全文获取类型
收费全文 | 7375篇 |
免费 | 244篇 |
国内免费 | 12篇 |
专业分类
化学 | 5454篇 |
晶体学 | 138篇 |
力学 | 241篇 |
数学 | 365篇 |
物理学 | 1433篇 |
出版年
2023年 | 59篇 |
2022年 | 85篇 |
2021年 | 112篇 |
2020年 | 159篇 |
2019年 | 195篇 |
2018年 | 164篇 |
2017年 | 131篇 |
2016年 | 250篇 |
2015年 | 160篇 |
2014年 | 291篇 |
2013年 | 654篇 |
2012年 | 492篇 |
2011年 | 512篇 |
2010年 | 315篇 |
2009年 | 281篇 |
2008年 | 437篇 |
2007年 | 396篇 |
2006年 | 317篇 |
2005年 | 297篇 |
2004年 | 259篇 |
2003年 | 206篇 |
2002年 | 181篇 |
2001年 | 101篇 |
2000年 | 100篇 |
1999年 | 80篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 79篇 |
1995年 | 54篇 |
1994年 | 65篇 |
1993年 | 73篇 |
1992年 | 56篇 |
1991年 | 51篇 |
1990年 | 49篇 |
1989年 | 49篇 |
1988年 | 46篇 |
1987年 | 56篇 |
1986年 | 49篇 |
1985年 | 56篇 |
1984年 | 70篇 |
1983年 | 49篇 |
1982年 | 48篇 |
1981年 | 54篇 |
1980年 | 42篇 |
1979年 | 45篇 |
1978年 | 29篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1975年 | 33篇 |
1973年 | 18篇 |
排序方式: 共有7631条查询结果,搜索用时 31 毫秒
141.
142.
K. Premchand B. Seetharami Reddy B. Mallikarjuna Rao K. Parthasaradhi M. V. R. Murti K. S. Rao 《Il Nuovo Cimento D》1993,15(1):73-76
Summary An attempt is made to study theK
α
L
1 satellite multiplet structure of chlorine atoms in different compounds by photon excitation. A comparison of the averageK
α
L
1 group energy shift from the diagram line with those based on Dirac-Fock evaluations with different number ofM-shell electrons shows a reasonable agreement. The
intensity ratios are found to be higher than those studied in Mg compounds in general. 相似文献
143.
144.
145.
S Chandrasekhar G Chandrashekar B Nagendra Babu K Vijeender K Venkatram Reddy 《Tetrahedron letters》2004,45(28):5497-5499
A facile synthesis of symmetrical and unsymmetrical ethers is achieved by reductive coupling of carbonyl compounds with alkoxysilanes. This reaction is performed using inert polymethylhydrosiloxane as the hydride source and B(C6F5)3 as the catalytic activator of the PMHS. 相似文献
146.
Dulal C. Ghosh 《Journal of Chemical Sciences》1984,93(1):33-46
A cndo/2D study of the charge distribution obtained through Mulliken population analysis in the ground state of the title
compounds shows that the features of charge distribution found by severalab initio calculations are fairly well reproduced by this method. The one-particle density, the interference density at the mid-point
of the bond axis and the kinetic part of the interference energy calculated through the deorthogonalized density matrices
over a wide range of intermolecular separation between the donor and the acceptor show that the one-particle density and the
interference density steadily grow with decreasing internuclear separation, while the kinetic interference energy starts with
negative value at large distance, then decreases and passes through a minima near but above the equilibrium distance and then
increases rapidly below it conforming to the characteristic general behaviour of the kinetic component of Morse curve. The
orbital pairwise interference density and the corresponding kinetic energy components reveal that the orbitals involved in
the covalent binding are σ2p AO of B and 2S and σ2p AO of N and C atoms in H3B-NH3 and H3B-CO respectively. 相似文献
147.
[reaction: see text] Chiral beta-syn-alkoxyhomoallylic alcohols derived from alkoxyallylboration of aldehydes upon oxidation provided the corresponding chiral ketones. Chelation-controlled nucleophilic addition to these ketones occurred in a highly stereoselective manner to afford anti-homoallylic tertiary alcohols. This methodology has been applied for the synthesis of the C(1)-C(11) subunit of C(8)-epi-fostriecin. 相似文献
148.
The synthesis and structure-activity-relationship (SAR) for a series of N-substituted piperazinyl carbamoyl 7-15 and piperazinyl acetyl 18-26 derivatives of tetrahydropapaverine have been carried out. The general synthetic methods of carbamoyl tetrahydropapaverine analogues involve N-substituted piperazines and carbamoyl imidazole tetrahydropapaverine as starting materials. Another route for synthesizing these compounds, involving the formation of carbamoyl imidazole piperazine has also been explored. Acylation of tetrahydropapaverine followed by substitution with various piperazinyl moities afforded the acetyl tetrahydropapaverine derivatives. Variously substituted piperazines have been used to monitor the effect of electron releasing and electron withdrawing substituents upon the antispasmodic activity of the molecules. Effect of varying electron densities on the antispasmodic activity, by altering the position of these groups on the benzene ring has also been monitored. Pharmacological methods involve the in vitro antispasmodic activity studies on a freshly removed guinea pig ileum using a force displacement transducer amplifier connected to a physiograph. Among the analogues synthesized in the present study, a promising compound 7, a potent muscle relaxant as compared to papaverine has been obtained. 相似文献
149.
S. Pitchumani C. Rami Reddy S. Rajadurai 《Journal of polymer science. Part A, Polymer chemistry》1982,20(2):277-282
Ethyl acrylate (EA) and n-butyl methacrylate (n-BMA) copolymers were prepared in solution and the composition of the copolymer samples was estimated by 1H-NMR spectroscopic techniques. Because the characteristic signals, which vary with the composition of the copolymer, were absent, the ratio of intensities of down-field protons to that of the total protons was used for the estimation of copolymer composition. Reactivity ratios were calculated from these values by using the Kelen-Tudos differential linear equation. 相似文献
150.
In catalytic concentrations (10?5?10?4 mol l?1) sulphur dioxide induces polymerization of MMA, particularly on photoactivation. The effective initiating species appears to be the monomer-SO2 complex rather than free SO2. A mechanism involving biradical initiation by decomposition of the initiating species, linear propagation in two directions, and significant termination of growing chains by chain transfer with initiating species has been suggested. The initiator transfer constant is 1.6 at 40°. 相似文献