全文获取类型
收费全文 | 5647篇 |
免费 | 212篇 |
国内免费 | 21篇 |
专业分类
化学 | 3592篇 |
晶体学 | 31篇 |
力学 | 197篇 |
数学 | 631篇 |
物理学 | 1429篇 |
出版年
2023年 | 35篇 |
2022年 | 60篇 |
2021年 | 77篇 |
2020年 | 83篇 |
2019年 | 85篇 |
2018年 | 53篇 |
2017年 | 55篇 |
2016年 | 125篇 |
2015年 | 119篇 |
2014年 | 147篇 |
2013年 | 212篇 |
2012年 | 313篇 |
2011年 | 371篇 |
2010年 | 172篇 |
2009年 | 190篇 |
2008年 | 338篇 |
2007年 | 304篇 |
2006年 | 325篇 |
2005年 | 322篇 |
2004年 | 254篇 |
2003年 | 200篇 |
2002年 | 178篇 |
2001年 | 113篇 |
2000年 | 74篇 |
1999年 | 51篇 |
1998年 | 49篇 |
1997年 | 63篇 |
1996年 | 88篇 |
1995年 | 68篇 |
1994年 | 65篇 |
1993年 | 80篇 |
1992年 | 58篇 |
1991年 | 29篇 |
1990年 | 32篇 |
1989年 | 39篇 |
1988年 | 39篇 |
1987年 | 33篇 |
1986年 | 37篇 |
1985年 | 28篇 |
1984年 | 34篇 |
1983年 | 26篇 |
1982年 | 30篇 |
1981年 | 26篇 |
1980年 | 31篇 |
1978年 | 27篇 |
1975年 | 31篇 |
1968年 | 50篇 |
1967年 | 117篇 |
1966年 | 110篇 |
1965年 | 71篇 |
排序方式: 共有5880条查询结果,搜索用时 156 毫秒
991.
992.
993.
Michael Frotscher Wilhelm Klein Joseph Bauer Chang‐Ming Fang Jean‐Franois Halet Anatoliy Senyshyn Carsten Baehtz Barbara Albert Prof. Dr. 《无机化学与普通化学杂志》2007,633(15):2626-2630
The literature states different compositions (M/B = 1:2 vs. 2:5) and structures for diborides of molybdenum and tungsten. Using X‐ray and neutron powder diffraction as well as energy and wavelength dispersive electron microprobe analysis, the Mo/B and W/B systems were now reinvestigated. Molybdenum diboride crystallizes as a stoichiometric compound Mo2B4 (formerly described as Mo2B5) in space group (No. 166, a, b = 3.01375(2) Å, c = 20.9541(3) Å), and as a non‐stoichiometric compound MoB2?x (formerly described as MoB2) in P6/mmm (No. 191, a, b = 3.043(2) Å, c = 3.067(2) Å), whereas stoichiometric tungsten diboride W2B4 (formerly described as W2B5) is found to crystallize in space group P63/mmc (No. 194, a, b = 2.9864(4) Å, c = 13.896(2) Å). These results seem to be supported by DFT calculations which show the instability of a hypothetic W2B5. 相似文献
994.
Pointillart F Train C Villain F Moulin CC Gredin P Chamoreau LM Gruselle M Aullon G Alvarez S Verdaguer M 《Journal of the American Chemical Society》2007,129(5):1327-1334
Tris(bipyridine)ruthenium(II) is used as a templating agent to insert palladium(II) into three-dimensional oxalate-based networks. The templated-assembly of [Ru(bpy)(3)][Pd(2)(ox)(3)] (Pd(2)) and [Ru(bpy)(3)][PdMn(ox)(3)] (PdMn) is described. The latter compound is structurally characterized by powder X-ray diffraction and X-ray absorption spectroscopy. These techniques reveal an unusual 6-fold oxygen environment around the Pd(II) atoms with two short (2.02 Angstrom) and four long (2.17 Angstrom) Pd-O distances. As stated by magnetometry, this environment is associated with a triplet ground state (S = 1) of the palladium(II) ion: when the temperature is decreased, the chiMT product shows a monotonous decrease from 5.54 cm(3) K mol(-1) at 300 K, a value which is slightly lower than the one expected for independent paramagnetic Pd(II) (S = 1, g = 2) and Mn(II) (S = 5/2, g = 2) ions. This thermal variation is due to antiferromagnetic exchange interactions between the two spin bearers. Nevertheless, no long-range magnetic order is detected down to 2 K. These results are confirmed by an analysis of the [MII(C(2)O(4))(3)](4-) (M = Ni, Pd, Pt) complex and of a [Pd(II){mu-(C(2)O(4))Mn(II)(OH(2))(4)}(3)](2+) tetranuclear model using density functional theory. 相似文献
995.
