全文获取类型
收费全文 | 714篇 |
免费 | 25篇 |
国内免费 | 1篇 |
专业分类
化学 | 587篇 |
晶体学 | 14篇 |
力学 | 9篇 |
数学 | 67篇 |
物理学 | 63篇 |
出版年
2023年 | 5篇 |
2022年 | 16篇 |
2021年 | 19篇 |
2020年 | 12篇 |
2019年 | 8篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 16篇 |
2015年 | 26篇 |
2014年 | 18篇 |
2013年 | 28篇 |
2012年 | 55篇 |
2011年 | 81篇 |
2010年 | 27篇 |
2009年 | 27篇 |
2008年 | 46篇 |
2007年 | 53篇 |
2006年 | 48篇 |
2005年 | 43篇 |
2004年 | 43篇 |
2003年 | 28篇 |
2002年 | 29篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1988年 | 4篇 |
1986年 | 1篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1938年 | 1篇 |
1934年 | 1篇 |
1905年 | 1篇 |
排序方式: 共有740条查询结果,搜索用时 15 毫秒
681.
Shaw BF Schneider GF Arthanari H Narovlyansky M Moustakas D Durazo A Wagner G Whitesides GM 《Journal of the American Chemical Society》2011,133(44):17681-17695
A previous study, using capillary electrophoresis (CE) [J. Am. Chem. Soc. 2008, 130, 17384-17393], reported that six discrete complexes of ubiquitin (UBI) and sodium dodecyl sulfate (SDS) form at different concentrations of SDS along the pathway to unfolding of UBI in solutions of SDS. One complex (which formed between 0.8 and 1.8 mM SDS) consisted of native UBI associated with approximately 11 molecules of SDS. The current study used CE and (15)N/(13)C-(1)H heteronuclear single quantum coherence (HSQC) NMR spectroscopy to identify residues in folded UBI that associate specifically with SDS at 0.8-1.8 mM SDS, and to correlate these associations with established biophysical and structural properties of this well-characterized protein. The ability of the surface charge and hydrophobicity of folded UBI to affect the association with SDS (at concentrations below the CMC) was studied, using CE, by converting lys-ε-NH(3)(+) to lys-ε-NHCOCH(3) groups. According to CE, the acetylation of lysine residues inhibited the binding of 11 SDS ([SDS] < 2 mM) and decreased the number of complexes of composition UBI-(NHAc)(8)·SDS(n) that formed on the pathway of unfolding of UBI-(NHAc)(8) in SDS. A comparison of (15)N-(1)H HSQC spectra at 0 mM and 1 mM SDS with calculated electrostatic surface potentials of folded UBI (e.g., solutions to the nonlinear Poisson-Boltzmann (PB) equation) suggested, however, that SDS binds preferentially to native UBI at hydrophobic residues that are formally neutral (i.e., Leu and Ile), but that have positive electrostatic surface potential (as predicted from solutions to nonlinear PB equations); SDS did not uniformly interact with residues that have formal positive charge (e.g., Lys or Arg). Cationic functional groups, therefore, promote the binding of SDS to folded UBI because these groups exert long-range effects on the positive electrostatic surface potential (which extend beyond their own van der Waals radii, as predicted from PB theory), and not because cationic groups are necessarily the site of ionic interactions with sulfate groups. Moreover, SDS associated with residues in native UBI without regard to their location in α-helix or β-sheet structure (although residues in hydrogen-bonded loops did not bind SDS). No correlation was observed between the association of an amino acid with SDS and the solvent accessibility of the residue or its rate of amide H/D exchange. This study establishes a few (of perhaps several) factors that control the simultaneous molecular recognition of multiple anionic amphiphiles by a folded cytosolic protein. 相似文献
682.
