全文获取类型
收费全文 | 1682篇 |
免费 | 88篇 |
国内免费 | 3篇 |
专业分类
化学 | 1425篇 |
晶体学 | 9篇 |
力学 | 12篇 |
数学 | 143篇 |
物理学 | 184篇 |
出版年
2024年 | 2篇 |
2023年 | 32篇 |
2022年 | 99篇 |
2021年 | 107篇 |
2020年 | 66篇 |
2019年 | 56篇 |
2018年 | 60篇 |
2017年 | 31篇 |
2016年 | 99篇 |
2015年 | 56篇 |
2014年 | 80篇 |
2013年 | 114篇 |
2012年 | 113篇 |
2011年 | 133篇 |
2010年 | 79篇 |
2009年 | 67篇 |
2008年 | 110篇 |
2007年 | 83篇 |
2006年 | 81篇 |
2005年 | 77篇 |
2004年 | 62篇 |
2003年 | 39篇 |
2002年 | 26篇 |
2001年 | 13篇 |
2000年 | 6篇 |
1999年 | 15篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 13篇 |
1995年 | 3篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
1964年 | 1篇 |
1961年 | 1篇 |
1960年 | 2篇 |
排序方式: 共有1773条查询结果,搜索用时 46 毫秒
11.
Natalia N. Sergeeva 《Tetrahedron letters》2006,47(35):6169-6172
The palladium-catalyzed reaction of RLi with various 5,10,15,20-tetrasubstituted porphyrins offers a convenient synthetic route to chlorins and porphodimethenes (calixphyrins). The reactions utilized various Pd catalysts and CuI and yielded either 2,3-substituted chlorins or 5,10-disubstituted porphodimethenes in yields ranging from 20-40%. The reaction of octaethylporphyrin with t-BuLi in the presence of a Pd-catalyst generated the corresponding 5,10-porphodimethene in 72% yield. 相似文献
12.
Karin Krüger Natalia V. Shakhlevich Yuri N. Sotskov Frank Werner 《The Journal of the Operational Research Society》1995,46(12):1481-1497
We consider a scheduling problem where a set of n jobs has to be processed on a set of m machines and arbitrary precedence constraints between operations are given. Moreover, for any two operations i and j values a ij >0 and a ji >0 may be given where a ij is the minimal difference between the starting times of operations i and j when operation i is processed first. Often, the objective is to minimize the makespan but we consider also arbitrary regular criteria. Even the special cases of the classical job shop problem J//Cmax belong to the set of NP-hard problems. Therefore, approximation or heuristic algorithms are necessary to handle large-dimension problems. Based on the mixed graph model we give a heuristic decomposition algorithm for such a problem, i.e. the initial problem is partitioned into subproblems that can be solved exactly or approximately with a small error bound. These subproblems are obtained by a relaxation of a subset of the set of undirected edges of the mixed graph. The subproblems are successively solved and a proportion of the results obtained for one subproblem is kept for further subproblem definitions. Numerical results of the algorithm presented here are given. 相似文献
13.
Carolina Menna Natalia Calonghi Lanfranco Masotti Paolo Neyroz 《Journal of fluorescence》1993,3(1):33-43
This report describes a method to conjugate lucifer yellow to the external surface of liposomes. The heterobifunctional cross-linking reagentN-succinimidyl 3-(2-pyridyldithio)propionate has been used to activate DMPE molecules. The DMPE-dithiopyridine product has been mixed with DMPC to prepare liposome vesicles. These have been reduced by DTT and finally reacted with lucifer yellow-iodoacetamide to produce the fluorescence-labeled vesicles. The quenching of their fluorescence intensity by Kl is consistent with fully exposed fluorophores. The decay of the fluorescence intensity of the lipid-bound lucifer yellow is biexponential (1=7.9 ns; 2=1.1 ns), with a relative yield of 0.16. When the fluorescent liposomes are mixed with cells, the lucifer yellow-DMPE derivative is transferred. Boar spermatozoa and peripheral human blood lymphocytes have been used as cellular models. The extent of incorporation is dependent on the incubation time and temperature. At 36°C, lucifer yellow fluorescence appears in the spermatozoa cells after 10 min of incubation and reaches its maximum at about 60 min. The fluorescent phospholipid derivative seems to incorporate specifically into membrane structures. The highest labeling ratio is observed with integer, scarcely motile, spermatozoa. A poorer labeling yield (15%) is found with lymphocytes. Interestingly, photobleaching due to epiillumination of the labeled cells is apparently negligible and cells are clearly visible after irradiation times ranging from several minutes to few hours.A preliminary account of this work was presented at the Quarto Simposio su Biotecnologie Biochimiche, Capri, 28–30 June 1992. 相似文献
14.
