全文获取类型
收费全文 | 5515篇 |
免费 | 148篇 |
国内免费 | 18篇 |
专业分类
化学 | 4032篇 |
晶体学 | 18篇 |
力学 | 63篇 |
数学 | 972篇 |
物理学 | 596篇 |
出版年
2020年 | 49篇 |
2019年 | 50篇 |
2016年 | 92篇 |
2015年 | 92篇 |
2014年 | 101篇 |
2013年 | 167篇 |
2012年 | 160篇 |
2011年 | 225篇 |
2010年 | 113篇 |
2009年 | 105篇 |
2008年 | 213篇 |
2007年 | 209篇 |
2006年 | 212篇 |
2005年 | 242篇 |
2004年 | 187篇 |
2003年 | 163篇 |
2002年 | 162篇 |
2001年 | 76篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 51篇 |
1997年 | 72篇 |
1996年 | 70篇 |
1995年 | 73篇 |
1994年 | 53篇 |
1993年 | 53篇 |
1992年 | 67篇 |
1991年 | 47篇 |
1990年 | 69篇 |
1989年 | 41篇 |
1988年 | 64篇 |
1987年 | 66篇 |
1986年 | 41篇 |
1985年 | 97篇 |
1984年 | 56篇 |
1983年 | 60篇 |
1982年 | 68篇 |
1981年 | 65篇 |
1980年 | 52篇 |
1979年 | 66篇 |
1978年 | 68篇 |
1977年 | 51篇 |
1976年 | 75篇 |
1975年 | 74篇 |
1974年 | 53篇 |
1973年 | 47篇 |
1972年 | 40篇 |
1970年 | 45篇 |
1967年 | 39篇 |
1966年 | 43篇 |
排序方式: 共有5681条查询结果,搜索用时 15 毫秒
51.
We use a driving field, of the type first suggested by Evans, to generate a steady heat current in the simplest possible system, a two-dimensional periodic fluid of three hard disks. Hard-disk motion equations can be conveniently derived from repulsive constant-force or linear-force potentials by considering the infinitely repulsive limit of these potentials. We show that the isoenergetic and isokinetic forms of the nonequilibrium equations of motion generate steady-state heat conductivities differing by terms of order 1/N, whereN is the number of particles. The resulting conductivities appear to vary as the logarithm of the driving field strength. Even at low fields, the three-body periodic-system results lie well below Enskog's infinite-system prediction. 相似文献
52.
Michael Eyer Karl Schlögl Michael Widhalm 《Monatshefte für Chemie / Chemical Monthly》1984,115(12):1429-1442
Chiral title compounds have been resolved by medium pressure liquid chromatography (MPLC) on triacetylcellulose in ethanol. Whereas a quantitative separation was achieved for16 and20, a recycling technique had to be applied to1,4,6,10 and15 leading to a 100% e.e. for1,6 and15 and to ca. 60% e.e. for4 and10.An unambiguous assignment of the chirality (+)-(R)
a
-(S)
m
for tricarbonylchromium-6,6-dimethyl-diphenicacid dimethylester (4) was deduced both from the photochemical decomplexation to (-)-5 with the known chirality (R)
a
(whose e.e. was determined by MPLC on triacetylcellulose) and by comparison of the1H-n.m.r. spectra of4 and5 thereby proving the structure 4 for the former.Other chiralities both for mono and bis(tricarbonylchromium)biphenyl complexes were established by chemical correlations and comparison of the CD spectra with those of key compounds.
Herrn Prof. Dr.H. Tuppy mit besten Wünschen zum 60. Geburtstag gewidmet. 相似文献
53.
Field-portable, high-speed GC/TOFMS 总被引:1,自引:0,他引:1
Syage JA Nies BJ Evans MD Hanold KA 《Journal of the American Society for Mass Spectrometry》2001,12(6):648-655
This work is focused on developing a fast gas chromatograph, time-of-flight mass spectrometer (GC/TOFMS) for man-portable field use. The goal is to achieve a total system solution for meeting performance, size, weight, power, cost, and ruggedness requirements for a laboratory in the field. The core technology will also be adaptable to specific applications including real-time point detection for hazardous chemical releases (e.g., chemical weapons), for biological agent signature identification, and for mobile monitoring platforms (e.g., air, ship, truck). Previously we presented results of a feasibility demonstration for a 30-lb field-portable TOFMS system. In this work we present recent progress in integrating a low-power, high-speed GC and show the capability for accurately recording fast GC transients for targeted compound detection using a quadrupole ion trap, time-of-flight instrument (QitTof). 相似文献
54.
