全文获取类型
收费全文 | 5738篇 |
免费 | 152篇 |
国内免费 | 42篇 |
专业分类
化学 | 4255篇 |
晶体学 | 34篇 |
力学 | 159篇 |
数学 | 657篇 |
物理学 | 827篇 |
出版年
2021年 | 72篇 |
2020年 | 85篇 |
2019年 | 75篇 |
2018年 | 55篇 |
2017年 | 54篇 |
2016年 | 106篇 |
2015年 | 97篇 |
2014年 | 99篇 |
2013年 | 230篇 |
2012年 | 358篇 |
2011年 | 436篇 |
2010年 | 205篇 |
2009年 | 166篇 |
2008年 | 364篇 |
2007年 | 351篇 |
2006年 | 384篇 |
2005年 | 402篇 |
2004年 | 301篇 |
2003年 | 248篇 |
2002年 | 263篇 |
2001年 | 87篇 |
2000年 | 97篇 |
1999年 | 64篇 |
1998年 | 63篇 |
1997年 | 53篇 |
1996年 | 95篇 |
1995年 | 70篇 |
1994年 | 55篇 |
1993年 | 46篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 29篇 |
1989年 | 28篇 |
1988年 | 28篇 |
1987年 | 30篇 |
1986年 | 24篇 |
1985年 | 43篇 |
1984年 | 47篇 |
1983年 | 39篇 |
1982年 | 37篇 |
1981年 | 51篇 |
1980年 | 45篇 |
1979年 | 25篇 |
1978年 | 49篇 |
1977年 | 32篇 |
1976年 | 34篇 |
1975年 | 43篇 |
1974年 | 31篇 |
1973年 | 31篇 |
1972年 | 22篇 |
排序方式: 共有5932条查询结果,搜索用时 15 毫秒
941.
Both vanadium and molybdenum cofactor clusters are found in nitrogenase. In biomimetic research, many fewer heterometal MFe3S4 cubane-type clusters have been synthesized with M = V than with M = Mo because of the well-established structural relationship of the latter to the molybdenum coordination unit in the enzyme. In this work, a series of single cubane and edge-bridged double cubane clusters containing the cores [VFe3(mu3-S)4]2+ and [V2Fe6(mu3-S)6(mu4-S)2]2+ have been prepared by ligand substitution of the phosphine clusters [(Tp)VFe3S4(PEt3)3]1+ and [(Tp)2V2Fe6S8(PEt3)4]. The single cubanes [(Tp)VFe3S4L3]2- and double cubanes [(Tp)2V2Fe6S8L4]4- (L= F-, N3-, CN-, PhS-) are shown by X-ray structures to have trigonal symmetry and centrosymmetry, respectively. Single cubanes form the three-member electron transfer series [(Tp)VFe3S4L3]3-,2-,1-. The ligand dependence of redox potentials and electron distribution in cluster cores as sensed by 57Fe isomer shifts (delta) have been determined. Comparison of these results with those previously determined for the analogous molybdenum clusters (Pesavento, Berlinguette, and Holm Inorg. Chem. 2007, 46, 510) allows detection of the influence of heterometal M on the properties. At constant M and variable L, redox potentials are lowest for pi-donor ligands and largest for cyanide and relate approximately with decreasing ferrous character in clusters with constant charge z = 2-. At constant L and z and variable M, EV > E(Mo) and delta(av)V < delta(av)Mo, demonstrating that M = Mo clusters are more readily oxidized and suggesting a qualitative relation between lower potentials (greater ease of oxidation) and ferrous character. 相似文献
942.
New cage models (MeAlO)n(Me3Al)m (n=16, m=6 or 7) isomeric with previously reported sheet models for the principle activator found in hydrolytic MAO (h-MAO) are compared at M06-2X and MN15 levels of theory using density functional theory with respect to their thermodynamic stability. Reactivity of the neutrals or corresponding anions with formula [(MeAlO)16(Me3Al)6Me]− towards chlorination, and loss of Me3Al is explored while reactivity of the neutrals towards formation of contact- and outer-sphere ion pairs from Cp2ZrMe2 and Cp2ZrMeCl is examined. The results suggest on balance that a cage model for this activator is less consistent with experiment than an isomeric sheet model, although the latter are more stable based on free energy. 相似文献
943.
944.
Fengfeng Ren Kody J. Day Dr. C. Scott Hartley 《Angewandte Chemie (International ed. in English)》2016,55(30):8620-8623
Simple discotic cores functionalized with reactive arms have been assembled into two‐ and three‐tiered covalent stacks through imine formation. The targets are obtained in good yields, but competing formation of misassembled byproducts highlights some of the challenges inherent to the thermodynamically controlled assembly of rigid, compact, three‐dimensional architectures. The structures comprise a central stack of arenes surrounded by a triple helix of interconnected arms. The racemization rate is strongly dependent on the number of tiers, suggesting cooperative conformational coupling in these multi‐tiered structures. 相似文献
945.
