全文获取类型
收费全文 | 36083篇 |
免费 | 2306篇 |
国内免费 | 1188篇 |
专业分类
化学 | 24820篇 |
晶体学 | 356篇 |
力学 | 1390篇 |
综合类 | 111篇 |
数学 | 3566篇 |
物理学 | 9334篇 |
出版年
2023年 | 302篇 |
2022年 | 528篇 |
2021年 | 760篇 |
2020年 | 766篇 |
2019年 | 757篇 |
2018年 | 582篇 |
2017年 | 585篇 |
2016年 | 1162篇 |
2015年 | 1019篇 |
2014年 | 1351篇 |
2013年 | 2209篇 |
2012年 | 2770篇 |
2011年 | 2965篇 |
2010年 | 1900篇 |
2009年 | 1665篇 |
2008年 | 2331篇 |
2007年 | 2178篇 |
2006年 | 1978篇 |
2005年 | 1806篇 |
2004年 | 1613篇 |
2003年 | 1295篇 |
2002年 | 1313篇 |
2001年 | 939篇 |
2000年 | 744篇 |
1999年 | 550篇 |
1998年 | 391篇 |
1997年 | 403篇 |
1996年 | 463篇 |
1995年 | 354篇 |
1994年 | 372篇 |
1993年 | 362篇 |
1992年 | 341篇 |
1991年 | 280篇 |
1990年 | 200篇 |
1989年 | 192篇 |
1988年 | 172篇 |
1987年 | 149篇 |
1986年 | 128篇 |
1985年 | 199篇 |
1984年 | 124篇 |
1983年 | 103篇 |
1982年 | 130篇 |
1981年 | 96篇 |
1980年 | 86篇 |
1978年 | 85篇 |
1977年 | 91篇 |
1976年 | 99篇 |
1975年 | 104篇 |
1974年 | 81篇 |
1973年 | 103篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
911.
The conversion of carbamates into ureas using aluminum amide complexes is reported. This reaction is a convenient method to prepare bi-, tri- and tetra-substituted ureas from carbamate-protected primary or secondary amines by reaction with primary or secondary amines in the presence of stoichometric quantities of trimethylaluminum. A reactivity trend of the various carbamates was observed and methyl and benzyl carbamates were reacted selectively in the presence of t-butyl carbamates. 相似文献
912.
Jiang W Wanner J Lee RJ Bounaud PY Boger DL 《Journal of the American Chemical Society》2002,124(19):5288-5290
A convergent total synthesis of the ramoplanin A2 and ramoplanose aglycon is disclosed. Three key subunits composed of residues 3-9 (heptapeptide 15), pentadepsipeptide 26, and pentapeptide 34 (residues 10-14) were prepared, sequentially coupled, and cyclized to provide the 49-membered depsipeptide core of the aglycon. Key to the preparation of the pentadepsipeptide 26 incorporating the backbone ester was the asymmetric synthesis of an orthogonally protected L-threo-beta-hydroxyasparagine and the development of effective and near-racemization free conditions for esterification of its hindered alcohol (EDCI, DMAP, 0 degrees C). The coupling sites were chosen to maximize the convergency of the synthesis including that of the three subunits, to prevent late stage racemization of carboxylate-activated phenylglycine-derived residues, and to enlist beta-sheet preorganization of an acyclic macrocyclization substrate for 49-membered ring closure. As such, macrocyclization at the chosen Phe(9)-D-Orn(10) site may benefit from both beta-sheet preorganization as well as closure at a D-amine terminus. Deliberate late stage incorporation of the subunit bearing the labile depsipeptide ester and a final stage Asn(1) side chain introduction provides future access to analogues of the aglycons which themselves are reported to be equally potent or more potent than the natural products in antimicrobial assays. 相似文献
913.
Gel analysis system purchasers have a large variety of digital imaging products to choose from today. Understanding the specifications presented by the manufacturers is important to ensure that the system fits the needs ofthe researcher using the system. The background information contained in this paper explains the terminology and many of the specifications along with their significance. 相似文献
914.
