全文获取类型
收费全文 | 279篇 |
免费 | 4篇 |
国内免费 | 3篇 |
专业分类
化学 | 213篇 |
晶体学 | 7篇 |
力学 | 3篇 |
数学 | 30篇 |
物理学 | 33篇 |
出版年
2023年 | 1篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 2篇 |
2013年 | 17篇 |
2012年 | 15篇 |
2011年 | 12篇 |
2010年 | 8篇 |
2009年 | 9篇 |
2008年 | 22篇 |
2007年 | 13篇 |
2006年 | 22篇 |
2005年 | 25篇 |
2004年 | 21篇 |
2003年 | 14篇 |
2002年 | 17篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 11篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1966年 | 1篇 |
1955年 | 2篇 |
1939年 | 1篇 |
1937年 | 2篇 |
1936年 | 1篇 |
1928年 | 1篇 |
1924年 | 1篇 |
排序方式: 共有286条查询结果,搜索用时 15 毫秒
81.
Jaap J.D. de Jong Theodora D. Tiemersma-Wegeman Wesley R. Browne Kingo Uchida Ben L. Feringa 《Tetrahedron》2008,64(36):8324-8335
The synthesis and characterisation of a series of chiral and achiral low molecular weight organogelators (LMWGs) based on bis-amide substituted dithienylethene photochromic switches is reported. The LMWGs gelate a range of solvents depending on the specific functionalisation of the hydrogen bonding amide groups. In mixtures of chiral and achiral LMWGs the stereochemical outcome of the chiral aggregation is determined by the chiral LMWG molecules in most cases. However, for the first time we demonstrate that the stereochemical outcome of the aggregation can be influenced by the achiral LWMG molecules in some cases. Furthermore specific π-π (and/or van der Waals) interactions of chiral LMWGs 1-3o with the solvent allow the solvent to influence the control of chirality of aggregation. This influence of the solvent has a dramatic effect on whether four- or two-gel states are available. 相似文献
82.
83.
Jaap S. Sinninghe Damst W. Irene C. Rijpstra Jan W. de Leeuw Gerard W. M. Lijmbach 《Journal of separation science》1994,17(6):489-500
Five crude oils with varying sulfur contents (0.1 – 4.7%) were characterized on a molecular level for organically-bound sulfur. Aromatic fractions were analyzed by GC-(MS) and asphaltene and polar fractions were analyzed by flash pyrolysis-GC-(MS). The polar fractions were also desulfurized with Raney Ni and the hydrocarbons formed were analyzed by GC-MS. Major sulfur compounds in the aromatic fractions were identified as alkylbenzo- and alkyldiben-zothiophenes. The flash pyrolyzates of the asphaltene contained alkylthiophenes and alkylbenzothiophenes as major compounds, depending on the thermal maturity of the oil. Generally, the sulfur-rich crude oils contained relatively more sulfur compounds. The flash pyrolyzates of polar fractions contained a variety of sulfur compounds (alkylthiolanes, alkylthianes, terpenoid sulfides, alkylbenzothiophenes) with substantial differences between different crude oils. Raney Ni desulfurization of the polar fraction yielded hydrocarbons dominated by n-alkanes, but isoprenoid alkanes, n-alkylcyclohexanes, mid-chain methylalkanes, tricyclic terpanes, hopanes and steranes were also present. These hydrocarbons show a potential to fingerprint crude oils since their distribution patterns are more characteristic than those of the hydrocarbons present in the saturated hydrocarbon fraction. 相似文献
84.
Faysal Benaskar Volker Engels Narendra Patil Jan Meuldijk Lumbertus A. Hulshof Jaap. C. Schouten Andrew E.H. Wheatley 《Tetrahedron letters》2010,51(2):248-5988
The action of nanoparticulate copper catalysts with a mean particle size of 10 nm in the Ullmann ether synthesis is reported using multimode microwave heating and employing stable chloropyridine salts and unactivated phenol, with stabilized copper nanoparticles outperforming other copper catalysts in terms of stability and reusability. 相似文献
85.
Bogdan I. Florea Martijn Verdoes Nan Li Wouter A. van der Linden Paul P. Geurink Hans van den Elst Tanja Hofmann Arnoud de Ru Peter A. van Veelen Keiji Tanaka Katsuhiro Sasaki Shigeo Murata Hans den Dulk Jaap Brouwer Ferry A. Ossendorp Alexei F. Kisselev Herman S. Overkleeft 《Chemistry & biology》2010,17(8):795-801
86.
Martijn J.J. MulderChristophe Faulmann Jaap G. HaasnootJan Reedijk 《Journal of solid state chemistry》2002,168(2):390-395
Combination of the [Ni(bpy)3]2+ cation complex and the [Pd(dmit)2]− anion (dmit=C3S52−=1,3-dithiole-2thione-4,5-dithiolate) has resulted in the paramagnetic [Ni(bpy)3][Pd(dmit)2]·CH3CN compound, a suitable precursor for a molecular magnetic conductor. Its crystal structure consists of a Pd(dmit)2 anion arrangement that is quite different from segregated stack layers often found for M(dmit)2−based compounds. The reduction of the [Pd(dmit)2]- to the 2− charged anion in the title compound most probably is the result of a charge disproportionation between Pd(dmit)2 anions. 相似文献
87.