Grégoire G Jouvet C Dedonder C Sobolewski AL 《Journal of the American Chemical Society》2007,129(19):6223-6231
In recent experiments, the excited-state lifetimes of protonated aromatic amino acids (TrpH+ and TyrH+) have been recorded by means of pump-probe photodissociation technique. The lifetime of TyrH+ is much longer than that of TrpH+, which has been initially rationalized on the basis of a simple phenomenological model. Besides, specific photofragments including the formation of radical cation after hydrogen loss are observed for TrpH+ that are not found for TyrH+. The ab initio calculations reported here for TrpH+ and TyrH+ using a coupled-cluster method are meant to track the rich photochemistry of these protonated amino acids following UV excitation. 相似文献
996.
A reaction cascade comprising a rhodium-catalyzed C-H activation, a subsequent hydrometalation of an alkylidene cyclopropane in vicinity, regioselective C-C bond activation of the flanking cyclopropane ring, followed by reductive elimination of the resulting metallacycle, opens a new entry into functionalized cycloheptene derivatives. This crossover of C-H activation and higher order cycloaddition has been performed in two different formats, either using alkylidenecyclopropanes with a lateral vinylpyridine moiety or with a pending aldehyde group as the trigger. The reaction tolerates various functional groups, leaves chiral centers alpha to the reacting sites unaffected, and proceeds with excellent stereoselectivity. Labeling experiments support the proposed mechanism explaining the observed net cycloisomerization process. 相似文献
997.
Poater A Solans-Monfort X Clot E Copéret C Eisenstein O 《Journal of the American Chemical Society》2007,129(26):8207-8216
Density functional theory (DFT, B3PW91) calculations have been carried out on the reactivity of ethene with model systems M(NR)(=CHCH3)(X)(Y) for M = Mo or W, R = methyl or phenyl, X = CH2CH3, OCH3, or OSiH3, and Y = CH2CH3, OCH3, or OSiH3, which are representative of experimental olefin metathesis catalysts, and the results are compared to those previously obtained for Re(CCH3)(=CHCH3)(X)(Y). The general pathway comprises four steps: olefin coordination, [2+2] cycloaddition, cycloreversion, and olefin de-coordination. Two key factors have been found to control the detailed shape of the energy profiles: the energy of distortion of the tetrahedral catalyst and the stability of the metallacycle intermediate, which is controlled by the M-C bond strength. The efficiency has been evaluated by calculating the turnover frequency (TOF) based on the steady-state approximation, and the most striking feature is that the unsymmetrical catalysts (X not equal to Y) are systematically more efficient for all systems (Mo, W, and Re). Overall, the Re complexes have been found to be less efficient than the Mo and W catalysts, except when Re is unsymmetrically substituted: it is then calculated to be as efficient as the best Mo and W catalysts. 相似文献
998.
Dommergue A Bahlmann E Ebinghaus R Ferrari C Boutron C 《Analytical and bioanalytical chemistry》2007,388(2):319-327
Snow surfaces play an important role in the biogeochemical cycle of mercury in high-latitude regions. Snowpacks act both as
sources and sinks for gaseous compounds. Surprisingly, the roles of each environmental parameter that can govern the air–surface
exchange over snow are not well understood owing to the lack of systematic studies. A laboratory system called the laboratory
flux measurement system was used to study the emission of gaseous elemental mercury from a natural snowpack under controlled
conditions. The first results from three snowpacks originating from alpine, urban and polar areas are presented. Consistent
with observations in the field, we were able to reproduce gaseous mercury emissions and showed that they are mainly driven
by solar radiation and especially UV-B radiation. From these laboratory experiments, we derived kinetic constants which show
that divalent mercury can have a short natural lifetime of about 4–6 h in snow. 相似文献
999.
Masoum S Malabat C Jalali-Heravi M Guillou C Rezzi S Rutledge DN 《Analytical and bioanalytical chemistry》2007,387(4):1499-1510
Support vector machines (SVMs) were used as a novel learning machine in the authentication of the origin of salmon. SVMs have the advantage of relying on a well-developed theory and have already proved to be successful in a number of practical applications. This paper provides a new and effective method for the discrimination between wild and farm salmon and eliminates the possibility of fraud through misrepresentation of the country of origin of salmon. The method requires a very simple sample preparation of the fish oils extracted from the white muscle of salmon samples. (1)H NMR spectroscopic analysis provides data that is very informative for analysing the fatty acid constituents of the fish oils. The SVM has been able to distinguish correctly between the wild and farmed salmon; however ca. 5% of the country of origins were misclassified. 相似文献
1000.
de Caro D Faulmann C Valade L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(6):1650-1663
Molecular materials built from coordination complexes exhibit properties that can be explained through intermolecular electronic interactions driven by the ligand moieties. The nature of the ligand in the precursor molecules governs the connectivity of the magnetic phases and the possibility of producing them by using a gas-phase process. Metallocenium, metal bisdithiolate materials, and solvated and solvent-free [M(tcne)2] (tcne=tetracyanoethylene) magnets illustrate such features. 相似文献