全文获取类型
收费全文 | 2070篇 |
免费 | 103篇 |
国内免费 | 14篇 |
专业分类
化学 | 1741篇 |
晶体学 | 10篇 |
力学 | 27篇 |
数学 | 189篇 |
物理学 | 220篇 |
出版年
2024年 | 2篇 |
2023年 | 36篇 |
2022年 | 108篇 |
2021年 | 97篇 |
2020年 | 75篇 |
2019年 | 78篇 |
2018年 | 52篇 |
2017年 | 43篇 |
2016年 | 116篇 |
2015年 | 102篇 |
2014年 | 78篇 |
2013年 | 132篇 |
2012年 | 168篇 |
2011年 | 190篇 |
2010年 | 113篇 |
2009年 | 84篇 |
2008年 | 108篇 |
2007年 | 107篇 |
2006年 | 98篇 |
2005年 | 85篇 |
2004年 | 64篇 |
2003年 | 57篇 |
2002年 | 44篇 |
2001年 | 11篇 |
2000年 | 13篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1925年 | 1篇 |
1924年 | 1篇 |
排序方式: 共有2187条查询结果,搜索用时 15 毫秒
21.
Ab initio and DFT calculations reveal that acyl radicals add to imines and electron-rich olefins through simultaneous SOMO --> pi*, pi --> SOMO, and HOMO --> pi*C=O interactions between the radical and the radicalophile. At the CCSD(T)/aug-cc-pVDZ//QCISD/cc-pVDZ level, energy barriers of 15.6 and 17.9 kJ mol(-1) are calculated for the attack of the acetyl radical at the carbon and nitrogen ends of methanimine, respectively. These barriers are 17.1 and 20.4 kJ mol(-1) at BHandHLYP/cc-pVDZ. In comparison, barriers of 34.0 and 23.4 kJ mol(-1) are calculated at BHandHLYP/cc-pVDZ for reaction of the acetyl radical at the 1- and 2-positions in aminoethylene, repectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO --> pi*imine, pi imine--> SOMO, and LPN --> pi*C=O interactions are worth 90, 278, and 138 kJ mol-1, respectively, in the transition state (2) for reaction of acetyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving aminoethylene. These multiorbital interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions. NBO analyses for the remaining systems in this study support the hypothesis that the acetyl radical is ambiphilic in nature. 相似文献
22.
Jean-Ulrich Mullot Sara Karolak Anne Fontova Bruno Huart Yves Levi 《Analytical and bioanalytical chemistry》2009,394(8):2203-2212
Pollution of the environment by pharmaceuticals is a subject of growing scientific and societal concern. However, few quantitative
data have been reported concerning hospital wastewater contamination. Among the different molecules used at hospital, antineoplastic
drugs appear to be of special interest, and 5-fluorouracil (5-FU) can be considered as a key compound of this therapeutic
class. To monitor this pharmaceutical in hospital wastewater, a highly specific and selective method was developed using gas
chromatography tandem mass spectrometry after solid-phase extraction. This sensitive method (limit of quantification = 40 ng L−1) was then applied to assess sewage contamination of a middle-size hospital with oncology service located in Paris, France.
Native 5-FU was detectable in 12 of the 14 analysed samples. In positive samples, concentration range was measured from 0.09
to 4.0 μg L−1. Finally, a predicting model for the hospital wastewater concentrations is presented, and results of this model are discussed. 相似文献
23.
Znabet A Blanken S Janssen E de Kanter FJ Helliwell M Turner NJ Ruijter E Orru RV 《Organic & biomolecular chemistry》2012,10(5):941-944
An efficient combination of MAO-N-catalyzed desymmetrization of cyclic meso-amines with Ugi-Smiles multicomponent chemistry produced optically pure N-aryl proline amides. This method represents the first report of a fully asymmetric Ugi-Smiles process. 相似文献
24.
Sara Mattsson 《Tetrahedron letters》2007,48(14):2497-2499
When treated with amine bases such as triethylamine and various lithium salts in wet solvents, esters are efficiently hydrolyzed to the corresponding acids in good yields. Esters incorporating an α- or β-heteroatom with respect to the ester carbonyl group are hydrolyzed rapidly even at room temperature. To further demonstrate the usefulness of this method, one example is provided where hydrolysis of acetylated camphorsultam is mediated by LiBr. 相似文献
25.
Electrostatic interactions in proteins can be probed experimentally through determination of residue-specific acidity constants. We describe here triple-resonance NMR techniques for direct determination of lysine and arginine side-chain protonation states in proteins. The experiments are based on detection of nonexchangeable protons over the full range of pH and temperature and therefore are well suited for pKa determination of individual amino acid side chains. The experiments follow the side-chain 15Nzeta (lysine) and 15Nepsilon or 13Czeta (arginine) chemical shift, which changes due to sizable changes in the heteronuclear electron distribution upon (de)protonation. Since heteronuclear chemical shifts are overwhelmed by the charge state of the amino acid side chain itself, these methods supersede 1H-based NMR in terms of accuracy, sensitivity, and selectivity. Moreover, the 15Nzeta and 15Nepsilon nuclei may be used to probe changes in the local electrostatic environment. Applications to three proteins are described: apo calmodulin, calbindin D9k, and FKBP12. For apo calmodulin, residue-specific pKa values of lysine side chains were determined to fall between 10.7 and 11.2 as a result of the high net negative charge on the protein surface. Ideal two-state titration behavior observed for all lysines indicates the absence of significant direct charge interactions between the basic residues. These results are compared with earlier studies based on chemical modification. 相似文献
26.
