of structural systems 总被引:1,自引:0,他引:1
Zhiping Qiu Di Yang Isaac Elishakoff 《International Journal of Solids and Structures》2008,45(10):2850-2860
The probabilistic reliability approach is the most widely used method for reliability analysis. The recent research shows that the reliabilities of structural systems strongly depend on the parameters of the probability model. It is possible that the little error in the estimation of the parameters may lead to the remarkable error of the resulting probability. In this study, we introduce the interval approach into the conventional reliability theory. We present a novel approach which allows us to obtain the system failure probability interval from the statistical parameter intervals of the basic variables. This approach is a combination of the two techniques, namely the classical reliability theory and the interval analysis. In the end of this paper, we show the feasibility of the proposed approach through two examples of the truss systems. 相似文献
190.