Stephen Taylor Scott Glasgow James Taylor Jan Vecer 《Methodology and Computing in Applied Probability》2016,18(3):847-867
Heat kernel perturbation theory is a tool for constructing explicit approximation formulas for the solutions of linear parabolic equations. We review the crux of this perturbative formalism and then apply it to differential equations which govern the transition densities of several local volatility processes. In particular, we compute all the heat kernel coefficients for the CEV and quadratic local volatility models; in the later case, we are able to use these to construct an exact explicit formula for the processes’ transition density. We then derive low order approximation formulas for the cubic local volatility model, an affine-affine short rate model, and a generalized mean reverting CEV model. We finally demonstrate that the approximation formulas are accurate in certain model parameter regimes via comparison to Monte Carlo simulations. 相似文献
946.
Structure Elucidation of a Complex Faldaprevir‐(±)‐α‐Tocopherol Addition Product Reveals Reactivity of Common Excipient Vitamin E‐TPGS 下载免费PDF全文
Nina C. Gonnella Shirley A. Rodriguez Scott Pennino Daniel Norwood 《Journal of heterocyclic chemistry》2016,53(6):1878-1891
Vitamin E‐TPGS (d ‐α‐tocopheryl polyethylene glycol succinate) is a common excipient used in drug formulations. This excipient is formed by esterification of Vitamin E succinate with polyethylene glycol. As part of an oral formulation for active pharmaceutical ingredient (API) Faldaprevir, Vitamin E‐TPGS was found to decompose under ambient conditions producing free (±)‐α‐Tocopherol which subsequently formed an adduct composed of α‐Tocopherol and the API. The addition product was isolated using liquid chromatography with collections onto solid‐phase extraction cartridges, and full structure elucidation was achieved using mass spectrometry and nuclear magnetic resonance spectroscopy. The results revealed a regioselective addition of α‐Tocopherol to the API that likely occurs through the formation of a stabilized ortho‐quinone methide intermediate. This finding demonstrates the propensity of the common excipient (Vitamin E‐TPGS) to generate chemically active intermediates that may react with formulation ingredients. 相似文献
947.
948.
Dr. Jie Wang Dr. Tian Qin Dr. Tie‐Gen Chen Dr. Laurin Wimmer Jacob T. Edwards Dr. Josep Cornella Benjamin Vokits Dr. Scott A. Shaw Prof. Phil S. Baran 《Angewandte Chemie (International ed. in English)》2016,55(33):9676-9679
A transformation analogous in simplicity and functional group tolerance to the venerable Suzuki cross‐coupling between alkyl‐carboxylic acids and boronic acids is described. This Ni‐catalyzed reaction relies upon the activation of alkyl carboxylic acids as their redox‐active ester derivatives, specifically N‐hydroxy‐tetrachlorophthalimide (TCNHPI), and proceeds in a practical and scalable fashion. The inexpensive nature of the reaction components (NiCl2?6 H2O—$9.5 mol?1, Et3N) coupled to the virtually unlimited commercial catalog of available starting materials bodes well for its rapid adoption. 相似文献
949.
Daniel J. Scott Dr. Nicholas A. Phillips Joshua S. Sapsford Arron C. Deacy Dr. Matthew J. Fuchter Dr. Andrew E. Ashley 《Angewandte Chemie (International ed. in English)》2016,55(47):14738-14742
Despite the rapid development of frustrated Lewis pair (FLP) chemistry over the last ten years, its application in catalytic hydrogenations remains dependent on a narrow family of structurally similar early main‐group Lewis acids (LAs), inevitably placing limitations on reactivity, sensitivity and substrate scope. Herein we describe the FLP‐mediated H2 activation and catalytic hydrogenation activity of the alternative LA iPr3SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr3Sn+, and is rapidly and easily prepared from simple, inexpensive starting materials. This highly thermally robust LA is found to be competent in the hydrogenation of a number of different unsaturated functional groups (which is unique to date for main‐group FLP LAs not based on boron), and also displays a remarkable tolerance to moisture. 相似文献
950.
This paper describes our synthesis of the A-E subunit of gambieric acid (GA) in addition to the synthesis of the A-ring and the C-E tricycle. The use of an enol ether-olefin RCM strategy to couple the A and C-E subunits and, in the process, generate the B-ring is noteworthy. 相似文献