J. V. Crivello J. L. Lee D. A. Conlon 《Journal of polymer science. Part A, Polymer chemistry》1987,25(12):3293-3309
All aromatic polyimides bearing diarylsulfide linkages in the main chain were prepared either by condensation of a sulfur containing dianhydride with an aromatic diamine or the condensation of a sulfur containing aromatic diamine with a dianhydride. Phenylation with diphenyl iodonium salts was then used to convert the diarylsulfide groups to triarylsulfonium salts. The resulting photosensitive polyimides were shown to undergo main chain cleavage during photolysis using UV irradiation. These new polyimides are candidates for positive working, high temperature photoresist materials. 相似文献
915.
A Chelex-100 resin column has been employed for the preliminary concentration of trace elements in water samples before their determination by neutron-activation analysis. The column, filled with a 1:1 mixture of the resin (50-100 mesh) and Pyrex glass powder of the same mesh-size, is shown to maintain a constant flow-rate and give reproducible results. By a combination of preconcentration and neutron-activation analysis it is possible to determine Ba, Ca, Cd, Ce, Co, Cr, Cu, Fe, La, Mg, Mn, Sc, U, V and Zn in sea-water and/or fresh water simultaneously at the parts per milliard level. 相似文献
916.
Jung-Hoon Oh Mun-Chual Rho Hyeong Kyu Lee Jae Nyoung Kim Sang-Hun Jung 《Tetrahedron letters》2004,45(38):7077-7079
Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated. 相似文献
917.
Yu-Ping Zhang Kwang-Pill Lee Hyun-Joo Noh Ali Md Showkat Seong-Ho Choi 《Microchemical Journal》2004,78(2):99-106
An appropriate optimization strategy should be used to find a desired resolution or selectivity with a minimum number of experiments in a limited time, which could assure the baseline separation of all target compounds. It was usually realized by means of a specialized computer program. In this paper, mapping optimization method and overlapping resolution mapping were compared for the optimization of a reversed-phase high-performance liquid chromatography (HPLC) isocratic separation of neutral compounds. The calculated resolutions and separation time of 7 to 10 experiments are fitted by different equations, which were used to build a contour plot with a minimum effective resolution and maximum retention time as a function of a mobile phase composition. The balance between resolution and analysis time could be easily realized by the overlapping of the final overlapping resolution mapping and analysis time mapping. The validity of the two methods was confirmed by some typical experiments. The models are simple, visual, and common without theoretical arithmetic. 相似文献
918.
The self-assembly of metallosupramolecules from reactions of flexible 2-pyridyl ligands and silver salts is described. When 1,3-bis(2-pyridyl)propane (L1), tris[(2-pyridyl)methyl]methane (L2), and 1,3-bis(2-pyridyl)-2-tolylpropane (L3) are used in combination with silver ions, novel discrete metallocyclic complexes are formed in crystals. Moreover, the self-assembly of 1,3-bis(2-pyridyl)-2-phenylpropane (L4) with silver nitrate yields a coordination polymer. The examination of its solution shows that this coordination polymer is formed via the solution-based discrete metallocyclic species. 相似文献
919.
Much improved catalytic carbon-carbon bond-forming reactions between aryl chlorides and Grignard reagents has been achieved using nickel(II) ion on nanoporous carbon. 相似文献
920.
Keeyung Lee 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,40(1):164-166
The electronic structure and magnetism of 13 atom clusters of ruthenium, rhodium and palladium having face centered cubic(fcc) geometry has been studied using a Gaussian orbital basis and the local spin density approximation. Calculations were done for the lattice spacings relevant to the bulk crystal lattice. Using the fixed moment states as input potentials, as many as 3 self-consistent states were obtained for these clusters. The 3 converged states of Rh13 cluster is found to have magnetic moments of 0.69 μB , 1.00 μB and 1.46 μB . Out of these states, 0.69 μB moment state is found to be the ground state. But the total energy difference between the 0.69 μB and 1.00 μB state is very small. The 1.46 μB moment state coincides with the state reported previously by other authors which was obtained using the discrete variational method. The experimentally observed moment was around 0.47 μB . Our calculated moment is closer to the experimentally observed moment than the previously reported moment, but is still a bit larger. Ru13 cluster is also found to have large moments, and 3 self-consistent states are also obtained for this cluster. The 3 magnetic moments of the Ru13 cluster are 0.46 μB , 0.62 μB and 1.08 μB . Out of these states, 0.62 μB moment state is found to be the ground state. For the Pd13 cluster, in addition to the nonmagnetic state previously reported, a state with magnetic moment of 0.46 μB is also found to exist indicating possible magnetism in cluster phase. 相似文献