全文获取类型
收费全文 | 3067篇 |
免费 | 127篇 |
国内免费 | 16篇 |
专业分类
化学 | 1878篇 |
晶体学 | 6篇 |
力学 | 130篇 |
数学 | 629篇 |
物理学 | 567篇 |
出版年
2024年 | 3篇 |
2023年 | 34篇 |
2022年 | 94篇 |
2021年 | 128篇 |
2020年 | 73篇 |
2019年 | 91篇 |
2018年 | 71篇 |
2017年 | 53篇 |
2016年 | 119篇 |
2015年 | 112篇 |
2014年 | 129篇 |
2013年 | 195篇 |
2012年 | 231篇 |
2011年 | 255篇 |
2010年 | 152篇 |
2009年 | 110篇 |
2008年 | 209篇 |
2007年 | 175篇 |
2006年 | 164篇 |
2005年 | 166篇 |
2004年 | 117篇 |
2003年 | 106篇 |
2002年 | 95篇 |
2001年 | 38篇 |
2000年 | 34篇 |
1999年 | 28篇 |
1998年 | 15篇 |
1997年 | 29篇 |
1996年 | 21篇 |
1995年 | 17篇 |
1994年 | 17篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 5篇 |
1973年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有3210条查询结果,搜索用时 15 毫秒
61.
Moret I Gambaro A Piazza R Corami F Ravazzi C Andreoli C Truzzi C Lambertucci L Scarponi G 《Annali di chimica》2004,94(5-6):373-387
During the period March 1997-March 1998 dimethyl sulphide (DMS), dimethylsulphoniopropionate (DMSP) and carbon disulphide (CS2) were determined weekly in the water of the Lagoon of Venice, Italy (at three stations located in the Giudecca Canal, the San Secondo Canal and the Rio di San Nicolò). At the same time, the following hydrological and biological variables were also measured: tide height, temperature, transmittance, fluorescence, pH, salinity, chlorinity, sulphate, ammonia, nitrite, nitrate, phosphate, silicate, chlorophyll a, phaeopigments, phytoplankton (abundance and biomass). Principal component analysis (PCA), applied as a dimension reduction tool, made it possible to summarize multivariate information in a small number of components, which highlighted the relationships between the temporal evolutions of the sulphur compounds with hydrological and biological variables in the seasonal biogeochemical cycle of the lagoon. In particular the overall temporal cycle, which begins with the development of biological activity in late winter and spring, followed by the predominance of degradation processes during the late summer and the remineralization of nutrients in autumn, is clearly described in the plane of the first two principal components, together with the interrelationships between all the relevant variables. 相似文献
62.
Laurent Salvi Hélène Cattey Yves Mugnier Antonio Otero 《Journal of organometallic chemistry》2004,689(22):3473-3480
Electrochemical reduction of niobocene dichloride (η5-C5H4SiMe3)2NbCl21 formulated as in the presence of 3,4-diaminobenzoic acid yields to the complex [Nb(η5-C5H4SiMe3)2(κ2-O,O-OOC(C6H3)(NH2)2)] 3. When CN(2,6-Me2C6H3) formulated as xylylisonitrile (CNXylyl) is added to a complex 3 solution, a substitution reaction takes place to lead to the complex [Nb(η5-C5H4SiMe3)2(κ1-O-OOC(C6H3)(NH2)2)(CN(2,6-Me2C6H3)) 4 after 3 h. An alternative way to yield quantitatively and nearly instantaneously 4 consists in a previous oxidation of 3 in the presence of CNXylyl. Hence, we present here a new example of electron-transfer-catalyzed (ETC) ligand substitution of carboxylato niobocene complex induced by electrochemical oxidation. The structure of the complexes, the formation mechanism are described using electrochemical and spectroscopic data. Electrochemical simulation have been done to verify experimental results and to complete them with a kinetic study. 相似文献
63.
6-Trifluoromethyl-12-acylindolo[1,2-c]quinazolines are prepared in high yield through the palladium-catalyzed reaction of bis(o-trifluoroacetamidophenyl)acetylene with aryl or vinyl halides and triflates. The reaction, which tolerates a variety of important functional groups, probably involves the formation of a 3-acyl-2-(o-trifluoroacetamidophenyl)indole intermediate, followed by its cyclization to the indoloquinazoline product. [reaction: see text] 相似文献
64.
