全文获取类型
收费全文 | 7115篇 |
免费 | 481篇 |
国内免费 | 331篇 |
专业分类
化学 | 3003篇 |
晶体学 | 29篇 |
力学 | 514篇 |
综合类 | 149篇 |
数学 | 1826篇 |
物理学 | 2406篇 |
出版年
2023年 | 56篇 |
2022年 | 73篇 |
2021年 | 98篇 |
2020年 | 109篇 |
2019年 | 141篇 |
2018年 | 105篇 |
2017年 | 153篇 |
2016年 | 142篇 |
2015年 | 119篇 |
2014年 | 199篇 |
2013年 | 299篇 |
2012年 | 219篇 |
2011年 | 387篇 |
2010年 | 365篇 |
2009年 | 506篇 |
2008年 | 419篇 |
2007年 | 374篇 |
2006年 | 349篇 |
2005年 | 259篇 |
2004年 | 178篇 |
2003年 | 191篇 |
2002年 | 181篇 |
2001年 | 118篇 |
2000年 | 123篇 |
1999年 | 104篇 |
1998年 | 91篇 |
1997年 | 75篇 |
1996年 | 71篇 |
1995年 | 53篇 |
1994年 | 46篇 |
1993年 | 34篇 |
1992年 | 39篇 |
1991年 | 29篇 |
1990年 | 21篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1986年 | 17篇 |
1985年 | 414篇 |
1984年 | 284篇 |
1983年 | 234篇 |
1982年 | 206篇 |
1981年 | 149篇 |
1980年 | 116篇 |
1979年 | 133篇 |
1978年 | 75篇 |
1977年 | 87篇 |
1976年 | 87篇 |
1975年 | 88篇 |
1974年 | 116篇 |
1973年 | 130篇 |
排序方式: 共有7927条查询结果,搜索用时 15 毫秒
91.
U. Berg R. Isaksson J. Sandström U. Sjöstrand A. Eiglsperger A. Mannschreck 《Tetrahedron letters》1982,23(41):4237-4240
(+)- and (?)- as well as (+)- and (?)- were partially separated by liquid chromatography on triacetylcellulose; racemizations resulted in the barriers (Table 2) to rotation via planarized transition states. 相似文献
92.
The crystal structure of a second high-pressure copper vanadate phase, CuVO3(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (Rw = 0.051). The space group is rhombohedral, , with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [aR = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu+V5+O3 is proposed. This represents a unique example of Cu+ in an octahedral environment. 相似文献
93.
Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio Te/Th. 相似文献
94.
Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu2AIIBIVX4, where AII = Zn, Mn, Fe, or Co, BIV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu2MnBX4 compounds decrease with increasing covalency of the MnX bond, and those of Cu2FeGeS4 and Cu2CoGeS4 reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu2MnGeS4, Cu2CoGeS4, and Cu2FeGeS4 indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu2ZnGeS4 doped nominally with 0.1% Mn reveal that Mn2+ experiences an axial distortion and that the bond ionicity is the same as in ZnS. 相似文献
95.
Several aspects of past and current studies in the area of iminium salt photochemistry are discussed. Investigations of olefin-iminium salt photoaddition and photocyclization reactions are reviewed and conclusions about electron-transfer pathways for fluorescence quenching and reaction are discussed. The results of recent studies of alcohol and ether photoaddition to 2-phenyl-1-pyrrolinium salts are presented. These C-C bond forming processes occur in moderate yields to produce β-amino alcohol or ether products. In addition, alcohols and ethers serve as efficient quenchers of pyrrolinium salt fluorescence. Rate constants for quenching appear to be dependent upon both the oxidation potential of the alcohols and ethers and the availability of C-H bond α to oxygen. This data along with deuterium isotope effects on quenching combine to suggest a common mechanism for both fluorescence quenching and photoaddition. The nature of this mechanism is tested using the comparative quenching effeciencies of the tertiary alcohols t-butyl alcohol and 1,2,2-trimethyl-1-cyclopropanol. The latter alcohol having a weak C-C bond adjacent to the hydroxyl function quenches the fluorescence of 2-phenyl-1-pyrrolium salts at a rate two orders of magnitude greater than for t-butyl alcohol. The observations made are interpreted in terms of a sequential electron-proton transfer mechanism for quenching and photoaddition. Lastly, the relationship of iminium salt photochemical studies to other investigations of electron-transfer photochemistry is discussed. 相似文献
96.
N.E. Kolobova A.B. Antonova O.M. Khitrova M.Yu. Antipin Yu.T. Struchkov 《Journal of organometallic chemistry》1977,137(1):69-78
The novel complexes CpRe(CCHPh)(CO)2 and Cp2Re2(μ-CCHPh)(CO)4 containing a terminal and a bridging phenylvinylidene ligand respectively and the binuclear complex Cp(CO)2Re[CC(Ph)C(Ph)CH2]Re(CO)2Cp were obtained in the reaction of CpRe(CO)3 with PhCCH.According to an X-ray study of the latter complex the unusual bridging ligand is η1-bonded to one Re atom and η2-bonded to the other. 相似文献
97.
James Y. Becker Larry L. Miller Frank R. Stermitz 《Journal of Electroanalytical Chemistry》1976,68(2):181-191
The electrochemical oxidation of the alkaloid laudanosine (Ia) to O-methylflavinantine (II) has been studied in acetonitrile solvent. Using cyclic voltammetry, rotating disc voltammetry and preparative electrolyses on several alkaloids, simple aliphatic amines and aromatic compounds, some aspects of the mechanism of this coupling reaction are elucidated. The first anodic wave for laudanosine at platinum has Ep=0.55 V vs. Ag/Ag+. The electrode rapidly becomes partially passivated at potentials above 0.5 V. This is due to a film which “dissolves” below 0.5 V, at a rate independent of the potential. It is shown that the reaction (Ia)→(II) proceeds at 0.5 V by initial oxidation of the amine moiety. If acids such as sodium bicarbonate are added to the anolyte the amine is protonated causing the first wave to disappear. Oxidation at 1.1 V under these acidic conditions produces the same product, but more rapidly and in significantly higher yield because electrode filming and side reactions resulting from the amine oxidation are abrogated. 相似文献
98.
Mary McPartlin 《Polyhedron》1984,3(12):1321-1328
The recently developed electron-counting rules for condensed polyhedral metal carbonyl clusters have been utilised to develop a systematic approach for enumerating the structural alternatives for metal cluster compounds which are isoelectronic. The more usual cluster degradation and aggregation processes are represented in a matrix form which relates the change in electron count to the number of new bonds formed (or broken). The matrices for cluster degradation and aggregation are represented in a complementary fashion so that the search for isoelectronic clusters greatly simplified. The utility of the approach is illustrated by reference to some recent examples from metal carbonyl cluster chemistry. 相似文献
99.
C. Mioskowski A. Solladie-Cavallo G. Solladie 《Journal of organometallic chemistry》1978,149(3):C63-C65
Attachment of a Cr(CO)3 moiety to one of the aromatic rings in rigid diaryl ketones of very low prochirality followed by addition of a chiral and optically pure reagent leads to satisfactory asymmetric induction ~40%. Asymmetric induction is smaller in the case of acyclic systems. 相似文献
100.
The kinetics of the reaction between organocobaloximes, RCo(DH)2H2O, and iodine have been investigated. They reveal the participation of an RCo(DH)2H2O · I2 intermediate which undergoes intramolecular transalkylation and acts as an electrophile towards a second organocobaloxime molecule. The trend in reactivity as the R group is varied is discussed. 相似文献