全文获取类型
收费全文 | 830篇 |
免费 | 45篇 |
国内免费 | 6篇 |
专业分类
化学 | 534篇 |
晶体学 | 1篇 |
力学 | 22篇 |
数学 | 180篇 |
物理学 | 144篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 33篇 |
2021年 | 46篇 |
2020年 | 37篇 |
2019年 | 39篇 |
2018年 | 35篇 |
2017年 | 25篇 |
2016年 | 46篇 |
2015年 | 33篇 |
2014年 | 32篇 |
2013年 | 50篇 |
2012年 | 72篇 |
2011年 | 63篇 |
2010年 | 25篇 |
2009年 | 41篇 |
2008年 | 40篇 |
2007年 | 54篇 |
2006年 | 30篇 |
2005年 | 25篇 |
2004年 | 29篇 |
2003年 | 13篇 |
2002年 | 11篇 |
2001年 | 10篇 |
2000年 | 14篇 |
1999年 | 6篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 1篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1985年 | 5篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1974年 | 2篇 |
1952年 | 1篇 |
1938年 | 1篇 |
排序方式: 共有881条查询结果,搜索用时 187 毫秒
831.
Joana Pinto Vera L. M. Silva Artur M. S. Silva Rosa M. Claramunt Dionisia Sanz M. Carmen Torralba M. Rosario Torres Felipe Reviriego Ibon Alkorta José Elguero 《Magnetic resonance in chemistry : MRC》2013,51(4):203-221
The structures of three azines derived from 2‐formylimidazole, 4(5)‐formylimidazole, and 4(5)‐formyl‐5(4)‐methylimidazole have been determined in solution and in the solid state. Density Functional Theory (DFT) Polarizable Continuum Model (PCM) calculations (geometries, energies, and chemical shifts), NMR [solution and cross polarization magic‐angle spinning (CPMAS)], and X‐ray crystallography [azine of 4(5)‐formylimidazole] have been used. The configuration around the central C = N bonds has been determined and some insights about prototropic tautomerism and conformation have been gained. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
832.
Antonio Alonso-Ayuso Felipe Carvallo Laureano F. Escudero Monique Guignard Jiaxing Pi Raghav Puranmalka Andrés Weintraub 《European Journal of Operational Research》2014
Deterministic mine planning models along a time horizon have proved to be very effective in supporting decisions on sequencing the extraction of material in copper mines. Some of these models have been developed for, and used successfully by CODELCO, the Chilean state copper company. In this paper, we wish to consider the uncertainty in a very volatile parameter of the problem, namely, the copper price along a given time horizon. We represent the uncertainty by a multistage scenario tree. The resulting stochastic model is then converted into a mixed 0–1 Deterministic Equivalent Model using a compact representation. We first introduce the stochastic model that maximizes the expected profit along the time horizon over all scenarios (i.e., as in a risk neutral environment). We then present several approaches for risk management in a risk averse environment. Specifically, we consider the maximization of the Value-at-Risk and several variants of the Conditional Value-at-Risk (one of them is new), the maximization of the expected profit minus the weighted probability of having an undesirable scenario in the solution provided by the model, and the maximization of the expected profit subject to stochastic dominance constraints recourse-integer for a set of profiles given by the pairs of target profits and bounds on either the probability of failure or the expected profit shortfall. We present an extensive computational experience on the actual problem, by comparing the risk neutral approach, the tested risk averse strategies and the performance of the traditional deterministic approach that uses the expected value of the uncertain parameters. The results clearly show the advantage of using the risk neutral strategy over the traditional deterministic approach, as well as the advantage of using any risk averse strategy over the risk neutral one. 相似文献
833.
Given a finite group \(G\) which possesses a non-abelian simple normal subgroup \(N\) having exactly four \(G\) -class sizes, we prove that \(N\) is isomorphic to PSL \((2, 2^a)\) with \(a\ge 2\) . Thus, we obtain an extension for normal subgroups of the classic N. Itô’s theorem which characterizes those finite simple groups with exactly four class sizes. 相似文献
834.
