全文获取类型
收费全文 | 242641篇 |
免费 | 2409篇 |
国内免费 | 869篇 |
专业分类
化学 | 130161篇 |
晶体学 | 3689篇 |
力学 | 9889篇 |
综合类 | 2篇 |
数学 | 27044篇 |
物理学 | 75134篇 |
出版年
2020年 | 1764篇 |
2019年 | 2035篇 |
2018年 | 2469篇 |
2017年 | 2400篇 |
2016年 | 3816篇 |
2015年 | 2588篇 |
2014年 | 3889篇 |
2013年 | 11144篇 |
2012年 | 8065篇 |
2011年 | 9987篇 |
2010年 | 6563篇 |
2009年 | 6459篇 |
2008年 | 8970篇 |
2007年 | 9120篇 |
2006年 | 8364篇 |
2005年 | 7822篇 |
2004年 | 6978篇 |
2003年 | 6200篇 |
2002年 | 6129篇 |
2001年 | 7196篇 |
2000年 | 5357篇 |
1999年 | 4215篇 |
1998年 | 3547篇 |
1997年 | 3575篇 |
1996年 | 3287篇 |
1995年 | 3128篇 |
1994年 | 2944篇 |
1993年 | 2999篇 |
1992年 | 3293篇 |
1991年 | 3327篇 |
1990年 | 3143篇 |
1989年 | 3098篇 |
1988年 | 3159篇 |
1987年 | 3029篇 |
1986年 | 2924篇 |
1985年 | 3985篇 |
1984年 | 4145篇 |
1983年 | 3399篇 |
1982年 | 3749篇 |
1981年 | 3645篇 |
1980年 | 3517篇 |
1979年 | 3551篇 |
1978年 | 3777篇 |
1977年 | 3606篇 |
1976年 | 3814篇 |
1975年 | 3387篇 |
1974年 | 3516篇 |
1973年 | 3819篇 |
1972年 | 2327篇 |
1971年 | 1786篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The intramolecular Friedel-Crafts cyclisation of ω-arylalkanoic acids, which is in the benzene series a valuable synthetic route to bicyclic ketones with medium-sized and large rings, has been investigated with ω-(1-naphthyl)alkanoyl chlorides. The lower members, including naphthyl-caproic acid, close the ring at position 2 to form IV. The higher homologous acids show a preference for annellation at 7, making available a new type of heteronuclear naphthalene cyclic ketones (VII). During cyclisation of ω-(1-naphthyl) decanoic acid, the 1,4 ring closure competes with the 1,7 type.
Steric hindrance of resonance as a consequence of “medium ring torsion” was studied in different classes of naphthocyclenones. 相似文献
992.
L. A. Shchukina R. G. Vdovina Yu. B. Shvetsov A. V. Karpova 《Russian Chemical Bulletin》1962,11(2):286-288
Summary A preparative method was developed for obtaining L-and D-2-hydroxy-3-methylbutyric acids by the separation of racemic 2-hydroxy-3-methylbutyric acid with the aid of D-and L-threo-2-amino-1-p-nitrophenyl-1,3-propanediols. 相似文献
993.
A rapid and selective method has been developed for the determination of cadmium in environmental samples by thermal neutron activation analysis, employing substoichiometric solvent extraction technique. Alcoholic solution of 2-mercaptobenzothiazole /2-HMBT/ has been used for the substoichiometric extraction of Cd/II/ from an aqueous solution of pH 7.0 into methyl iso-butyl ketone /MIBK/. 相似文献
994.
995.
996.
V. I. Khleskov B. N. Burykin R. É. Garibov 《Theoretical and Experimental Chemistry》1985,21(2):139-145
A quantum-chemical analysis of the electronic structure of two low-spin complexes, viz., bis(imidazole)hemichrome [Fe(III)P]2Im (S = 1.2) and bis(imidazole)hemochrome [Fe(II)P]·2Im (S = 0), which simulate hemi- and hemoproteins (P denotes porphyrin, Im denotes imidazole, and S is the spin of the iron ion), has been carried out by the extended Hückel method with SCF convergence with respect to the charges of the atoms. The data on the electronic structure of the complexes have been used for calculations of the quadrupole splitting in the57Fe Mössbauer spectra of these compounds. An analysis of the contributions of the valence electrons of the Fe atom and the ligands to the quadrupole splitting as a function of the structure of the iron ion, its electronic environment, and the parameters of the first coordination sphere has been carried out. Good agreement between the calculated and experimental values of the quadrupole splitting has been obtained. An analysis of the orbital interactions has been carried out, and data on the classification and composition of the MO's have been presented.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 146–153, March–April, 1985. 相似文献
997.
998.
R. S. Él'kinson A. V. Eremeev É. E. Liepin'sh 《Chemistry of Heterocyclic Compounds》1985,21(5):521-525
The reaction of 2H-azirine with a number of dihydroxy thiols and dithiols leads to aziridin-2-yl alkyl sulfides. The latter readily isomerize to cyclic sulfides the size of the ring of which is determined by the length of the alkyl sulfide chain.For Communication 1, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 623–627, May, 1985. 相似文献
999.
1000.