全文获取类型
收费全文 | 441篇 |
免费 | 7篇 |
专业分类
化学 | 282篇 |
晶体学 | 28篇 |
力学 | 8篇 |
数学 | 33篇 |
物理学 | 97篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 13篇 |
2015年 | 7篇 |
2014年 | 18篇 |
2013年 | 24篇 |
2012年 | 48篇 |
2011年 | 36篇 |
2010年 | 29篇 |
2009年 | 27篇 |
2008年 | 30篇 |
2007年 | 30篇 |
2006年 | 22篇 |
2005年 | 18篇 |
2004年 | 11篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1883年 | 1篇 |
排序方式: 共有448条查询结果,搜索用时 46 毫秒
131.
C. H. Collins C. A. Bertran A. L. Pires Valente Priscila de A. Leone A. L. M. Murta K. E. Collins 《Chromatographia》1988,26(1):168-170
Summary Separation of perhalogenated compounds resulting from thermolysis, photolysis or radiolysis of mixtures of halogens in carbon tetrachloride was carried out by programmed temperature gas chromatography. Plots of relative retentions (to CBr4) vs the number of bromine atoms present for known compounds of the general formula CxBryClz (x=1 and 2; y+z=4 or 6) gave three linear correlations (all with r>0.99). These correlations permitted projections of the identities of reaction products for which no standards were available. These projections were later confirmed by CG-MS. *** DIRECT SUPPORT *** A0732166 00010 相似文献
132.
José E. Cortés-Figueroa Jeffrey D. Zubkowski Edward J. Valente 《Journal of chemical crystallography》1998,28(3):217-220
The x-ray crystal structure of the complex 2-PDOW(CO)4 (five-membered ring, PDO = 2, 2, 4, 7, 7-pentamethyl-3,6-dithiaoctane) is reported. The complex crystallizes in the monoclinic crystal system, space group P21/c, [#14] with unit cell parameters a = 14.002(14) Å, b = 9.340(10) Å, c = 15.094(12) Å, = 92.67(4)°, V = 1972(3) Å3; Z = 4. The arrangement of the ligands around the metal atom is distorted from octahedral geometry. Large C–O bond distances and short W–C bond distances of the carbonyl groups located at a trans position with respect to PDO is indicative of a trans influence. The W–S(1) and W–S(2) bond distances of 2.545(3) and 2.545(2) Å, respectively, are shorter than observed for closely related complexes. 相似文献
133.
Mononitration of indane produces a mixture of 4- and 5- nitroindanes. Crystallization from mixtures occurs after distillation improves composition of a major component to above 80%. 4-Nitroindane: triclinic, space group
(#2),a=7.332(4) Å,b=8.304(4) Å,c=8.358(4) Å, =61.43(4)°, =67.60(4)°, =70.15(4)°,V=405.4(4) Å3,Z=2. Non-H-atoms are nearly planar, aliphatic H's are eclipsed. 5-Nitroindane: monoclinic, space groupP21/c (#14),a=10.946(8) Å,b=15.643(10) Å,c=9.415(6) Å, =92.34(5)°,V=1611(2) Å3,Z=8. Non-H-atoms in the two molecules differ in torsion of the nitro group with respect to indane and fold of the nonbenzylic methylene group. Semiempirical calculations (PM3) suggest that distorsion from planarity may be associated with the two lowest energy vibrational modes. Uv, ir, ms, proton, and13C-nmr spectra are correlated with the solid state structures. 相似文献
134.
J.S. Valente P. Navarro J.J. Delgado E. Torres-García 《Applied Surface Science》2008,254(10):3006-3013
In the present work, a detailed study on structural and textural properties, as well as a thermo-kinetic characterization of two manganese cryptomelane-type oxides, was carried out. A suitable methodology was stated for determining the several stages taking place during the surface molecules’ removal. Thus, further insights were provided about the chemical nature of the sites present at the solid's surface. Relative strengths and the number of basic sites at the surface were estimated. The results show that the removal of the adsorbed (H2O, CO2) molecules from the surface is essential to disclose the oxide surface features. The morphological properties and thermo-kinetic characterization indicate the existence of at least three types of basic sites, owing activation energy values ranging from 140 to 190 kJ/mol. The dispersion observed in the activation energy data, during the CO2 loss, suggests that the interaction occurs on different sites. Therefore, both samples showed a heterogeneous surface; this is related to the energetic nature of the sites, as well as the basic strengths of the surface manganese cryptomelane oxides. 相似文献
135.
