全文获取类型
收费全文 | 2052篇 |
免费 | 109篇 |
国内免费 | 12篇 |
专业分类
化学 | 1298篇 |
晶体学 | 5篇 |
力学 | 95篇 |
综合类 | 1篇 |
数学 | 441篇 |
物理学 | 333篇 |
出版年
2023年 | 16篇 |
2022年 | 28篇 |
2021年 | 45篇 |
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 42篇 |
2017年 | 28篇 |
2016年 | 83篇 |
2015年 | 75篇 |
2014年 | 64篇 |
2013年 | 122篇 |
2012年 | 131篇 |
2011年 | 143篇 |
2010年 | 94篇 |
2009年 | 91篇 |
2008年 | 124篇 |
2007年 | 138篇 |
2006年 | 115篇 |
2005年 | 116篇 |
2004年 | 100篇 |
2003年 | 98篇 |
2002年 | 76篇 |
2001年 | 26篇 |
2000年 | 23篇 |
1999年 | 27篇 |
1998年 | 13篇 |
1997年 | 21篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 14篇 |
1990年 | 16篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 11篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1970年 | 3篇 |
1935年 | 3篇 |
排序方式: 共有2173条查询结果,搜索用时 0 毫秒
41.
4-Hydroxy-5,5-dimethylimidazolines tethered at N-1 to an aryl sulfide undergo an unprecedented acid-catalysed domino reaction, involving double methyl transposition, heterocyclisation, isomerisation of thiazetidinium ion and, finally, pi-cyclisation. In this way a one-pot synthesis of original tricyclic N,S-acetals was developed. The same triheterocyclic products can be prepared also starting from the corresponding 5-hydroxy isomers (in this case the cascade of reactions does not involve methyl transposition). 相似文献
42.
Luc Maurette 《Tetrahedron》2004,60(44):10077-10098
Three different kinds of ring carbo-mers of [5]cyclitol ethers were targeted as challenging examples of functional [5]pericyclynes. Three tertiary pentaaryl-carbo-[5]cyclitol methyl ethers were synthesized through a [11+4] ring-closing double addition of triphenyl- and tri-p-anisyl-undecatetrayn-diides to dibenzoylacetylene. These compounds, obtained as oily mixtures of stereoisomers, are stable and can behave as acetylenic ligands of one or two Co2(CO)6 units. NMR analysis reveals that the broad diasteroisomeric dispersity of a triether, is consistently reduced in the symmetrized pentaether. Three bis-secondary triaryl-carbo-[5]cyclitol methyl ethers with adjacent CH(OR) vertices were synthesized through a similar [11+4] ring-closing process, where the same tetrayn-diides add to both the carbaldehyde ends of the (η2-OCH-CC-CHO)Co2(CO)6 complex. Despite the possibility of tautomeric isomerization, the occurrence of two adjacent bis-propargylic carbinol vertices does not diminish the stability of the [5]pericyclyne framework. Finally, two bis-secondary carbo-[5]cyclitol methyl ethers with non-adjacent CH(OH) vertices were synthesized through an alternative [10+5] ring-closing process. The bis-secondary carbo-[5]cyclitols are regarded as isohypsic equivalents of the challenging [C,C]5carbo-cyclopentadienyl cation. A diphenyl-hexaoxy-[5]pericyclyne with two non-adjacent secondary carbinol vertices was also prepared through a [10+5] ring-closing strategy: this molecule is an isohypsic equivalent of the previously calculated zwitterionic carbo-cyclopentadienone, which could be observed as a DCI/NH3-MS fragment after treatment with SnCl2/HCl. Analytical HPLC showed that the C11 triphenyl-undecatetrayne precursor of the [11+4] strategy was obtained as a statistical 1:2:1 mixture of the three possible diastereoisomers. Semi-preparative HPLC allowed for the resolution of this mixture. The pure major diastereoisomer was employed to prepare a partly resolved sample of pentamethoxy-pentaphenyl-[5]pericyclyne. Analytical HPLC showed that the latter corresponds to the statistical distribution of the expected three residual diastereoisomers. Semi-preparative HPLC finally afforded samples of diastereoisomerically pure pentamethoxy-[5]pericyclyne as crystalline solids. 相似文献
43.
The total synthesis of (±) dihydroreynosin and of (±) 1-oxocostic acid (R = H) are illustrative examples for a short entry into 1-oxygenated eudesmanes via an easily accessible key intermediate . 相似文献
44.
Christophe Hoarau Alexis Du Fou de Kerdaniel Pierre Grandclaudon Francis Marsais 《Tetrahedron letters》2005,46(49):8573-8577
The efficient and regioselective palladium-catalyzed C-2 arylation of ethyl 4-oxazolecarboxylate 1 with phenyliodide is described. The different parameters (solvent, base, ligand and catalyst) for the optimal conditions of this arylation process have been screened. 相似文献
45.
Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method. 相似文献
46.
47.
P. Luc R. Vetter C. Bauche-Arnoult J. Bauche 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1994,31(3):145-148
High accuracy measurements of hyperfine structure due to47Ti and49Ti in the 3d 2 4s 2 a 3 F 2?3d 2 4s4p z 5 D 1 absorption line at σ=18482.772 cm?1 have been performed by use of a Doppler-free experiment, where a beam of titanium atoms is crossed by a CW single mode tunable dye laser. They have allowed for the determination of isotope shifts between46Ti,47Ti,48Ti,49Ti and50Ti. By use of accurate values of mean square nuclear charge radii for the even isotopes, it has been possible to separate mass shifts from field shifts and to determine accurate values for the mean square nuclear charge radii of47Ti and49Ti. The field shift presents a marked odd-even staggering. 相似文献
48.
Samuel Melaku Ilse Gelaude Frank Vanhaecke Luc Moens Richard Dams 《Mikrochimica acta》2003,142(1-2):7-12
Microwave digestion reduction-aeration and pyrolysis combined with cold vapour atomic absorption and cold vapour atomic fluorescence
are compared for the determination of total mercury in several biological and environmental matrices. The biological samples
were digested in a mixture of HNO3/H2O2, the environmental samples in a mixture of HNO3/HClO4. After reduction with SnCl2, the mercury was collected by two-stage gold amalgamation. After microwave digestion reduction-aeration, detection limits
of 1.4 ng g−1 and 0.6 ng g−1 were obtained for cold vapour atomic absorption spectrometry (CVAAS) and cold vapour atomic fluorescence spectrometry (CVAFS),
respectively, for 250 mg of environmental samples. For biological samples (500 mg) the detection limits were 0.7 ng g−1 (CVAAS) and 0.4 ng g−1 (CVAFS). After pyrolysis, detection limits of 3.5 ng g−1 and 1.6 ng g−1 for CVAAS and CVAFS, respectively, were obtained for a 10 mg sample. Pyrolysis can only be applied when the organic content
of the sample is not too high. Accurate results were obtained for 8 certified reference materials of both environmental and
biological origin. In addition, a real sludge sample was analysed.
Author for correspondence. E-mail: richard.dams@rug.ac.be
Received September 18, 2002; accepted December 3, 2002
Published online May 5, 2003 相似文献
49.
Morales D Pérez J Riera L Riera V Miguel D Mosquera ME García-Granda S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4132-4143
Reactions of the complex [MoCl(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1) (phen=1,10-phenanthroline) with potassium arylamides were used to synthesize the amido complexes [Mo(N(R)Ar)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (R=H, Ar=Ph, 2 a; R=H, Ar=p-tolyl, 2 b; R=Me, Ar=Ph; 2 c). For 2 b the Mo-N(amido) bond length (2.105(4) A) is consistent with it being a single bond, with which the metal attains an 18-electron configuration. The reaction of 2 b with HOTf affords the amino complex [Mo(eta(3)-C(3)H(4)-Me-2)(NH(2)(p-tol))(CO)(2)(phen)]OTf (3-OTf). Treatment of 3-OTf with nBuLi or KN(SiMe(3))(2) regenerates 2 b. The new amido complexes react with CS(2), arylisothiocyanates and maleic anhydride. A single product corresponding to the formal insertion of the electrophile into the Mo-N(amido) bond is obtained in each case. For maleic anhydride, ring opening accompanied the formation of the insertion product. The reaction of 2 b with maleimide affords [Mo(eta(3)-C(3)H(4)-Me-2)[NC(O)CH=CHC(O)](CO)(2)(phen)] (7), which results from simple acid-base metathesis. The reaction of 2 b with (p-tol)NCO affords [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(eta(2)-MoO(4))] (8), which corresponds to oxidation of one third of the metal atoms to Mo(VI). Complex 8 was also obtained in the reactions of 2 b with CO(2) or the lactide 3,6-dimethyl-1,4-dioxane-2,5-dione. The structures of the compounds 2 b, 3-OTf, [Mo(eta(3)-C(3)H(4)-Me-2)[SC(S)(N(H)Ph)](CO)(2)(phen)] (4), [Mo(eta(3)-C(3)H(4)-Me-2)[SC(N(p-tol))(NH(p-tol))](CO)(2)(phen)] (5 a), and [Mo(eta(3)-C(3)H(4)-Me-2)[OC(O)CH=CHC(O)(NH(p-tol))](CO)(2)(phen)] (6), 7, and 8 (both the free complex and its N,N'-di(p-tolyl)urea adduct) were determined by X-ray diffraction. 相似文献
50.
The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process. 相似文献