We present a systematic analysis of the integral operators of potential theory that arise when solving the Helmholtz or Maxwell equations in the exterior (or interior) of a sphere in the frequency domain. After obtaining expressions for the signatures of layer potentials in the spherical harmonic or vector spherical harmonic basis, we turn to a consideration of various integral equations that have been proposed in the literature for problems of acoustic and electromagnetic scattering. The selection of certain parameters in “combined field” and “Calderon-preconditioned” formulations is shown to have a significant impact on condition number, extending earlier work by Kress and others. 相似文献
835.
María Magdalena Centeno José Daniel Martínez Mary Lorena Araujo Felipe Brito Edgar Del Carpio Lino Hernández Vito Rocco Lubes 《Journal of solution chemistry》2014,43(6):1011-1018
Solution equilibria of the ternary systems Ni(II)–picolinic acid (Hpic) and the amino acids aspartic acid (H2asp), glutamic acid (H2glu), cysteine (H2cys) and histidine (Hhis), where the amino acids are denoted as H i L, have been studied pH-metrically. The formation constants of the resulting mixed ligand complexes have been determined at 25 °C using a ionic strength 1.0 mol·dm?3 NaCl. In the Ni(II)–Hpic–H2asp and Ni(II)–Hpic–H2glu systems, the complexes [Ni(pic)H2L]+, Ni(pic)HL, [Ni(pic)L]? and [Ni(pic)L(OH)]2? were detected. In the Ni(II)–Hpic–H2cys system the complexes [Ni(pic)H2L]+ and [Ni(pic)L]? are present. Additionally, in the Ni(II)–Hpic–Hhis system the species [Ni(Hpic)HL]2+, Ni(pic)L and [Ni(pic)L(OH)]? were identified. The species distribution diagrams as functions of pH are briefly discussed. 相似文献
836.
Detection and Quantification of Sulfonamide Antibiotic Residues in Milk Using Scanning Electrochemical Microscopy 下载免费PDF全文
Felipe Conzuelo Lutz Stratmann Stefanie Grützke José M. Pingarrón Wolfgang Schuhmann 《Electroanalysis》2014,26(3):481-487
The use of scanning electrochemical microscopy (SECM) for the qualitative and quantitative determination of sulfapyridine (SPY) in milk is described. A direct competitive immunoassay was performed involving an antibiotic horseradish peroxidase (HRP)‐labeled analog and using selective capture antibodies immobilized on the surface of Protein G‐modified glassy carbon plates. SECM detection was accomplished by means of the sample generator/tip collector (GC) mode involving the reduction of benzoquinone (BQ) generated upon the HRP‐catalyzed oxidation of hydroquinone (HQ) at the modified substrate surface in the presence of H2O2. The detection limit for SPY in milk samples was as low as 0.13 ng mL?1. 相似文献
837.
Valentina Pirro Paolo Oliveri Christina Ramires Ferreira Andrés Felipe González-Serrano Zoltan Machaty Robert Graham Cooks 《Analytica chimica acta》2014
The development of sensitive measurements to analyze individual cells is of relevance to elucidate specialized roles or metabolic functions of each cell under physiological and pathological conditions. Lipids play multiple and critical roles in cellular functions and the application of analytical methods in the lipidomics area is of increasing interest. In this work, in vitro maturation of porcine oocytes was studied. Two independent sources of chemical information (represented by mass spectra in the positive and negative ion modes) from single oocytes (immature oocytes, 24-h and 44-h in vitro matured oocytes) were acquired by using desorption electrospray ionization-mass spectrometry (DESI-MS). Low and mid-level data fusion strategies are presented with the aim of better exploring the large amount of chemical information contained in the two mass spectrometric lipid profiles. Data were explored by principal component analysis (PCA) within the two multi-block approaches to include information on free fatty acids, phospholipids, cholesterol-related molecules, di- and triacylglycerols. After data fusion, clearer differences among immature and in vitro matured porcine oocytes were observed, which provide novel information regarding lipid metabolism throughout oocyte maturation. In particular, changes in TAG composition, as well as increase in fatty acid metabolism and membrane complexity were evidenced during the in vitro maturation process. This information can assist the improvement of in vitro embryo production for porcine species. 相似文献
838.