The rate-determining deprotonation of 5-nitrobenzisoxazole (Kemp elimination) by hydroxide is efficiently catalyzed by vesicles formed from dimethyldioctadecylammonium chloride (C(18)()C(18)()(+)()). Gradual addition of sodium didecyl phosphate (C(10)()C(10)()(-)()) leads to the formation of catanionic vesicles, which were characterized by cryo-electron microscopy, and their main phase transition temperatures (DSC) and zeta-potentials. Increasing percentages of C(10)()C(10)()(-)() in the vesicular bilayers decrease the catalysis of the Kemp elimination. A detailed kinetic analysis, supported by consideration of substrate binding site polarities and counterion binding percentages, suggest that the catalytic effects of C(18)()C(18)()(+)()/C(10)()C(10)()(-)() catanionic vesicles are primarily determined by the binding of catalytically active hydroxide ions to the vesicular surface area. The formation of neutral microdomains between 10 and 30 mol % of C(10)()C(10)()(-)() in the bilayer, as revealed by DSC, is not apparent from the catalytic effects found for these vesicles. Interestingly, the catalytic effects observed for 50 mol % C(10)()C(10)()(-)() in the catanionic vesicles indicate an asymmetric distribution of C(18)()C(18)()(+)() and C(10)()C(10)()(-)() over the bilayer leaflets. The overall kinetic results illustrate the highly complex mix of factors which determines catalytic effects on reactions occurring in biological cell membranes. 相似文献
88.
Hoek-van den Hil EF Beekmann K Keijer J Hollman PC Rietjens IM van Schothorst EM 《Analytical and bioanalytical chemistry》2012,402(3):1389-1392
Flavonoids are bioactive food compounds with potential lipid-lowering effects. Commercially available enzymatic assays are
widely used to determine free fatty acid (FFA) and triglyceride (TG) levels both in vivo in plasma or serum and in vitro in
cell culture medium or cell lysate. However, we have observed that various flavonoids interfere with peroxidases used in these
enzymatic assays, resulting in incorrect lower FFA and TG levels than actually present. Furthermore, addition of isorhamnetin
or the major metabolite of the flavonoid quercetin in human and rat plasma, quercetin-3-O-glucuronide, to murine serum also resulted in a significant reduction of the detected TG levels, while a trend was seen for
FFA levels. It is concluded that when applying these assays, vigilance is needed and alternative analytical methods, directly
assessing FFA or TG levels, should be used for studying the biological effects of flavonoids on FFA and TG levels. 相似文献
89.
Maria Barysz Andrzej J. Sadlej Jaap G. Snijders 《International journal of quantum chemistry》1997,65(3):225-239
A series of nonsingular two-component relativistic Hamiltonians is derived from the Dirac Hamiltonian by first performing the free-particle Foldy–Wouthuysen transformation and then a block-diagonalizing transformation. The latter is defined in terms of operators which can be determined iteratively through arbitrary order in α, leading to transformed Hamiltonians with the two-component block accurate through α2k, k=1, 2, 3,… . These Hamiltonians give relativistic energies which differ from Dirac's energies only in terms higher than α2k. Their relation to other nonsingular methods of relativistic quantum chemistry (the Douglas–Kroll method, the regular Hamiltonian schemes) is discussed. By removing the spin-dependent operators, the derived Hamiltonians can be written in spin-free one-component form. The computational effort involved is essentially the same as in the case of the Douglas–Kroll scheme and amounts to relatively easy modification of the core Hamiltonian. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 225–239, 1997 相似文献
90.
Eva Benick Radoslav Novakovsky Jn Hrouzek Jn Krupcík Pat Sandra Jaap De Zeeuw 《Journal of separation science》1996,19(2):95-98
A simple dual-column gas chromatographic system with a six-port switching valve has been used to separate the atropisomers of PCB congeners 84, 91, and 95 in technical PCB formulations and in extracts of soil and river sediment. A capillary column coated with a methylphenylsiloxane stationary phase (CP-Sil 8) was used as the first column, for retention window selection, and a permethylated β-cyclodextrin (ChirasilDex) capillary column as the main separation column. Because peak overlap could not be eliminated by optimization of column temperature, the enantiomeric ratios of PCB congeners could not be determined from the original chromatograms. The correct enantiomer ratio was determined from the peak areas obtained by deconvolution of the chromatograms. Whereas the PCB atropisomers considered were present in equal concentrations in the technical PCB formulations, analysis of a river sediment sample confirmed different residual concentrations of the atropisomers of congener 95. 相似文献