全文获取类型
收费全文 | 1141篇 |
免费 | 175篇 |
国内免费 | 122篇 |
专业分类
化学 | 902篇 |
晶体学 | 23篇 |
力学 | 124篇 |
综合类 | 4篇 |
数学 | 23篇 |
物理学 | 362篇 |
出版年
2024年 | 3篇 |
2023年 | 14篇 |
2022年 | 35篇 |
2021年 | 41篇 |
2020年 | 38篇 |
2019年 | 60篇 |
2018年 | 37篇 |
2017年 | 44篇 |
2016年 | 65篇 |
2015年 | 47篇 |
2014年 | 57篇 |
2013年 | 138篇 |
2012年 | 79篇 |
2011年 | 43篇 |
2010年 | 58篇 |
2009年 | 75篇 |
2008年 | 54篇 |
2007年 | 72篇 |
2006年 | 63篇 |
2005年 | 45篇 |
2004年 | 55篇 |
2003年 | 44篇 |
2002年 | 41篇 |
2001年 | 27篇 |
2000年 | 25篇 |
1999年 | 20篇 |
1998年 | 25篇 |
1997年 | 16篇 |
1996年 | 19篇 |
1995年 | 14篇 |
1994年 | 15篇 |
1993年 | 6篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1438条查询结果,搜索用时 15 毫秒
991.
Ahmed Ould Aliyenne 《Tetrahedron letters》2008,49(9):1473-1475
Chiral 3-N-mesitylenesulfonyl-1,3-oxazolidin-2-ones 4a-e derived from (l)- and (d)-amino acids 1a-e undergo lateral lithiation with lithium diisopropylamide and TMEDA in anhydrous THF to provide new optically-active 1,2-benzothiazin-3-one 1,1-dioxide derivatives 5a-e with yields ranging from 63% to 79%. 相似文献
992.
Yuexing Zhang Xue Cai Dongdong Qi Ping Yao Yongzhong Bian Dr. Jianzhuang Jiang Prof. 《Chemphyschem》2008,9(5):781-792
The effects of alkyloxy substituents attached to one phthalocyanine ligand of three heteroleptic bis(phthalocyaninato) yttrium complexes Y(Pc)[Pc(α‐OCH3)4] ( 1 ), Y(Pc)[Pc(α‐OCH3)8] ( 2 ), and Y(Pc)[Pc(β‐OCH3)8] ( 3 ), as well as their reduction products {Y(Pc)[Pc(α‐OCH3)4]}? ( 4 ), {Y(Pc)[Pc(α‐OCH3)8]}? ( 5 ), and {Y(Pc)[Pc(β‐OCH3)8]}? ( 6 ) [H2Pc(α‐OCH3)4=1,8,15,22‐tetrakis(methyloxy)phthalocyanine; H2Pc(α‐OCH3)8=1,4,8,11,15,18,22,25‐octakis(methyloxy)phthalocyanine; H2Pc(β‐OCH3)8=2,3,9,10,16,17,23,24‐octakis(methyloxy)phthalocyanine] are studied by DFT calculations. Good consistency is found between the calculated results and experimental data for the electronic absorption, IR, and Raman spectra of 1 and 3 . Introduction of electron‐donating methyloxy groups on one phthalocyanine ring of the heteroleptic double‐deckers induces structural deformation in both phthalocyanine ligands, electron transfer between the two phthalocyanine rings, changes in orbital energy and composition, shift of electronic absorption bands, and different vibrational modes of the unsubstituted and substituted phthalocyanine ligands in the IR and Raman spectra in comparison with the unsubstituted homoleptic counterpart Y(Pc)2. The calculations reveal that incorporation of methyloxy substituents at the nonperipheral positions has greater influence on the structure and spectroscopic properties of bis(phthalocyaninato) yttrium double‐deckers than at the peripheral positions, which increases with increasing number of substituents. Nevertheless, the substituent effect of alkyloxy substituents at one phthalocyanine ligand of the double‐decker on the unsubstituted phthalocyanine ring and on the whole molecule and the importance of the position and number of alkyloxy substituents are discussed. In addition, the effect of reducing 1 – 3 to 4 – 6 on the structure and spectroscopic properties of the bis(phthalocyaninato) yttrium compounds is also discussed. This systemic DFT study is not only useful for understanding the structure and spectroscopic properties of bis(phthalocyaninato) rare earth metal complexes but also helpful in designing and preparing double‐deckers with tunable structure and properties. 相似文献
993.
994.
Blas JR López-Bes JM Márquez M Sessler JL Luque FJ Orozco M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(4):1108-1116
Molecular dynamics simulations show that calix[4]pyrrole (CP) and octafluorocalix[4]pyrrole (8F-CP) are extremely flexible molecules. CP mainly adopts the 1,3-alternate conformation in all the solvents, although the percentage of alternative conformations increases in polar solvents, especially those with good hydrogen-bonding acceptor properties. However, in the case of 8F-CP, the cone conformation is the most populated in some solvents. Transitions between conformers are common and fast, and both CP and 8F-CP can adopt the cone conformation needed for optimum interaction with anions more easily than would be predicted on the basis of previous gas-phase calculations. Furthermore, the present studies show that when a fluoride anion is specifically placed initially in close proximity to CP and 8F-CP in their respective 1,3-alternate conformations, an extremely fast change to the cone conformation is observed in both cases. The results suggest that preorganization does not represent a major impediment to anion-binding for either CP or 8F-CP, and that ion-induced conformational changes can follow different mechanisms depending on the solvent and the chemical substituents present on the calix[4]pyrrole beta-pyrrolic positions. 相似文献
995.
