首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   498篇
  免费   12篇
  国内免费   2篇
化学   312篇
晶体学   9篇
力学   26篇
综合类   1篇
数学   23篇
物理学   141篇
  2022年   12篇
  2021年   14篇
  2020年   8篇
  2019年   18篇
  2018年   10篇
  2017年   12篇
  2016年   7篇
  2015年   11篇
  2014年   17篇
  2013年   38篇
  2012年   29篇
  2011年   43篇
  2010年   12篇
  2009年   18篇
  2008年   24篇
  2007年   19篇
  2006年   24篇
  2005年   21篇
  2004年   23篇
  2003年   13篇
  2002年   17篇
  2001年   6篇
  2000年   7篇
  1999年   9篇
  1998年   4篇
  1997年   5篇
  1996年   3篇
  1995年   4篇
  1994年   13篇
  1993年   3篇
  1992年   7篇
  1991年   5篇
  1990年   2篇
  1989年   6篇
  1985年   6篇
  1984年   5篇
  1983年   3篇
  1980年   3篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1974年   4篇
  1968年   3篇
  1966年   1篇
  1959年   2篇
  1956年   1篇
  1935年   1篇
  1934年   3篇
  1933年   1篇
  1932年   1篇
排序方式: 共有512条查询结果,搜索用时 15 毫秒
491.
A synthesis of sulfoximine-substituted medium-ring nitrogen heterocycles (MRNHs) having a high degree of substitution has been developed. Its key steps are the modular asymmetric synthesis of sulfoximine-substituted N-tethered trienes and their Ru-catalyzed ring-closing metathesis (RCM) reaction. The highly substituted N-tethered trienes were obtained enantio- and diastereopure through 1) the diastereoselective aminoalkylation of sulfoximine-substituted allyltitanium complexes with N-tert-butylsulfonyliminoester, 2) N-allylation of homoallylic N-sulfonyl amines, 3) allylation, hydroxylalkylation, and formylation of α-lithioalkenylsulfoximines, and 4) allylation of α-formylalkenylsulfoximines. The Ru-catalyzed RCM reaction of the sulfoximine-substituted 1,7,10- and 1,7,12-trienes stereoselectively afforded the corresponding nine-, ten-, and eleven-membered MRNHs in good yields. An interesting difference in reactivity was noted in the case of a sulfoximine-substituted 1,7,10-triene and its corresponding 1,10-diene. While the triene readily underwent a RCM reaction, the diene reacted only in the presence of Ti(OiPr)(4) under formation of the corresponding MRNH. The feasibility of a removal of the sulfoximine auxiliary and the N-sulfonyl protecting group from the MRNHs were demonstrated through reduction and cleavage, respectively, of a nine-membered heterocycle, both of which proceeded readily and gave the corresponding cyclic alkene and amine, respectively.  相似文献   
492.
Ravinder Kaur  Ranju Mahajan 《Optik》2011,122(5):375-380
Laboratory as well as PIC simulation experiments reveal strong flow of energetic electrons co-moving with laser beam in laser plasma interaction. Equation governing the evolution of complex envelope in slowly varying envelope approximation is nonlinear parabolic equation. Variational approach is used to solve this problem and a Lagrangian for the problem is set up. Assuming a trial Gaussian profile, authors solve the reduced Lagrangian problem for beam width and curvature. Two scale lengths for inhomogenity along the direction of propagation, one for nonlinearity and other for diffraction management are introduced. Self-focusing, self-modulation as well as self-trapping of the laser-electron-beam plasma system is studied under variety of parameters.  相似文献   
493.
This article deals with the cloud point studies of ethoxylated sorbitan ester, Tween in the presence of glycols, and salts. Cloud point temperature of Tween 20 (5 mol dm?3) and Tween 80 (1 mol dm?3) has been determined in the presence of various salts in addition to glycols. The glycols chosen for these studies were triethylene glycol (TEG), and ethylene glycol mono butylether (EGMBE). At concentration below 0.02 M these salts have no significant effect on cloud point temperature in all cases. The cloud point temperature of Tween+TEG and Tween+EGMBE found to decrease in the presence of sodium chloride (NaCl) and potassium chloride (KCl). The cloud point temperature of Tween 20 + TEG and Tween 20 + EGMBE found to increase in the presence of urea and nicotinamide. The change in cloud point was found to be more in case of nicotinamide as compare to urea. The influence of an additive on the cloud point depends on how it affects the intermicellar interactions. An effort has been made to understand the interaction between solvent and additives leading to a change in solubility of Tween.  相似文献   
494.