The enzymatic elongation of acyl-CoA esters by malonyl-CoA, respectively de novo synthesis of fatty acids from malonyl-CoA by liver microsomes and non particulate fraction of α-tocopherol deficient rats is diminished versus controls. However, liver microsomes of vitamin E deficient rats synthesize more eicosatetraenic acid from γ-linolenic acid and more γ-linolenic acid from linolic acid than do those of tocopherol supplemented animals. It has often been shown that the liver phosphatides of tocopherol deficient rats contain more arachidonic acid than those of controls, a fact which can be explained now by increased activity of the enzymes involved in the biosynthesis of arachidonic acid. Certain polyenic fatty acids are more rapidly synthesized in the absence of naturally occuring antioxidants. Some enzymes of the respiratory chain have also been examined. No vitamin E deficiency effect has been found on enzymes such as gluconate-6-P-dehydrogenase, isocitrate-dehydrogenase, malate-dehydrogenase and lactate dehydrogenase contained in the non particulate fraction. Sonicated mitochondria of tocopherol deficient rats show a greater activity towards cytochrome, oxydase and β-hydroxy-acetyl-CoA-dehydrogenase than controls, possibly due to ultrastractural alteration of this particle. 相似文献
55.
Karl Heinz Dötz Barbara Fügen-Köster Dietmar Neugebauer 《Journal of organometallic chemistry》1979,182(4):489-498
1-Diethylaminopropyne reacts with pentacarbonyl[methoxy(2,2-diphenylethenyl)carbene]chromium in a stereoselective way to give pentacarbonyl-[diethylamino-E-(2-methoxy-1-methyl-4,4-diphenyl-1,3-butadienyl)carbene]-chromium via insertion of the alkyne into the metal-carbene bond. The reaction of the ynamine with pentacarbonyl[methylamino(phenyl)carbene]chromium results in insertion of the alkyne and loss of carbon monoxide to give cis-tetracarbonyl[3-aza-1-methyl-2-phenyl-2-butenyl)diethylaminocarbene]-chromium. Its structure was established by oxidative degradation in an aqueous medium to give 2-benzoyl-N,N-diethylpropanamide and finally confirmed by an X-ray analysis. The new compounds are characterized spectroscopically. 相似文献
56.
Herwig G. Winkler Hagen T. Spath Prof. Dr. Karl Torkar 《Monatshefte für Chemie / Chemical Monthly》1975,106(2):535-545
The phase diagrams of the systems CsN3/Zn(N3)2 and KN3/Zn(N3)2 have been obtained employing the microscopic technique ofL. Kofler andA. Kofler. Within the system CsN3/Zn(N3)2 three eutectics at 148°C, 142°C, and 210°C were found. Besides Cs2Zn(N3)4, melting incongruently in the interval 153°C to 170°C, there exist two further compounds of the most probable composition Cs3Zn2(N3)7 and CsZn2(N3)5, melting congruently at 170°C and 210°C, resp. In the system KN3/Zn(N3)2 there exist two eutectics at 203°C and 172°C and two compounds, one of them, i.e. K2Zn(N3)4, melting congruently at 206°C, the other one, with composition KZn3(N3)7 or KZn4(N3)9, melting incongruently at 210°C.
Mit 8 Abbildungen
Herrn Professor Dr.Heribert Grubitsch zum 70. Geburtstag gewidmet. 相似文献
Mit 8 Abbildungen
Herrn Professor Dr.Heribert Grubitsch zum 70. Geburtstag gewidmet. 相似文献
57.