Havva?DemirelliEmail author Fitnat?K?seoglu Nusret?Kavak 《Journal of solution chemistry》2004,33(11):1467-1479
The formation and protonation constants of 17 Schiff bases-derived 2-hydroxyaniline with some substituted benzaldehydes, and the stability constants of Cu(II) complexes of these Schiff bases, have been determined potentiometrically in 20, 40, and 60% dioxane–water media. The data from the potentiometric titrations were evaluated with the BEST computer program. For all Schiff bases studied, it was observed that the log KOH values related to the protonation equilibria of the phenolic oxygen are increased, and the log KNH values related to the protonation equilibria of the azomethine nitrogen are decreased, as the dioxane content is increased. The variation of these constants is discussed on the basis of specific solute–solvent interactions and structural changes of Schiff bases from water to the dioxane–water media. Also, titrimetric-pH investigation of substituted benzilidene-2-hydroxyaniline systems has revealed the formation of stable mono-Schiff base complexes with the metal ion Cu(II). 相似文献
996.
Tuning anion‐π interaction via halogen substituent effects in cyanuric acids and its derivatives 下载免费PDF全文
Shuangli Du Bingqiang Wang Jian Zhang Caiyun Zhang 《International journal of quantum chemistry》2015,115(17):1147-1152
Several anion‐π complexes of isocyanuric acid, thioderivatives and their halogen substituted derivatives with chloride anion have been studied. The geometric and energetic features, charges transfer from chloride anion to the aromatic rings and “atoms‐in‐molecules” analysis are performed and discussed for these complexes. The results show that the strength of the anion‐π interaction between cyanuric derivatives and chloride anion can be tuned by halogen‐substituting. The localized molecular orbital energy decomposition analyses shows that, in the total interaction, exchange and electrostatic energies are the dominant stabilizing forces, and the polarization energies also make a favorable contribution. Finally, solvent effect significantly weakens the anion‐π interaction between the isocyanuric derivatives and chloride anion, especially in polar solvents. © 2015 Wiley Periodicals, Inc. 相似文献
997.
Bo Lin LIN Yao FU Lei LIU Qing Xiang GUO Department of Chemistry University of Science Technology of China Hefei 《中国化学快报》2003,14(10)
C4'-H bond dissociation enthalpies of nucleosides were predicted using theoretical methods to a precision of 1-2 Kcal/mol. It was found that the stability of the C4' nucleoside radical is slightly dependent on the base. The orders of stability are dA < dG < dT < dC for deoxynucleosides and U < G < A = C for nucleosides. 相似文献
998.
Frances E. Bugden Dr. Guy J. Clarkson Dr. Mark D. Greenhalgh 《European journal of organic chemistry》2023,26(5):e202201459
The regioselective lithiation of 1,2,3-triazoles provides an opportunity to introduce additional functionality, however this simple functionalisation strategy using triazoles bearing electron-withdrawing N-substituents has not been investigated until now. Herein, we demonstrate that the lithiated triazole intermediates can readily decompose, even at −78 °C. In addition, lithiation-deuteration studies reveal lithiation can take place competitively on both the triazole and the electron-withdrawn aryl ring. Careful control of reaction conditions is therefore required to i) minimise decomposition pathways; and ii) facilitate regioselective functionalisation of the triazole. 相似文献
999.
Fumio Hamazu Sumio Akashi Tatsuya Koizumi Toshikazu Takata Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1991,29(13):1845-1851
Benzyl o-, m-, and p-substituted phenyl methyl sulfonium salts ( 2b – 2g ) were synthesized and their activities as cationic initiators were evaluated in the bulk polymerization of phenyl glycidyl ether (PGE). Especially, their activities were estimated with respect to the effect of substituents on the aryl groups. In the polymerizations of PGE with a series of benzyl p-substituted phenyl methyl sulfonium salts, the order of their activities was found to be 2c (CH3OCOO) > 2b (CH3COO) > 2d (CH3O) ~ 2a (HO). In particular, 2c was the most active initiator of all, capable of initiating the polymerization of PGE even at room temperature. In the polymerizations with 2a, 2e (m-Cl), 2f (o-CH3), and 2g (m-CH3), the activity of 2e was the highest of all while those of 2a, 2f , and 2g were almost the same. These results strongly suggested that the electron-withdrawing group placed on the aryl group undoubtedly enhanced the activity of the sulfonium salts as the cationic initiators. 相似文献
1000.
The title compounds, the P(3)‐axially and P(3)‐equatorially substituted cis‐ and trans‐configured 9‐benzyl‐3‐fluoro‐2,4‐dioxa‐9‐aza‐3‐phosphadecalin 3‐oxides (=9‐benzyl‐3‐fluoro‐2,4‐dioxa‐9‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides=7‐benzyl‐2‐fluorohexahydro‐4H‐1,3,2‐dioxaphosphorino[4,5‐c]pyridine 2‐oxides) were prepared (ee >99%) and fully characterized (Schemes 2 and 4). The absolute configurations were deduced from that of their precursors, the enantiomerically pure ethyl 1‐benzyl‐3‐hydroxypiperidine‐4‐carboxylates and 1‐benzyl‐3‐hydroxypiperidine‐4‐methanols which were unambiguously assigned. Being configuratively fixed and conformationally constrained phosphorus analogues of acetylcholine, the title compounds represent acetylcholine mimetics and are suitable probes for the investigation of molecular interactions with acetylcholinesterase. As determined by kinetic methods, all of the compounds are moderate irreversible inhibitors of the enzyme. 相似文献