Journal of Fluorescence - A Schiff base bis(thiophen-2-yl-methylene)benzene-1, 4-diamine (L) was synthesized and used for selective and sensitive detection of Fe3+. L exhibited enhanced...  相似文献   
495.
Jiang et al. proposed an algorithm to solve the inverse minimum cost flow problems under the bottleneck-type weighted Hamming distance [Y. Jiang, L. Liu, B. Wuc, E. Yao, Inverse minimum cost flow problems under the weighted Hamming distance, European Journal of Operational Research 207 (2010) 50–54]. In this note, it is shown that their proposed algorithm does not solve correctly the inverse problem in the general case due to some incorrect results in that article. Then, a new algorithm is proposed to solve the inverse problem in strongly polynomial time. The algorithm uses the linear search technique and solves a shortest path problem in each iteration.  相似文献   
496.
刘晓波  李帅  张阿漫 《爆炸与冲击》2022,42(1):014202-1-014202-13
水下爆炸冲击波是舰船抗冲击评估中重要的载荷成分,也是水中结构物毁伤程度快速预报的关键和依据。通过小当量实验发现,由于传统 Taylor 平板理论公式忽略了冲击波波速的非线性变化 ,导致其在预报近距离水下爆炸冲击波壁压脉宽时出现偏差。为此,给出了比例爆距R/W1/3为0.11~5.30 m/kg1/3 (R为爆距,W为炸药质量)下的冲击波速度拟合公式,对传统Taylor理论公式进行修正。修正后,在R/W1/3=0.11 m/kg1/3下,壁压脉宽及冲量偏差大幅减小;在R/W1/3≥0.21 m/kg1/3下,两者偏差均小于12%。此外,在处理水下近场和中远场爆炸问题时,发现数值耗散会导致壁压峰值被明显削弱,于是提出了一种可行的数值策略消除计算中数值耗散导致的削弱效应,结果与修正的Taylor平板理论公式吻合良好,峰值偏差均小于9%。改进后的冲击波壁压理论公式及数值计算方法可为舰船抗爆抗冲击领域提供理论和技术支撑。  相似文献   
497.
A new poly(vinyl chloride) PVC membrane electrode to determine monomer concentrations of dodecylbenzenesulphonate ions (DBS) based on a neutral ion-pair carrier complex of dodecyltrimethylammonium–dodecylbenzenesulphonate (DTA+–DBS), is reported here. The electrode exhibits a slope of 51.25 mV per decade for DBS ions. The DBS ion selective electrode (ISE) can determine monomer units down to concentrations as low as 3.32 × 10−4 M. The effect of three kinds of additives, i.e. alcohols, glycols and triblock polymers on the performance of the surfactant selective electrode is studied systematically. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed interference method (FIM) at 1.0 × 10−2 M concentration of foreign anions. The sensor responds well to the surfactant ions in the presence of additives at lower concentration. The Gibbs free energy of micelle formation (ΔGm) of sodium dodecylbenzenesulphonate (SDBS) in the presence of various additives is calculated and found to vary differently with respect to the increase in the amount of additives. The sensor worked in the acidic pH range with a short response time of 30 s. The lifetime of the sensor is more than three months. The sensor was further used to determine the amount of DBS in local detergents. This method of determining anionic surfactants was found to be quite accurate when compared with classical methods.  相似文献   
498.
