全文获取类型
收费全文 | 4565篇 |
免费 | 121篇 |
国内免费 | 16篇 |
专业分类
化学 | 2882篇 |
晶体学 | 13篇 |
力学 | 236篇 |
数学 | 797篇 |
物理学 | 774篇 |
出版年
2023年 | 50篇 |
2022年 | 138篇 |
2021年 | 132篇 |
2020年 | 109篇 |
2019年 | 97篇 |
2018年 | 82篇 |
2017年 | 91篇 |
2016年 | 185篇 |
2015年 | 143篇 |
2014年 | 139篇 |
2013年 | 245篇 |
2012年 | 258篇 |
2011年 | 286篇 |
2010年 | 171篇 |
2009年 | 150篇 |
2008年 | 279篇 |
2007年 | 268篇 |
2006年 | 221篇 |
2005年 | 236篇 |
2004年 | 162篇 |
2003年 | 156篇 |
2002年 | 112篇 |
2001年 | 51篇 |
2000年 | 47篇 |
1999年 | 45篇 |
1998年 | 32篇 |
1997年 | 31篇 |
1996年 | 53篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 39篇 |
1986年 | 23篇 |
1985年 | 39篇 |
1984年 | 38篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 26篇 |
1980年 | 27篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 28篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1858年 | 11篇 |
排序方式: 共有4702条查询结果,搜索用时 21 毫秒
991.
992.
Giancarlo D'Ambrosio Mario Miragliuolo Francesco Sannino 《Zeitschrift fur Physik C Particles and Fields》1993,59(3):451-456
We discuss the decayK
s
+– in the framework of chiral perturbation theory. The direct emission in this decay can be an useful test of meson dynamics. 相似文献
993.
Giovanni Piccardi Francesco Pergola Maria Luisa Foresti Rolando Guidelli 《Journal of Electroanalytical Chemistry》1977,84(2):235-253
Polarographic current-potential characteristics and current-time curves for the reduction of methylene blue (MB) to methylene blue leucoform (MBL) in a pH 7.9 aqueous phosphate buffer have been examined in detail over a wide concentration range. It has thus been shown that the so called “normal” or “main” reduction wave of MB actually consists of two separate steps, the former with a half-wave potential practically coinciding with the formal potential E0=?0.250 V/SCE of the MB/MBL couple and the latter with a half-wave potential of about ?0.310 V/SCE. As soon as the well-known MB adsorption prewave has attained its maximum height (which occurs at a MB bulk concentration c0*?5×10?5M), a further slight increase in c0* causes the appearance of the wave with E1/2=?0.310 V. The height of the linear potential-sweep voltammetric peak corresponding to the latter wave increases proportionally to the sweep rate, thus revealing the “adsorption” nature of this wave. A comparison with a previous chronocoulometric investigation of the MB/MBL system has permitted us to conclude that the wave with E1/2=?0.310 V is due to reduction of the MB molecules which, after having reached the surface of the dropping electrode by diffusion, are adsorbed at the top of the adsorbed monolayer of MBL in direct contact with the electrode and remain in this adsorbed state after reduction. For c0*>7×10?5M the wave with E1/2?E0=?0.250 V starts to develop. This wave is due to the electroformation of MBL molecules which diffuse back into the solution. The shape of polarographic current-potential characteristics and current-time curves has been accounted for semiquantitatively through an approximate solution of the corresponding diffusional problem. 相似文献
994.
A 13C nmr study of a diazo, of two diazonium salts and of some other model pyrazole compounds is reported. It is found that the C-4 diazo carbon is more shielded than the normal sp2 pyrazole hybridized carbon, while its diazonium salt is more deshielded. The electron density on the C-4 of the diazopyrazole is probably due to the positive charge on the terminal nitrogen. Thus the 13C chemical shift values allowed us to understand the chemical behaviour of a compound of potential antitumor activity. 相似文献
995.
Cappelli A Pericot Mohr Gl Anzini M Vomero S Donati A Casolaro M Mendichi R Giorgi G Makovec F 《The Journal of organic chemistry》2003,68(24):9473-9476
A new polymer based on a functionalized benzofulvene moiety has been synthesized by spontaneous polymerization of the monomer in the solid state. This polymer shows a very high molar mass, high solubility in the most common organic solvents, and thermoreversible polymerization properties. An interesting application in synthesis is reported. 相似文献
996.
Veaceslav Kulci?ki Nicon Ungur Margherita Gavagnin Francesco Castelluccio Guido Cimino 《Tetrahedron》2007,63(32):7617-7623
The functionalization of the B-ring of the scalarane framework has been achieved for the first time by a radical relay halogenation (RRH) synthetic method. The known scalaranic methyl ester, which was prepared by a procedure with an overall yield higher than those reported in the literature, has been used as the starting substrate. Some theoretical considerations explaining the course of RRH reaction are also presented. 相似文献
997.
From the ciliate Euplotes focardii, collected from Ross Sea coastal waters, Antarctica, a new-skeleton diterpenoid, epoxyfocardin, was isolated as a 85:15 mixture of hemiacetals 8a and 8b . The absolute configuration of 8a/8b was determined from Mosher's esters 11a/11b and 12a/12b . Focardin 9a/9b , most likely a biogenetic precursor of 8a/8b , was also isolated as a minor component. Focardin, and particularly epoxyfocardin, proved to be toxic towards representatives of ciliate communities from Antarctic, temperate, tropical, and equatorial environments, constituting the first example of ecologically relevant metabolites from ciliate species that inhabit polar ecosystems. 相似文献
998.
[reaction: see text] A stereoselective/stereospecific synthesis of polysubstituted tetrahydronaphthols based on the Michael addition of ortho-lithiated stilbene oxides to alpha,beta-unsaturated Fischer carbene complexes followed by an unusual cyclization of the corresponding intermediate in a 6-endo-tet mode is described. 相似文献
999.
Boltzmann-like distributions appear in many properties and energy-related quantities of proteins. A few examples are hydrophobicity, various types of side-chain/side-chain interactions, proline isomerization, hydrogen bonds, internal cavities, interactions at the level of specific atom types, and the propensity of the phi/'phi' ratio. Here, we conjecture that the Boltzmann hypothesis also holds for the intra-residue energy distribution. We confirm the conjecture by calculating the energies of 41,672 residues of the structures of highly resolved proteins, where at least 12 out of 20 naturally occurring amino acids follow Boltzmann's law. We further examine the entire set of all residue energies and find that the convolution of the individual distributions gives a Poisson function, which is followed by approximately 50% of individual proteins' structures. 相似文献
1000.