全文获取类型
收费全文 | 23520篇 |
免费 | 834篇 |
国内免费 | 125篇 |
专业分类
化学 | 15606篇 |
晶体学 | 243篇 |
力学 | 695篇 |
数学 | 2180篇 |
物理学 | 5755篇 |
出版年
2023年 | 149篇 |
2022年 | 416篇 |
2021年 | 569篇 |
2020年 | 428篇 |
2019年 | 433篇 |
2018年 | 382篇 |
2017年 | 360篇 |
2016年 | 675篇 |
2015年 | 578篇 |
2014年 | 815篇 |
2013年 | 1393篇 |
2012年 | 1699篇 |
2011年 | 1905篇 |
2010年 | 1206篇 |
2009年 | 1047篇 |
2008年 | 1605篇 |
2007年 | 1387篇 |
2006年 | 1431篇 |
2005年 | 1207篇 |
2004年 | 1071篇 |
2003年 | 898篇 |
2002年 | 848篇 |
2001年 | 527篇 |
2000年 | 473篇 |
1999年 | 329篇 |
1998年 | 210篇 |
1997年 | 205篇 |
1996年 | 270篇 |
1995年 | 184篇 |
1994年 | 197篇 |
1993年 | 202篇 |
1992年 | 151篇 |
1991年 | 123篇 |
1990年 | 125篇 |
1989年 | 97篇 |
1988年 | 68篇 |
1987年 | 72篇 |
1986年 | 50篇 |
1985年 | 74篇 |
1984年 | 50篇 |
1983年 | 42篇 |
1982年 | 63篇 |
1981年 | 62篇 |
1979年 | 46篇 |
1978年 | 45篇 |
1977年 | 34篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 36篇 |
1973年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A new cyclization route, triggered by epoxide opening, has been performed to provide the key intermediates for isoindolobenzapine alkaloids, lennoxamine and chilenine. The epoxide was prepared by the Stille reaction using vinyltributylstannane and the following dioxirane treatment. Cyclization under the treatment of BF3 · OEt2 provided an azepine moiety, and the oxidative cyclization toward the known precursor for the alkaloids has been achieved by reaction with a stoichiometric amount of Pd(OAc)2. This formal synthesis suggests a new route to the alkaloids. 相似文献
992.
The palladium-catalyzed arylation and vinylation of 2,3-dihydrofuran with aryl and alkenyl iodonium salts afforded 2-phenyl- or 2-alkenyl-2,5-dihydrofurans at room temperature in an aqueous medium. 相似文献
993.
The reaction of ninhydrin with 1,2,3- and 1,2,4-trimethylbenzene in the presence of H2SO4 or AlCl3 afforded 2-monoaryl and 2,2-diaryl-1,3-indanedione derivatives as the major products. With 1,3,5-trimethylbenzene as the arene nucleophile, either a reduction product or an indenoindanone derivative was obtained depending upon the catalyst employed in the reaction. 相似文献
994.
A series of self-healable polyurethane (SHPU)/modified graphene (MG) nanocomposites were synthesized from poly(tetramethylene glycol) (PTMG) and 4,4′-methylene diphenyl diisocyanate (MDI) with minute amounts (0–1 wt%) of MG which was chemically modified graphene oxide (GO) with phenyl isocyanate and reduced in the presence of phenylhydrazine. 相似文献
995.
Yongseon Kim Jaehyuk Lim Shinhoo Kang 《International journal of quantum chemistry》2013,113(2):148-154
The dissolution phenomenon of Mn ions in LiMn2O4 (LMO) cathode material for lithium ion batteries (LIBs) was investigated by a first principle calculation using the discrete variational Xα molecular orbital method. It was found that the oxidation number of Mn ions easily increases at high temperatures due to the empty levels of Mn 3d orbitals located in the vicinity of the Fermi energy level of LMO crystal. The changes of density of states (DOS) and Mn‐O bonding properties with doping were examined. Analysis of DOS showed that the substitution of elements with a smaller oxidation number than Mn was found effective in keeping Mn ions at higher oxidation states. From the calculation of bonding properties, the dissolution of Mn was found to be strongly correlated with the covalent nature of Mn‐O bond. Based on the results, we concluded that increasing the covalent character of Mn‐O bond is effective to minimize the dissolution of Mn ions, along with suppressing the formation of Jahn‐Teller‐active Mn3+ by inducing Mn ions at high oxidation state with proper selection of doping elements. © 2012 Wiley Periodicals, Inc. 相似文献
996.
N. N. Medvedev V. P. Voloshin A. V. Kim A. V. Anikeenko A. Geiger 《Journal of Structural Chemistry》2013,54(2):271-288
This paper is a review of our recent computational studies of volumetric characteristics using computer models of dilute solutions. Partial molar volume (PMV) and its components are calculated for simple and complex molecules in water (methane, noble gases, surfactants, polypeptides). Advantages and disadvantages of various computational methods are discussed. It is proposed to use the Voronoi-Delaunay technique to determine the reasonable boundary between a solute molecule and solvent molecules and to identify the PMV components related to the molecule, the boundary layer, and the solvent. It is noted that the observed increase in PMV with temperature for large molecules is due to an increase in the volume of voids in the boundary layer, i.e., due to the “thermal volume.” In this case, the solvent gives a negative contribution to the PMV. In contrast, for simple molecules (methane), the contribution from the solvent is positive and is the main factor in the increase in the PMV, which is associated with a specific change in water structure around a spherical hydrophobic particle outside the boundary layer. For surfactant molecules, the contribution from the solvent changes sign (from negative to positive) with increasing temperature. 相似文献
997.
Air oxidation of N-cyclopropylanilines was shown to occur under either ambient conditions or accelerated conditions (warming or shining light) in an open container. A subsequent fragmentation resulted in formation of the corresponding acetamide. While potential mechanisms have been previously proposed, simple aerobic oxidation to β-hydroxy-propionamides in the absence of a radical promoter has not been previously reported. 相似文献
998.
Zaesung No Shi Yong Ryu Seong-Kie Kim Jong Woong Ahn Bok Ju Song Keun Jae Kim 《合成通讯》2013,43(5):887-892
Thermal or base-promoted conversion of 5′-O-TBDMS-3′-O-(1H-imidazole-1-thiocarbonyl)thymidine (1) afforded 5′-O-TBDMS-2,3′-anhydro-thymidine (2), a pivotal intermediate for the transformation of the 3′-hydroxy group of 2′-deoxyribonucleosides, in high yield. 相似文献
999.
An efficient and simple amine‐catatalyzed azide conjugate addition to α,β‐unsaturated aldehydes has been developed. In the presence of a catalytic amount of tertiary amine with a 1:1 mixture of NaN3 and 37% HCl in CH2Cl2, α,β‐unsaturated aldehydes provided β‐azido aldehydes, which could be transformed into 1,3‐amino alcohols. 相似文献
1000.
Triciribine (TCN, 1) and its monophosphate (TCNP, 2) are tricyclic nucleotide derivatives that have potential antineoplastic activity. Triciribine inhibits the phosphorylation, activation, and signaling of Akt-1, -2, and -3, which may result in the inhibition of Akt-expressing tumor cell proliferation. Both TCN and TCNP have very low bioavailability, and the development of both drugs as intravenous (IV) treatments was halted because of the toxicity induced by the high doses needed for their use as general cytotoxic agents. This publication describes an expedient and straightforward total synthesis of amino acid prodrugs (3, 4) of TCN and TCNP. In our study, both the prodrugs significant improved the plasma exposure of the parent drugs and the prodrugs. 相似文献