全文获取类型
收费全文 | 203869篇 |
免费 | 1322篇 |
国内免费 | 429篇 |
专业分类
化学 | 105448篇 |
晶体学 | 3396篇 |
力学 | 10273篇 |
综合类 | 10篇 |
数学 | 21879篇 |
物理学 | 64614篇 |
出版年
2021年 | 2011篇 |
2020年 | 2033篇 |
2019年 | 2601篇 |
2018年 | 3672篇 |
2017年 | 3693篇 |
2016年 | 4840篇 |
2015年 | 2372篇 |
2014年 | 4330篇 |
2013年 | 8628篇 |
2012年 | 6902篇 |
2011年 | 8122篇 |
2010年 | 6493篇 |
2009年 | 6673篇 |
2008年 | 7677篇 |
2007年 | 7655篇 |
2006年 | 6741篇 |
2005年 | 5957篇 |
2004年 | 5660篇 |
2003年 | 5186篇 |
2002年 | 5177篇 |
2001年 | 5610篇 |
2000年 | 4149篇 |
1999年 | 3139篇 |
1998年 | 2707篇 |
1997年 | 2760篇 |
1996年 | 2545篇 |
1995年 | 2210篇 |
1994年 | 2327篇 |
1993年 | 2227篇 |
1992年 | 2524篇 |
1991年 | 2646篇 |
1990年 | 2556篇 |
1989年 | 2625篇 |
1988年 | 2533篇 |
1987年 | 2555篇 |
1986年 | 2412篇 |
1985年 | 3035篇 |
1984年 | 3105篇 |
1983年 | 2598篇 |
1982年 | 2562篇 |
1981年 | 2531篇 |
1980年 | 2336篇 |
1979年 | 2708篇 |
1978年 | 2674篇 |
1977年 | 2870篇 |
1976年 | 2875篇 |
1975年 | 2665篇 |
1974年 | 2602篇 |
1973年 | 2712篇 |
1972年 | 2086篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
861.
Chmutova G. A. Zverev V. V. Pudovik M. A. Khailova N. A. Pudovik A. N. 《Russian Journal of General Chemistry》2003,73(11):1696-1701
The stability of phosphorylated ureas and thioureas containing various substituents at the P and N atoms and the stability of their cyclization products, the corresponding diazaphospholidines and/or oxaza(thiaza)phospholines, was evaluated by PM3, MNDO, and DFT calculations. The thermal effects of cyclizations involving phosphorylated ureas (or thioureas) and of separate steps of their synthesis were calculated. According to the calculations, diazaphospholidine structures are preferably formed from substituted ureas and thiazaphospholine structures, from their thio analogs; formation of five-membered heterorings with the P-N bond is thermodynamically preferable as compared to their six-membered analogs. 相似文献
862.
p-Toluenesulfinic acid is an effective catalyst for condensation of acetoacetic ester with urea and aliphatic or aromatic aldehydes, leading to formation of 4-substituted 6-methyl-5-ethoxycarbonyl-1,2,3,4-tetrahydropyrimidin-2-ones.M. V. Lomonosov State Academy of Fine Chemical Technology, Moscow 117571, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 979–982, July, 1998. 相似文献
863.
L. N. Raevskaya N. I. Il’chenko Yu. I. Pyatnitskii N. V. Pavlenko 《Theoretical and Experimental Chemistry》1997,33(2):78-81
The catalytic transformation of methane into higher hydrocarbons was studied under unsteady conditions on oxides of various
metals (Pb, Fe, Nb, Ti, Cd, Mo, Mg, Zn, Zr, Bi, Mn), and also on Sr-, La-, Y-, and Mn-containing oxides of perovskite structure.
It is shown that the effectiveness of the catalysts not only depends on their chemical composition, but also is determined
to a considerable degree by the specific surface of the oxides.
L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of Ukraine, 252039 Kiev, Prosp. Nauki, 31, Ukraine.
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 2, pp. 91–95, March–April, 1997. 相似文献
864.
A. L. Vereshchagin N. N. Pogodaeva A. A. Semenov 《Chemistry of Heterocyclic Compounds》1987,23(12):1302-1304
A spectrophotometric study of the acid-base properties of 3-acylindoles showed that they were stronger acids than the alkylindoles. It was found that protonation of these compounds occurs at the carbonyl group. For indolylglyoxal, 3-acetylindole, and 1-methyl-3-acetylindole the pKb was determined; the pKafor indolylglyoxal and 3-acetylindole was measured in 50% aqueous methanol.Translated from Khimiya Geterotsiklicheskhikh Soedinenii, No. 12, pp. 1621–1624, December, 1987. 相似文献
865.
S. V. Ketchik L. M. Plyasova A. E. Seredkin V. V. Kostrov L. N. Morozov 《Reaction Kinetics and Catalysis Letters》1980,14(4):429-434
Structural peculiarities of the formation of copper-aluminium catalysts at copper concentrations <10 wt% and calcination temperatures of 573–1173 K have been studied.
<10% . 573–1173°K.相似文献
866.
The concentration dependence of the apparent molar volumes of lithium halides (and electrolytes in general) in alcohols (and
solvents permitting association in general) is, in the first instance, due to changes in the degree of association and to
the inherent difference between the apparent molar volumes of the ions and of the ion pairs. Previous publications on the
molar volumes of electrolytes in organic solvents, disregarding altogether ion pairing, appear to be incorrect. Data from
the literature for lithium chloride and lithium bromide in normal primary alcohols and several branched alcohols from C1 to C8 and data from our laboratory for lithium halides in 1-hexanol and 2-ethyl-1-hexanol served for the determination of φ
V
and φ
E
. Electrical and structural contributions to the values of these functions for the ions and for the ion pairs are discussed. 相似文献
867.
The free NH3 molecule and the [Zn(NH3)4]2+ ion were studied by the kinematic coupling approach. The pure effects of this coupling were found to be small, and some modifications had to be introduced in order to get a reasonable force field. The force constants deduced for the skeletal vibrations are comparable with those of a quasi-exact force field. Calculated frequencies for [68Zn(NH3)4]2+ and [64Zn(ND3)4]2+ are reported in addition to those of [64Zn(NH3)4]2+. Mean amplitudes of vibration for [64Zn(NH3)4]2+ are given. 相似文献
868.
Chloride1), borohydrides2) and hydrides3) of dicyclopentadienyl thallium (III) and diindenyl thallium (III) have already been reported. The present communication deals with a study on the preparation and characterization of difluorenyl thallium (III) chloride, -hydride and -borohydride. 相似文献
869.
Miss Itawati Pratanata L. R. Williams R. N. Williams 《Journal of mass spectrometry : JMS》1974,9(4):418-421
When subjected to electron-impact, chloromethyl aryl sulphones (II, X = Cl) fragment predominantly by the loss of CH2Cl from the molecular ion followed by the loss of SO2 and in most cases the appropriate metastable peaks are present to confirm the transitions. In addition, alkyl–oxygen bond formation in the molecular ion was revealed by the presence of prominent peaks corresponding to the [R? ?-SO]+ ions. In most spectra no evidence for aryl–oxygen bond formation could be found and thus the presence of the chlorine atom was able to effectively reverse the direction of skeletal rearrangement reported for aryl methyl sulphones. 相似文献
870.