A series of enyne-allenes, with and without benzannulation at the ene moiety and equipped with aromatic and carbonyl groups as internal triplet sensitizer units at the allene terminus, was synthesized. Both sets, the cyclohexenyne-allenes and benzenyne-allenes, underwent thermal C(2)-C(6) cyclization exclusively to formal ene products. In contrast, the photochemical C(2)-C(6) cyclization of enyne-allenes provided formal Diels-Alder and/or ene products, with higher yields for the benzannulated systems. A raise of the temperature in the photochemical cyclization of enyne-allene 1b' led to increasing amounts of the ene product in relation to that of the formal Diels-Alder product. Laser flash photolysis at 266 and 355 nm as well as triplet quenching studies for 1b,b' indicated that the C(2)-C(6) cyclization proceeds via the triplet manifold. On the basis of a density functional theory (DFT) study, a short-lived transient (tau = 30 ns) was assigned as a triplet allene, while a long-lived transient (tau = 33 micros) insensitive to oxygen was assigned as fulvene triplet diradical. An elucidation of the reaction mechanism using extensive DFT computations allowed rationalization of the experimental product ratio and its temperature dependence.  相似文献   
499.
A ruthenium(II) complex [Ru(PPh3)2(pytsc)2] {Hpytsc = pyridine‐2‐carbaldehydethiosemicarbazone, (C5H5N)C2(H)=N3‐N2(H)‐C1(=S)N1H} has been used as an ion carrier for the selective determination of silver(I) ions in solution. Silver(I) ion‐selective coated graphite based (CGE) and PVC polymeric membrane based (PME) electrodes exhibit Nernstian slope for silver(I) ions over a wide concentration range from 1.0 × 10−1 M to 5.0 × 10−6 M (with CGE) and 1.0 × 10−1 M to 2.0 × 10−5 M (with PME). The working pH range of these electrodes has been found to be from 1.2 to 7.2 for CGE and 2.2 to 6.5 for PME. The proposed CGE sensor exhibits better analytical features like sensitivity and selectivity towards different secondary ions in comparison to the corresponding PME with no interference from mercury(II) ions . These electrodes also act as indicator electrodes in potentiometric titration and have been successfully used for the determination of silver content in solution of real samples (1 gm dissolved in 100 mL of dilute nitric acid) such as silver ornaments and thin silver foils. Silver content determined by the use of ion selective electrode was found to vary in the concentration range from 1.20 x 10−2 M to 7.45 x 10−2 M and results were found to be comparable with those obtained from the traditional volumetric method of analysis. It is the first report of a metal‐ligand complex used as an ion carrier in ion selective electrode, which is selective for a metal ion other than the one used in the complex.  相似文献   
500.
An extended study of the scope and mechanism of the catalytic asymmetric aziridination of imines with ethyl diazoacetate mediated by catalysts prepared from the VANOL and VAPOL ligands and triphenylborate is described. Nonlinear studies with scalemic VANOL and VAPOL reveal an essentially linear relationship between the optical purity of the ligand and the product suggesting that the catalyst incorporates a single molecule of the ligand. Two species are present in the catalyst prepared from B(OPh)(3) and either VANOL or VAPOL as revealed by (1)H NMR studies. Mass spectral analysis of the catalyst mixture suggests that one of the species involves one ligand molecule and one boron atom (B1) and the other involves one ligand and two boron atoms (B2). The latter can be formulated as either a linear or cyclic pyroborate and the (11)B NMR spectrum is most consistent with the linear pyroborate structure. Several new protocols for catalyst preparation are developed which allow for the generation of mixtures of the B1 and B2 catalysts in ratios that range from 10:1 to 1:20. Studies with catalysts enriched in the B1 and B2 species reveal that the B2 catalyst is the active catalyst in the VAPOL catalyzed asymmetric aziridination reaction giving significantly higher asymmetric inductions and rates than the B1 catalyst. The difference is not as pronounced in the VANOL series. A series of 12 different imines were surveyed with the optimal catalyst preparation procedure with the finding that the asymmetric inductions are in the low to mid 90s for aromatic imines and in the mid 80s to low 90s for aliphatic imines for both VANOL and VAPOL catalysts. Nonetheless, the crystallinity of the N-benzhydryl aziridines is such that nearly all of the 12 aziridine products screened can be brought to >99 % ee with a single recrystallization.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号