Naka H Morey JV Haywood J Eisler DJ McPartlin M García F Kudo H Kondo Y Uchiyama M Wheatley AE 《Journal of the American Chemical Society》2008,130(48):16193-16200
The mechanisms by which directed ortho metalation (DoM) and postmetalation processes occur when aromatic compounds are treated with mixed alkylamido aluminate i-Bu3Al(TMP)Li (TMP-aluminate 1; TMP = 2,2,6,6-tetramethylpiperidide) have been investigated by computation and X-ray diffraction. Sequential reaction of ArC(=O)N(i-Pr)2 (Ar = phenyl, 1-naphthyl) with t-BuLi and i-Bu3Al in tetrahydrofuran affords [2-(i-Bu3Al)C(m)H(n)C(=O)N(i-Pr)2]Li x 3 THF (m = 6, n = 4, 7; m = 10, n = 6, 8). These data advance the structural evidence for ortho-aluminated functionalized aromatics and represent model intermediates in DoM chemistry. Both 7 and 8 are found to resist reaction with HTMP, suggesting that ortho-aluminated aromatics are incapable of exhibiting stepwise deprotonative reactivity of the type recently shown to pertain to the related field of ortho zincation chemistry. Density functional theory calculations corroborate this view and reveal the existence of substantial kinetic barriers both to one-step alkyl exchange and to amido-alkyl exchange after an initial amido deprotonation reaction by aluminate bases. Rationalization of this dichotomy comes from an evaluation of the inherent Lewis acidities of the Al and Zn centers. As a representative synthetic application of this high kinetic reactivity of the TMP-aluminate, the highly regioselective deprotonative functionalization of unsymmetrical ketones both under mild conditions and at elevated temperatures is also presented. 相似文献
839.
Dr. Tobias Brückner Benedikt Ritschel Prof. Dr. J. Oscar C. Jiménez-Halla Dr. Felipe Fantuzzi Dario Duwe Christian Markl Dr. Rian D. Dewhurst Maximilian Dietz Prof. Dr. Holger Braunschweig 《Angewandte Chemie (International ed. in English)》2023,62(5):e202213284
Carbene-stabilized diborynes of the form LBBL (L=N-heterocyclic carbene (NHC) or cyclic alkyl(amino)carbene (CAAC)) induce rapid, high yielding, intermolecular ortho-C−H borylation at N-heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC-stabilized diboryne is combined with pyridine, while a CAAC-stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H-shift resulting in a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine by heating. Use of the extended N-heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron-carbon exchange process. 相似文献
840.
Dr. Kyung Seob Song Patrick W. Fritz Dr. Daniel F. Abbott Lok Nga Poon Cristiano M. Caridade Dr. Felipe Gándara Prof. Dr. Victor Mougel Prof. Dr. Ali Coskun 《Angewandte Chemie (International ed. in English)》2023,62(38):e202309775
Phthalocyanines (PCs) are intriguing building blocks owing to their stability, physicochemical and catalytic properties. Although PC-based polymers have been reported before, many suffer from relatively low stability, crystallinity, and low surface areas. Utilizing a mixed-metal salt ionothermal approach, we report the synthesis of a series of metallophthalocyanine-based covalent organic frameworks (COFs) starting from 1,2,4,5-tetracyanobenzene and 2,3,6,7-tetracyanoanthracene to form the corresponding COFs named M-pPPCs and M-anPPCs, respectively. The obtained COFs followed the Irving–Williams series in their metal contents, surface areas, and pore volume and featured excellent CO2 uptake capacities up to 7.6 mmol g−1 at 273 K, 1.1 bar. We also investigated the growth of the Co-pPPC and Co-anPPC on a highly conductive carbon nanofiber and demonstrated their high catalytic activity in the electrochemical CO2 reduction, which showed Faradaic efficiencies towards CO up to 74 % at −0.64 V vs. RHE. 相似文献