Marta Abrantes Filipe A. Almeida Paz Anabela A. Valente Cláudia C.L. Pereira Sandra Gago Alírio E. Rodrigues Jacek Klinowski Martyn Pillinger Isabel S. Gonçalves 《Journal of organometallic chemistry》2009,694(12):1826-1833
The complex [(N-benzyloxycarbonylprolyl)cyclopentadienyl]-Mo(CO)3Me was prepared by the reaction of the amino acid-functionalized cyclopentadienyl-lithium reagent with Mo(CO)6, and subsequent methylation with CH3I. The complex was characterized by FTIR, 1H and 13C NMR, and elemental analysis. A single-crystal X-ray diffraction study showed that the complex is chiral, crystallizing in the orthorhombic P212121 space group with the Flack parameter refining to –0.007(16), which unequivocally confirms the presence of an enantiomerically pure compound. The complex was examined as a catalyst precursor in the liquid-phase epoxidation of trans-β-methylstyrene at 280–330 K, using either tert-butylhydroperoxide (tert-BuOOH), cumylhydroperoxide or urea-hydrogen peroxide adduct as oxidant, and, optionally, a co-solvent. With chloroform and tert-BuOOH, the catalytic activity surpasses that previously reported for chiral complexes of the type CpMo(CO)3X, giving the epoxide isomers ((R,R)-(+) and (S,S)-(?)-1-phenylpropylene oxide) in excellent selectivity, albeit with negligible enantiomeric excess. FTIR spectroscopy showed that the oxidative decarbonylation of the tricarbonyl complex with tert-BuOOH is fast under the reaction conditions used, and that the structural integrity of the amino acid-functionalized cyclopentadienyl group is retained during this process. The use of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate as a solvent for the catalyst generates a biphasic liquid–liquid reaction system that enables, at the end of a catalytic cycle, the separation and reutilization of the catalyst. 相似文献
136.
A high‐order Petrov–Galerkin finite element scheme is presented to solve the one‐dimensional depth‐integrated classical Boussinesq equations for weakly non‐linear and weakly dispersive waves. Finite elements are used both in the space and the time domains. The shape functions are bilinear in space–time, whereas the weighting functions are linear in space and quadratic in time, with C0‐continuity. Dispersion correction and a highly selective dissipation mechanism are introduced through additional streamline upwind terms in the weighting functions. An implicit, conditionally stable, one‐step predictor–corrector time integration scheme results. The accuracy and stability of the non‐linear discrete equations are investigated by means of a local Taylor series expansion. A linear spectral analysis is used for the full characterization of the predictor–corrector inner iterations. Based on the order of the analytical terms of the Boussinesq model and on the order of the numerical discretization, it is concluded that the scheme is fourth‐order accurate in terms of phase velocity. The dissipation term is third order only affecting the shortest wavelengths. A numerical convergence analysis showed a second‐order convergence rate in terms of both element size and time step. Four numerical experiments are addressed and their results are compared with analytical solutions or experimental data available in the literature: the propagation of a solitary wave, the oscillation of a flat bottom closed basin, the oscillation of a non‐flat bottom closed basin, and the propagation of a periodic wave over a submerged bar. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
137.
138.
We present results of an energy independent multipole analysis of π+ and π0 photoproduction on protons for photon energies between 160 MeV and 330 MeV. Each isospin part of the l ? 1 amplitudes has been parametrized; the 1 ? 2 amplitudes have been approximated by the Born terms. Unambiguous solutions for all the multipoles in the whole range of Eγ have been obtained. The continuity of the solutions as functions of the energy is very good. 相似文献
139.
R. Baldini-Celio G. Capon R. Del Fabbro M. Grilli E. Isrocci M. Locci C. Mencuccini G.P. Murtas M.S. Spano M. Spinetti V. Valente C. Bacci G. Penso B. Stella 《Physics letters. [Part B]》1975,58(4):475-477
The decay J/Ψ (3100)→ η(958) + γ has been searched for at tthe Adone e+e+- storage ring. No candidate event has been found. This allows to set an upper limit of 1.0 keV (90% C.L.) for this decay partial width. 相似文献
140.