Maximilian N. Kopylovich Kamran T. Mahmudov M. Fátima C. Guedes da Silva Luísa M.D.R.S. Martins Maxim L. Kuznetsov Telma F.S. Silva João J.R. Fraústo da Silva Armando J.L. Pombeiro 《Journal of Physical Organic Chemistry》2011,24(9):764-773
Trends between the Hammett's σp and related normal , inductive σI, resonance σR, negative and positive polar conjugation and Taft's σp° substituent constants and the distance, δN? H NMR chemical shift, oxidation potential (Ep/2°x, measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (pK, ΔG0, ΔH0 and ΔS0) of the dissociation process of unsubstituted 3‐(phenylhydrazo)pentane‐2,4‐dione (HL1) and its para‐substituted chloro (HL2), carboxy (HL3), fluoro (HL4) and nitro (HL5) derivatives were recognized. The best fits were found for σp and/or in the cases of , δN? H and Ep/2°x, showing the importance of resonance and conjugation effects in such properties, whereas for the above thermodynamic properties the inductive effects (σI) are dominant. HL2 exists in the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H‐bond with the distance of 2.588(3) Å. It was also established that the dissociation process of HL1–5 is non‐spontaneous, endothermic and entropically unfavourable, and that the increase in the inductive effect (σI) of para‐substitutents (? H < ? Cl < ? COOH < ? F < ? NO2) leads to the corresponding growth of the distance and decrease of the pK and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL1–5 acid dissociation process. The electrochemical behaviour of HL1–5 was interpreted using theoretical calculations at the DFT/HF hybrid level, namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron‐transfers. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
683.
Riveira MJ Gayathri C Navarro-Vázquez A Tsarevsky NV Gil RR Mischne MP 《Organic & biomolecular chemistry》2011,9(9):3170-3175
A new approach to the synthesis of cyclopenta[b]benzofuran derivatives via reaction of 1,3-dicarbonyl compounds with α,β,γ,δ-unsaturated aldehydes is described. The constitution and configuration of the new products have been firmly established by means of residual dipolar couplings (RDCs) and ab initio (13)C NMR chemical shift predictions. 相似文献
684.
Kopylovich MN Mac Leod TC Mahmudov KT Guedes da Silva MF Pombeiro AJ 《Dalton transactions (Cambridge, England : 2003)》2011,40(19):5352-5361
The zinc(II) complexes with ortho-hydroxy substituted arylhydrazo-β-diketonates [Zn(2)(CH(3)OH)(2)(μ-L(1))(2)] (5), [Zn{(CH(3))(2)SO}(H(2)O)(L(2))] (6), [Zn(2)(H(2)O)(2)(μ-L(3))(2)] (7) and [Zn(H(2)O)(2)(L(4))]·H(2)O (8) were synthesized by reaction of a zinc(II) salt with the appropriate hydrazo-β-diketone, HO-2-C(6)H(4)-NHN=C{C(=O)CH(3)}(2) (H(2)L(1), 1), HO-2-O(2)N-4-C(6)H(3)-NHN=C{C(=O)CH(3)}(2) (H(2)L(2), 2), HO-2-C(6)H(4)-NHN=CC(=O)CH(2)C(CH(3))(2)CH(2)C(=O) (H(2)L(3), 3) or HO-2-O(2)N-4-C(6)H(3)-NHN=[CC(=O)CH(2)C(CH(3))(2)CH(2)C(=O) (H(2)L(4), 4). They were fully characterized, namely by X-ray diffraction analysis that disclosed the formation of extensive H-bonds leading to 1D chains (5 and 6), 2D layers (7) or 3D networks (8). The thermodynamic parameters of the Zn(II) reaction with H(2)L(2) in solution, as well as of the thermal decomposition of 1-8 were determined. Complexes 5-8 act as diastereoselective catalysts for the nitroaldol (Henry) reaction. The threo/erythro diastereoselectivity of the β-nitroalkanol products ranges from 8:1 to 1:10 with typical yields of 80-99%, depending on the catalyst and substrate used. 相似文献
685.