Silvia Anthoine Dietrich Renate Lindauer Claire Stierlin Jürg Gertsch Dr. Ruth Matesanz Dr. Sara Notararigo José Fernando Díaz Dr. Karl‐Heinz Altmann Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(39):10144-10157
A series of epothilone B and D analogues bearing isomeric quinoline or functionalized benzimidazole side chains has been prepared by chemical synthesis in a highly convergent manner. All analogues have been found to interact with the tubulin/microtubule system and to inhibit human cancer cell proliferation in vitro, albeit with different potencies (IC50 values between 1 and 150 nM ). The affinity of quinoline‐based epothilone B and D analogues for stabilized microtubules clearly depends on the position of the N‐atom in the quinoline system, while the induction of tubulin polymerization in vitro appears to be less sensitive to N‐positioning. The potent inhibition of human cancer cell growth by epothilone analogues bearing functionalized benzimidazole side chains suggests that these systems might be conjugated with tumor‐targeting moieties to form tumor‐targeted prodrugs. 相似文献
27.
Tayebeh Hosseinnejad Saeed Dehghanpour Sara Basiri-nasab 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(11):2004-2011
The main goal of this research is to investigate the structural and thermochemical aspects of complexation between La3+ with tetrapropyl malonamide (TPMA) and tetrapropyl diglycolamide (TPDGA) ligands via density functional theory (DFT) methods. In this respect, the structural parameters of [La-TPMA]3+ and [La-TPDGA]3+ complexes have been calculated and compared with the available X-ray crystallographic data. These comparisons revealed that both calculated structural values using B3LYP and M06 are in a reliable agreement with X-ray crystal structure with a near accuracy. In the next step, the more efficiency of diglycolamides in comparison with malonamides in the extraction of La3+ have been analyzed by calculating thermochemical properties of the complexation. It should be stated that this issue has been observed in many experimental elucidations. In the next step, the inclusion of solvent effects on thermodynamical properties of complexation has been evaluated via polarized continuum model (PCM) calculations. In this context, enthalpy and Gibbs free energy changes have been determined in the presence of three solvents, chloroform, toluene and n-hexane. Our obtained results demonstrate that using n-hexane as solvent is more favorable thermodynamically than chloroform and toluene that confirms the previously observed experiments. Finally, the bond orders of some selected key bonds in TPMA and TPDGA ligands and their corresponded La3+ complexes have been evaluated comparatively to analyze the electronic features of coordination in [La-TPMA]3+ and [La-TPDGA]3+ complexes. 相似文献
28.
Anhar Abdel-Aziem Huda Refaat Mahmoud Rashdan Entesar Mohamed Ahmed Sara N. Shabaan 《Green Chemistry Letters and Reviews》2019,12(1):9-18
In the present study, a rapid, less expensive, clean and environmental friendly route to synthesis new pyrazoles, pyrazolopyridazines and condensed pyrimidines was developed via grinding of 2-(3-(dimethylamino)acryloyl)-3H-benzo[f]chromen-3-one (1) with different reagents. All the new compounds were characterized and established using elemental analysis and spectral data. Eight compounds were selected for in vitro antiproliferative against different human cancer cell lines entitled melanoma, cancers of the lung, leukemia, breast, brain, colon, prostate, ovary and kidney by the USA NCI. 相似文献
29.
Sara Sabater Jose A. Mata Eduardo Peris 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(20):6380-6385
A triazolyl‐di‐ylidene ligand has been used for the preparation of a homodimetallic complex of gold, and a heterodimetallic compound of gold and iridium. Both complexes have been fully characterized and their molecular structures have been determined by means of X‐ray diffraction. The catalytic properties of these two complexes have been evaluated in the reduction of nitroarenes by transfer hydrogenation using primary alcohols. The two complexes afford different reaction products; whereas the AuI–AuI catalyst yields a hydroxylamine, the IrIII–AuI complex facilitates the formation of an imine. 相似文献
30.
Ana Sánchez‐Iglesias Nathalie Claes Dr. Diego M. Solís Dr. Jose M. Taboada Prof. Sara Bals Prof. Luis M. Liz‐Marzán Dr. Marek Grzelczak 《Angewandte Chemie (International ed. in English)》2018,57(12):3183-3186
A limiting factor of solvent‐induced nanoparticle self‐assembly is the need for constant sample dilution in assembly/disassembly cycles. Changes in the nanoparticle concentration alter the kinetics of the subsequent assembly process, limiting optical signal recovery. Herein, we show that upon confining hydrophobic nanoparticles in permeable silica nanocapsules, the number of nanoparticles participating in cyclic aggregation remains constant despite bulk changes in solution, leading to highly reproducible plasmon band shifts at different solvent compositions. 相似文献