全文获取类型
收费全文 | 44134篇 |
免费 | 3622篇 |
国内免费 | 3227篇 |
专业分类
化学 | 31156篇 |
晶体学 | 325篇 |
力学 | 1073篇 |
综合类 | 641篇 |
数学 | 7561篇 |
物理学 | 10227篇 |
出版年
2023年 | 447篇 |
2022年 | 1673篇 |
2021年 | 1963篇 |
2020年 | 1619篇 |
2019年 | 1433篇 |
2018年 | 996篇 |
2017年 | 1008篇 |
2016年 | 1422篇 |
2015年 | 1450篇 |
2014年 | 1730篇 |
2013年 | 2756篇 |
2012年 | 1874篇 |
2011年 | 2044篇 |
2010年 | 1831篇 |
2009年 | 2280篇 |
2008年 | 2371篇 |
2007年 | 2487篇 |
2006年 | 1874篇 |
2005年 | 1249篇 |
2004年 | 1180篇 |
2003年 | 1217篇 |
2002年 | 3716篇 |
2001年 | 1300篇 |
2000年 | 894篇 |
1999年 | 692篇 |
1998年 | 723篇 |
1997年 | 515篇 |
1996年 | 571篇 |
1995年 | 494篇 |
1994年 | 522篇 |
1993年 | 520篇 |
1992年 | 542篇 |
1991年 | 353篇 |
1990年 | 322篇 |
1989年 | 280篇 |
1988年 | 282篇 |
1987年 | 198篇 |
1986年 | 226篇 |
1985年 | 331篇 |
1984年 | 246篇 |
1983年 | 161篇 |
1982年 | 297篇 |
1981年 | 474篇 |
1980年 | 427篇 |
1979年 | 478篇 |
1978年 | 379篇 |
1977年 | 283篇 |
1976年 | 238篇 |
1974年 | 107篇 |
1973年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
421.
422.
Thomas C. Amu 《Journal of solution chemistry》1981,10(1):69-78
Interdiffusion coefficient have been measured for 1,1,2,2-tetrabromoethane in 21 organic solvents in order to ascertain the influence of shape, size, viscosity, nature and degree of branching, of the solvent molecules on transport properties. None of the existing correlations for diffusion in binary liquid systems appear to fit all the experimental data. Steric effects due to the solvent molecules are shown to be very important in the interpretation of the diffusion process. 相似文献
424.
Monot Frédéric Benoit Yves Ballerini Daniel Vandecasteele Jean-Paul 《Applied biochemistry and biotechnology》1990,(1):375-386
Utilization of lipases for synthesis of esters of hydrophilic polyols has been investigated. The choice of a suitable solvent
is crucial in this type of reaction. An interesting case is fatty acid esters from neopentylpolyols, such as trimethylolpropane,
which are of great interest as high temperature lubricants. Enzymatic synthesis of trimethylolpropane tricaprylate was studied
as an alternative to chemical manufacturing. Triester production occurred only if the water produced by esterification was
continuously removed from the medium. In these condition, kinetics of appearance and transformation of mono-, di- and triesters
were determined in order to define optimal conditions. 相似文献
425.
Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances. 相似文献
426.
427.
428.
The Fermi and Coulomb holes of the 21
S state of the helium isoelectronic sequence are investigated. Several interesting differences between the results obtained and those which might be expected on the basis of the corresponding 23
S state are pointed out and discussed. 相似文献
429.
A. V. Belyakov A. V. Golubinskii L. V. Vilkov V. I. Shiryaev E. M. Styopina E. A. Kovalyova V. S. Nikitin 《Russian Chemical Bulletin》1993,42(2):346-349
The molecular structure of gaseous Br2Sn(CH2SiMe2)2O was studied by electron diffraction. The six-membered ring has a chair conformation whereas the entire molecule possessesC
s
symmetry. The existence of a boat conformer cannot be completely excluded. The results of theoretical calculations for a twisted-boat conformation are at variance with the experimental data. Steric strain caused by mutual repulsion of the two axial methyl groups is reduced to the tilt of the Me2Si fragments in opposite directions. This results in an increase (up to 26°C) in the angle formed by the bisector of the CM-Si-CM angle with the CcSiO plane. The main geometrical parameters are as follows:r
g (Å): Si-O 1.708(20); Si-CM 1.862(20); Si-Cc 1.882(9); Sn-C 2.108(26); Sn-Br 2.456(3); C-H 1.099(30); (degr.): C-Sn-C 105(2); Br-Sn-Br 107.9(1.2); Si-O-Si 129.6(3); CM-Si-CM 112; Si-C-H 113 (fixed value in accordance with experiment); Cc-Si-O 107(2); Sn-C-Si 109(2); torsion angles: (Si-C) 52(2); (Si-O) 62(1); (Cc-Sn) 54(1). The average amplitudes were fixed at the values calculated from the force field. Structural parameters of molecules with similar structures were analyzed and compared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 384–387, February, 1993. 相似文献
430.
M. M. Krayushkin M. A. Kalik L. G. Vorontsova E. Yu. Zvezdina M. G. Kurella 《Russian Chemical Bulletin》1993,42(7):1204-1208
9-Methyl-3,3a,4,5-tetrahydro-6H-thieno[2,3-b]thiocino[4,5-c]isoxazole and 9-hydroxyimino-2-methyl-7-methylene-6, 7,8,9-tetrahydro-5H-thieno[2,3-b]thiocine have been prepared for the first time by the oxidation of 2--alkenylthio-3-thiophenecarbaldoximes with NaOCl. X-ray structural investigations of thieno[2,3-b]thiocine and 2-homomethallylthio-5-methyl-3-thiophenecarbaldoxime have been carried out.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1262–1266, July, 1993. 相似文献