Kopylovich MN Mahmudov KT da Silva MF Kirillov AM Pombeiro AJ 《Dalton transactions (Cambridge, England : 2003)》2011,40(46):12472-12478
A novel para to meta shift of a nitro group at the phenyl ring of 3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione (H(2)L(1), 1), with formation of the new 3-(2-hydroxy-3,5-dinitrophenylhydrazo)pentane-2,4-dione (H(2)L(2), 2), occurs upon nitration of 1 with an equimolar amount of NaNO(2), under basic conditions. 2 acts as a polydentate ligand for the synthesis of the polymeric potassium [K(μ(5)-HL(2))](n) (3) and monomeric nickel(II) [Ni(H(2)O)(3)(L(2))]·H(2)O (4) compounds. They have been fully characterized, including single crystal X-ray analysis, and the complexes feature metal-organic (in 3) or supramolecular (in 4) 3D networks. The topological analysis of 3 reveals a uninodal 5-connected underlying net with the point symbol of (4(6).6(4)) and a very rare 5/4/t5 topology, which had not yet been observed in coordination polymers. 相似文献
686.
Deiana L Dziedzic P Zhao GL Vesely J Ibrahem I Rios R Sun J Córdova A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(28):7904-7917
The development, scope, and application of the highly enantioselective organocatalytic aziridination of α,β-unsaturated aldehydes is presented. The aminocatalytic azirdination of α,β-unsaturated aldehydes enables the asymmetric formation of β-formyl aziridines with up to >19:1 d.r. and 99% ee. The aminocatalytic aziridination of α-monosubstituted enals gives access to terminal α-substituted-α-formyl aziridines in high yields and up to 99% ee. In the case of the organocatalytic aziridination of disubstituted α,β-unsaturated aldehydes, the transformations were highly diastereo- and enantioselective and give nearly enantiomerically pure β-formyl-functionalized aziridine products (99% ee). A highly enantioselective one-pot cascade sequence based on the combination of asymmetric amine and N-heterocyclic carbene catalysis (AHCC) is also disclosed. This one-pot three-component co-catalytic transformation between α,β-unsaturated aldehydes, hydroxylamine derivatives, and alcohols gives the corresponding N-tert-butoxycarbonyl and N-carbobenzyloxy-protected β-amino acid esters with ee values ranging from 92-99%. The mechanisms and stereochemistry of all these catalytic transformations are also discussed. 相似文献
687.
688.
Afewerki S Breistein P Pirttilä K Deiana L Dziedzic P Ibrahem I Córdova A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(32):8784-8788
Branching out! The first co-catalytic enantioselective (up to 98:2 e.r.) β-alkylation of α,β-unsaturated aldehydes by combination of simple chiral amine and copper catalysts provides β-branched aldehydes in a one-pot protocol. The methodology was applied to the short total syntheses of bisabolane sesquiterpenes (S)-(+)-curcumene, (E)-(S)-(+)-3-dehydrocurcumene and (S)-(+)-tumerone. 相似文献
689.
690.
Maximilian N. Kopylovich Armando J.L. Pombeiro 《Coordination chemistry reviews》2011,255(1-2):339-355
The methods of synthesis of 1,3,5-triazapentadienes (also known as imidoylamidines), as well as the preparation of their complexes, are reviewed. The former methods include mainly the Pinner, Ley and Muller syntheses, the amination of N-imidoylimidoates or of nitriles bearing strong electron-withdrawing groups, the desulfurizing amination of N-tiobenzoylbenzamidines and the reaction of perfluoro-5-aza-4-nonene with primary amines. The synthetic procedures for the complexes involve not only the coordination of a pre-formed 1,3,5-triazapentadiene but also the generation in situ of such a species, namely by condensation of amidines, by direct one-pot template synthesis from nitriles, by nucleophilic addition of amidines or related reagents to coordinated nitriles, by nucleophilic additions to dicyanamidate or hydrolytic conversion of triazines. 相似文献