Unprotected 5-(4-nitrophenyl)-10,15,20-triphenylporphyrin and 5,10-bis(4-nitrophenyl)-15,20-diphenylporphyrin react in the presence of a base at low temperature with carbanions (which bear a leaving group X at the carbanionic center) affording vicarious nucleophilic substitution of hydrogen (VNS) products in good yields (50-89%). The reactivity is explained in terms of the predominance of the porphyrin N-anion resonance forms at this temperature. 相似文献
15.
Herein, we present results from MD simulations of the Michaelis complex formed between the dizinc beta-lactamase from B. fragilis and imipenem. We considered two catalytically important configurations, which differ in the presence or absence of a hydroxide bridge connecting the two zinc ions in the active site. The structural and dynamical effects induced by substrate binding, the specific roles of the conserved residues and the zinc-bound water molecules, the near attack conformers of the Michaelis complex, and so forth, are discussed in detail. The relative stability of the two configurations was estimated from QM linear scaling calculations on the enzyme-substrate complex combined with Poisson-Boltzmann electrostatic calculations and normal mode calculations. Importantly, we find that the two configurations have similar energies, indicating that these two structures could readily be interchanged, thereby facilitating catalysis. The configuration with the hydroxide bound to the two zinc ions is predicted to be the resting form of the enzyme, while the configuration without the bridge is the reactive form that was found to place the hydroxide in position to attack the carbonyl of the beta-lactam ring. Thus, we propose that the enzyme initiates catalysis by converting from the hydroxide bridge form into the configuration that lacks the hydroxide bridge. This interconversion increases the nucleophilicity of the hydroxide ion and exposes it to the beta-lactam carbonyl, which ultimately facilitates nucleophilic attack. The implications of the observed modes of binding, the possible influence of mutating the Lys184 and Asn193 residues on substrate binding, and the reaction mechanism are also discussed in detail. 相似文献
16.
Sitnikova NL Sprik R Wegdam G Eiser E 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7083-7089
We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months. 相似文献
17.
To investigate the molecular details of the phosphoryl-transfer mechanism catalyzed by cAMP-dependent protein kinase, we performed quantum mechanical (QM) calculations on a cluster model of the active site and molecular dynamics (MD) simulations of a ternary complex of the protein with Mg(2)ATP and a 20-residue peptide substrate. Overall, our theoretical results confirm the participation of the conserved aspartic acid, Asp(166), as an acid/base catalyst in the reaction mechanism catalyzed by protein kinases. The MD simulation shows that the contact between the nucleophilic serine side chain and the carboxylate group of Asp(166) is short and dynamically stable, whereas the QM study indicates that an Asp(166)-assisted pathway is structurally and energetically feasible and is in agreement with previous experimental results. 相似文献
18.
Isaac B. Bersuker Natalia N. Gorinchoi Victor Z. Polinger 《Theoretical chemistry accounts》1984,66(3-4):161-172
The vibronic origin of dynamic instability of molecular systems considered earlier, is here given a more complete and rigorous treatment. It is shown that the nonvibronic contribution to the curvature of the adiabatic potential arising due to nuclear displacements under fixed electronic density distribution, is always positive, and hence the only reason for dynamic instability is the pseudo Jahn-Teller effect. For some examples of special interest (planar equilateral NH3, planar square CH4 and linear H
3
+
) the molecular excited states, responsible for the instability of the ground state, are revealed by means of ab initio calculations. 相似文献
19.
20.
Trihalogermyl-furans and -thiophenes were prepared by inserting germanium dibromide (GeBr2) generated from the dibromogermane(II) dioxanate complex into the carbon-halogen bond of halo-furans and -thiophenes. Their ethanolysis and transesterification by triethanolamine yielded the germatranes which were subjected to psychotropic activity assays. The psychotropic properties of germatranes were found to depend on the type of the heterocycle and on the position of the germatrane moiety. 相似文献