全文获取类型
收费全文 | 254篇 |
免费 | 6篇 |
专业分类
化学 | 165篇 |
晶体学 | 1篇 |
力学 | 9篇 |
数学 | 47篇 |
物理学 | 38篇 |
出版年
2022年 | 14篇 |
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 7篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 17篇 |
2015年 | 9篇 |
2014年 | 5篇 |
2013年 | 17篇 |
2012年 | 10篇 |
2011年 | 24篇 |
2010年 | 18篇 |
2009年 | 18篇 |
2008年 | 21篇 |
2007年 | 20篇 |
2006年 | 7篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 4篇 |
2002年 | 5篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有260条查询结果,搜索用时 15 毫秒
71.
Silva M Antunes MA Dias M Domingos A dos Santos IC Marçalo J Marques N 《Dalton transactions (Cambridge, England : 2003)》2005,(20):3353-3358
Insertion of benzonitrile and acetonitrile into the U-C bond of [U(Tp(Me2))Cl(2)(CH(2)SiMe(3))](Tp(Me2)= HB(3,5-Me(2)pz)(3)) gives the ketimide complexes [U(Tp(Me2))Cl(2){NC(R)(CH(2)SiMe(3))}](R = Ph (1); Me (2)). The identity of complex was ascertained by a single-crystal X-ray diffraction study. In the solid state exhibits octahedral geometry with a short U-N bond length to the ketimide ligand. We also report herein the synthesis and the X-ray crystal structures of the uranium amide complexes [U(Tp(Me2))Cl(2)(NR(2))](R = Et (3); Ph (4)). A detailed comparison of the U-N bond lengths in these compounds with other known U-N (and Th-N) distances in amide and ketimide actinide(IV) complexes is performed, confirming the short character of the U-N bond length in 1. 相似文献
72.
Octavio A. C. Antunes Jairo Bordinhão Nakédia M. F. Carvalho James L. Wardell Pedro A. Arroyo Márcio E. Berezuk Lucio Cardozo-Filho Edward R. T. Tiekink 《Journal of chemical crystallography》2007,37(4):291-298
Two structures of the closely related piperazinium salts, [1,4-bis-(2-methoxy-2-carbonylethyl)piperazinium (2+)] chloride
(1) and [1,4-bis-(2-amidoethyl)piperazinium (2+)] perchlorate (2) reveal the persistence of a layer structure, interspersed with anions, held in place by N–H···Cl, C–H···O and C–H···Cl interactions
in the case of 1, and N–H···O and C–H···O interactions in the case of 2. Compound 1 crystallizes in the triclinic space group P-1 with a=6.7667(6) ?, b = 6.9467(6) ?, c=9.3880(18) ?, α=105.845(9)°, β=99.896(16)°, γ=103.600(9)°, and Z=1. Compound 2 crystallizes in the monoclinic space group P21/c with a = 9.0930(8) ?, b=10.0525(11) ? c=10.2104(13) ?, β=114.279(9)°, and Z=2. 相似文献
73.
There is no generally accepted definition for conditional Tsallis entropy. The standard definition of (unconditional) Tsallis entropy depends on a parameter that converges to the Shannon entropy as approaches 1. In this paper, we describe three proposed definitions of conditional Tsallis entropy suggested in the literature—their properties are studied and their values, as a function of , are compared. We also consider another natural proposal for conditional Tsallis entropy and compare it with the existing ones. Lastly, we present an online tool to compute the four conditional Tsallis entropies, given the probability distributions and the value of the parameter . 相似文献
74.