Herein, we report the 1,2-dialkylation of simple feedstock acrylates for the synthesis of valuable tertiary carboxylic acids by merging Giese-type radical addition with an Ireland–Claisen rearrangement. Key to success is the utilization of the reductive radical-polar crossover concept under photocatalytic reaction conditions to force the [3,3]-sigmatropic rearrangement after alkyl radical addition to allyl acrylates. Using readily available alkyl boronic acids as radical progenitors, this redox-neutral, transition-metal-free protocol allows the mild formation of two C(sp3)–C(sp3) bonds, thus providing rapid access to complex tertiary carboxylic acids in a single step. Moreover, this strategy enables the efficient synthesis of highly attractive α,α-dialkylated γ-amino butyric acids (GABAs) when α-silyl amines are used as radical precursors – a structural motif that was still inaccessible in related transformations. Depending on the nature of the radical precursors and their inherent oxidation potentials, either a photoredox-induced radical chain or a solely photoredox mechanism is proposed to be operative.A photocatalytic 1,2-dialkylation of α-substituted acrylates is enabled by a reaction cascade combining reductive radical-polar crossover with the established Ireland–Claisen rearrangement for the synthesis of valuable tertiary carboxylic acids. 相似文献
65.
Treatment of methyl 2-chloro-2-phenylhydrazonoacetate (1) with 1-phenylsulphonylpropyne (3) in the presence of triethylamine gives pyrazole derivatives due to cycloadditions of a nitrile imine intermediate with 3 and with its rearranged product phenylsulphonylallene (4). 相似文献
66.
New Chemotypes for the Inhibition of (p)ppGpp Synthesis in the Quest for New Antimicrobial Compounds
Crescenzo Coppa Luca Sorrentino Monica Civera Marco Minneci Francesca Vasile Sara Sattin 《Molecules (Basel, Switzerland)》2022,27(10)
Antimicrobial resistance (AMR) poses a serious threat to our society from both the medical and economic point of view, while the antibiotic discovery pipeline has been dwindling over the last decades. Targeting non-essential bacterial pathways, such as those leading to antibiotic persistence, a bacterial bet-hedging strategy, will lead to new molecular entities displaying low selective pressure, thereby reducing the insurgence of AMR. Here, we describe a way to target (p)ppGpp (guanosine tetra- or penta-phosphate) signaling, a non-essential pathway involved in the formation of persisters, with a structure-based approach. A superfamily of enzymes called RSH (RelA/SpoT Homolog) regulates the intracellular levels of this alarmone. We virtually screened several fragment libraries against the (p)ppGpp synthetase domain of our RSH chosen model RelSeq, selected three main chemotypes, and measured their interaction with RelSeq by thermal shift assay and STD-NMR. Most of the tested fragments are selective for the synthetase domain, allowing us to select the aminobenzoic acid scaffold as a hit for lead development. 相似文献
67.
68.
Abstract The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF3) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF3 as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ2), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ2 = 0°), and a maximum of about 15 kJ mol?1 when φ2 is 90°. In 1OCBF3 the barrier to rotation about the ring–O bond decreases substantially to being near zero. 相似文献
69.
70.
Paolo Muratore-Ginanneschi Carlos Mejía-Monasterio Luca Peliti 《Journal of statistical physics》2013,150(1):181-203
We derive the equations governing the protocols minimizing the heat released by a continuous-time Markov jump process on a one-dimensional countable state space during a transition between assigned initial and final probability distributions in a finite time horizon. In particular, we identify the hypotheses on the transition rates under which the optimal control strategy and the probability distribution of the Markov jump problem obey a system of differential equations of Hamilton-Jacobi-Bellman-type. As the state-space mesh tends to zero, these equations converge to those satisfied by the diffusion process minimizing the heat released in the Langevin formulation of the same problem. We also show that in full analogy with the continuum case, heat minimization is equivalent to entropy production minimization. Thus, our results may be interpreted as a refined version of the second law of thermodynamics. 相似文献