全文获取类型
收费全文 | 241篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
化学 | 203篇 |
晶体学 | 3篇 |
力学 | 3篇 |
数学 | 38篇 |
物理学 | 18篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 5篇 |
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 12篇 |
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 23篇 |
2012年 | 32篇 |
2011年 | 28篇 |
2010年 | 16篇 |
2009年 | 10篇 |
2008年 | 5篇 |
2007年 | 13篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 3篇 |
2003年 | 10篇 |
2002年 | 4篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1985年 | 2篇 |
排序方式: 共有265条查询结果,搜索用时 31 毫秒
201.
The ternary complexes [Pd(RaaiX)(SS)ClO4) where RaaiX is a N(1)-alkyl-2-(arylazo)imidazole (p-RC6H4N =NC3H2NN(1) X; X = Me, or Et, and R = H, Me or Cl) and SS = N,N-diethyldithiocarbamate or morpholinedithiocarbamate have been prepared and characterized by elemental analysis, i.r., u.v.-vis.
and 1H-n.m.r. data. Electrochemical studies show azo reduction. The complexes are thermally unstable and decompose to bis(dithiocarbamato)palladium(II) in solution.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
202.
The Rap-Stoermer reaction of salicylaldehydes with diverse phenacyl bromide and phenacyl iodides proceeded cleanly to afford various functionalized benzofurans in excellent yields under base-mediated solvent free microwave irradiation conditions. 相似文献
203.
204.
Minh Tho Nguyen Debasis Sengupta L. G. Vanquickenborne 《Chemical physics letters》1995,240(5-6):513-520
Potential energy surfaces of the reaction of SiH2 and C2H2 (and C2D2) have been calculated by means of ab initio molecular orbital theory at the QCISD/6-311G++(2df, 2p)//MP2/6-31G(d, p) level with corrections for the triple excitations to the QCISD energies. The barrier heights for the two reaction channels of the adduct, thus calculated, were further utilized for the dynamical calculation of the rate constants in the framework of quantum statistical Rice-Ramsperger-Kassel theory. Contributions of the rate constants of the various pathways to the total rate constant (KT) for the disappearance of the reactants are critically examined and compared with experiment. The pressure dependence of KT(C2H2) is primarily due to the formation of silirene. KT(C2D2) is consistently higher than KT(C2H2). The standard heat of formation of silirene is predicted to be 72.1 ± 3 kcal/mol. Rearrangement of silirene to vinylsilylene requires an activation energy smaller than that to silylacetylene. 相似文献
205.
In this article, we propose a failure rate based step-stress accelerated life testing (SSALT) model assuming that the time-to-event distribution belongs to a fairly general family of distributions and the underlying population consists of long term survivors. With increase in stress levels, it is expected that the mean time to the event of interest gets shortened leading to an order restriction among the mean times-to-event. We propose here a method of obtaining order restricted maximum likelihood estimators (MLEs) based on expectation maximization (EM) algorithm coupled with the method of generalized isotonic regression technique. Additionally, we address the testing of hypothesis problem for the presence of long term survivors in the underlying population based on both asymptotic and non-asymptotic approaches. To illustrate the effectiveness of the proposed method, extensive simulation experiments are carried out and a real data set is analyzed. 相似文献
206.
Milandip Karak Luiz C.A. Barbosa Celia R.A. Maltha Thiago M. Silva John Boukouvalas 《Tetrahedron letters》2017,58(29):2830-2834
Several α-unsubstituted β-arylbutenolides have been prepared in 69–92% yield by reductive dehalogenation of α-halo-β-arylbutenolides. The latter were assembled in a single-step from α,β-dihalobutenolides, which are accessible on a large-scale from biomass-derived furfural. Our dehalogenation protocol is illustrated by a new synthesis of the marine antibiotics rubrolide E and F, and 3″-bromorubrolide F. 相似文献
207.
208.
Density functional theory calculations were performed to elucidate the mechanism of the ruthenium-catalyzed hydroamidation of terminal alkynes, a powerful and sustainable method for the stereoselective synthesis of enamides. The results provide an explanation for the puzzling experimental finding that with tri-n-butylphosphine (P(Bu)3) as the ligand, the E-configured enamides are obtained, whereas the stereoselectivity is inverted in favor of the Z-configured enamides with (dicyclohexylphosphino)methane (dcypm) ligands. Using the addition of pyrrolidinone to 1-hexyne as a model reaction, various pathways were investigated, among which a catalytic cycle turned out to be most advantageous for both ligand systems that consists of: (a) oxidative addition, (b) alkyne coordination, (c) alkyne insertion (d) vinyl-vinylidene rearrangement, (e) nucleophilic transfer and finally (f) reductive elimination. The stereoselectivity of the reaction is decided in the nucleophilic transfer step. For the P(nBu)3 ligand, the butyl moiety is oriented anti to the incoming 2-pyrolidinyl unit during the nucleophilic transfer step, whereas for the dcypm ligand, steric repulsion between the butyl and cyclohexyl groups turns it into a syn orientation. Overall, the formation of E-configured product is favorable by 4.8 kcal mol–1 (Δ‡
GSDL) for the catalytic cycle computed with P(Bu)3 as ancillary ligand, whereas for the catalytic cycle computed with dcypm ligands, the Z-product is favored by 7.0 kcal mol–1 (Δ‡
GSDL). These calculations are in excellent agreement with experimental findings. 相似文献
209.
A concise asymmetric total synthesis of Amaryllidaceae alkaloid (+)-trans-dihydronarciclasine is accomplished starting from optically pure 7-azabicyclo[2.2.1]heptanone scaffold in 13 linear steps. Key features of the strategy include substrate-directed stereoselective installation of the trans B-C ring junction and regioselective Wacker-type internal olefin oxidation to provide a rapid access to all hydroxyl functionalities over the key cyclohexane ring in a stereocontrolled manner. 相似文献
210.