An extension to the rotating-sector method, which is usually applied to determine propagation and termination rate constants, is presented. The analytical treatment developed accounts for the simultaneous presence of a thermal initiation and of a first-order termination process. The applicability of the rotating-sector method is thus extended to situations where the rate in dark is higher than 5% of the rate in the presence of light, and more accurate estimates of the rate constants are obtained than before for any values of the “dark” rate. A previously published experiment on the application of the rotating-sector method to the autoxidation of styrene was reanalyzed. The estimates obtained for the propagation and the termination rate constants were 11% and 19% higher than the previous estimates, respectively. Finally, the improved rotating-sector method was also applied to the experimental determination of propagation (kp) and termination rate constants (2×kt) for both 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC) and 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) liposomes. The following results were obtained at 37°C: for PLPC kp =16.6 M−1s−1, and 2×kt=1.27×105 M−1s−1; for DLPC kp(intermolecular)=(13.3–13.9) M−1s−1, kp(intramolecular)=(4.7–5.4) s−1, and 2×kt=(0.99–1.05)×105 M−1s−1. The separation of the intermolecular and intramolecular propagation rate constants for DLPC was made possible both by a special adaptation of the rotating-sector equations to substrates with two oxidizable moieties, and by the experimental determination of the ratio between partially oxidized DLPC molecules (only one acyl is oxidized) and fully oxidized DLPC molecules (both acyls are oxidized). © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 753–767, 1998 相似文献
75.
W.S. Carvalho A.S. de Castro A.C.B. Antunes 《The European Physical Journal C - Particles and Fields》2000,17(1):173-177
A quarkonium–gluonium mixing scheme previously developed to describe the characteristic of the pseudoscalar mesons is applied
to axial and tensor mesons. The parameters of the model are determined by fitting the eigenvalues of a mass matrix. The corresponding
eigenvectors give the proportion of light quarks, strange quarks and glueball in each meson. However, the predictions of the
model for the branching ratios and electromagnetic decays are incompatible with the experimental results. These results suggest
the absence of gluonic components in the states of axial and tensor isosinglet mesons analyzed here.
Received: 12 November 1999 / Revised version: 27 April 2000 / Published online: 9 August 2000 相似文献
76.
This work exploits links between Data Envelopment Analysis (DEA) and multicriteria decision analysis (MCDA), with decision making units (DMUs) playing the role of decision alternatives. A novel perspective is suggested on the use of the additive DEA model in order to overcome some of its shortcomings, using concepts from multiattribute utility models with imprecise information. The underlying idea is to convert input and output factors into utility functions that are aggregated using a weighted sum (additive model of multiattribute utility theory), and then let each DMU choose the weights associated with these functions that minimize the difference of utility to the best DMU. The resulting additive DEA model with oriented projections has a clear rationale for its efficiency measures, and allows meaningful introduction of constraints on factor weights. 相似文献
77.
Suzuki–Miyaura reactions between halobenzenes and other substituted (hetero)arenes and boronic acids were carried out in PEG–water solution using Pd/BaSO4 as catalytic source in a ligand‐free system. High to moderate yields were generally obtained. The catalytic system could be recycled up to six times, showing a slight decrease in activity after each cycle. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
78.
A two-dimensional (in-plane) numerical model for surface waves propagation based on the non-linear dispersive wave approach described by Boussinesq-type equations, which provide an attractive theory for predicting the depth-averaged velocity field resulting from that wave-type propagation in shallow water, is presented. The numerical solution of the corresponding partial differential equations by finite-difference methods has been the subject of several scientific works. In the present work we propose a new approach to the problem: the spatial discretization of the system composed by the Boussinesq equations is made by a finite element method, making use of the weighted residual technique for the solution approach within each element. The model is validated by comparing numerical results with theoretical solutions and with results obtained experimentally. 相似文献
79.
This review presents the structure, biological activity, biosynthetic studies and, where applicable, references to syntheses of 81 marine alkaloids containing either tetra-, hexa- or octa-hydrogenated variants of pyrrolo[4,3,2-de]quinoline, pyrrolo[4,3,2-de]pyrrolo[2,3-h]quinoline and pyrido[2,3-h]pyrrolo[4,3,2-de]quinoline core skeletons. The literature describing the isolation of pyrroloiminoquinones, and related metabolites, from marine sponges is littered with taxonomic inconsistencies and recent efforts to clarify the taxonomy of the sponges that produce this group of metabolites are discussed. 相似文献
80.