Trilisa M. Perrine Rajat K. Chaudhuri Karl F. Freed 《International journal of quantum chemistry》2005,105(1):18-33
Simple and quadratic Padé resummation methods are applied to high‐order series from multireference many‐body perturbation theory (MR‐MBPT) calculations using various partitioning schemes (Møller–Plesset, Epstein–Nesbet, and forced degeneracy) to determine their efficacy in resumming slowly convergent or divergent series. The calculations are performed for the ground and low‐lying excited states of (i) CH2, (ii) BeH2 at three geometries, and (iii) Be, for which full configuration interaction (CI) calculations are available for comparison. The 49 perturbation series that are analyzed include those with oscillatory and monotonic divergence and convergence, including divergences that arise from either frontdoor or backdoor intruder states. Both the simple and quadratic Padé approximations are found to speed the convergence of slowly convergent or divergent series. However, the quadratic Padé method generally outperforms the simple Padé resummation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
58.
3,4-Dihydro-5(1H)-pyrromethenones are easier attacked at themeso-position by electrophiles than 5(1H)-pyrromethenones. This is demonstrated both by aMannich-type-substitution or deuterium-exchange-experiments and by the addition of O-, S-, and N-Nucleophiles to the exocyclic double bond of the model-dihydropyrromethenone (Z)-1 under very mild reaction conditions. Applying these results to the chemistry of 2,3-dihydro-bilatrienes-abc, their chemical characteristics—especially their tautomeric behavior and their dominant C-5-selectivity towards electrophiles—become better understandable. 相似文献
59.
Eva BeckerChristian Slugovc Eva RübaChristina Standfest-Hauser Kurt MereiterRoland Schmid Karl Kirchner 《Journal of organometallic chemistry》2002,649(1):55-63
The synthesis and some reactions of the Ru(II) and Ru(IV) half-sandwich complexes [RuCp(EPh3)(CH3CN)2]+ (E=P, As, Sb, Bi) and [RuCp(EPh3)(η3-C3H5)Br]+ have been investigated. The chemistry of this class of compounds is characterized by a competitive coordination of EPh3 either via a RuE or a η6-arene bond, where the latter is favored when the former is weaker, that is in going down the series. Thus in the case of Bi, the starting material [RuCp(CH3CN)3]+ does not react with BiPh3 to give [RuCp(BiPh3)(CH3CN)2]+ but instead gives only the η6-arene species [RuCp(η6-PhBiPh2)]+ and [(RuCp)2(μ-η6,η6-Ph2BiPh)]2+. Similarly, the EPh3 ligand can be replaced by an aromatic solvent or an arene substrate. Thus, the catalytic performance of [RuCp(EPh3)(CH3CN)2]+ for the isomerization of allyl-phenyl ethers to the corresponding 1-propenyl ethers is best with E=P, while the conversion drops significantly using the As and Sb derivatives. By the same token, only [RuCp(PPh3)(CH3CN)2]+ is stable in a non-aromatic solvent, whereas both [RuCp(AsPh3)(CH3CN)2]+ and [RuCp(SbPh3)(CH3CN)2]+ rearrange upon warming to [RuCp(η6-PhEPh2)]+ and related compounds. In addition, the potential of [RuCp(EPh3)(CH3CN)2]+ as precatalysts for the transfer hydrogenation of acetophenone and cyclohexanone has been investigated. Again aromatic substrates are clearly less suited than non-aromatic ones due to facile η6-arene coordination leading to catalyst's deactivation. 相似文献
60.
alpha-(N-Carbamoylalkyl)cuprates couple with enol triflates derived from carbocyclic and heterocyclic (i.e., piperidinones) beta-keto esters. Product yields are higher with the alkyl(cyano)cuprates [i.e., RCu(CN)Li, 56-93%] than with the dialkylcuprate reagents (i.e., R(2)CuLi.LiCN). An enol nonaflate works as well as the corresponding enol triflate. A facile synthetic route to gamma-amino alpha,beta-enoates not readily prepared from gamma-keto-alpha,beta-enoates is thus established. The gamma-amino-alpha,beta-enoates, available via N-Boc deprotection, can be cyclized to annulated pyrrolin-2-ones. 相似文献