全文获取类型
收费全文 | 156397篇 |
免费 | 1429篇 |
国内免费 | 418篇 |
专业分类
化学 | 84681篇 |
晶体学 | 2042篇 |
力学 | 6754篇 |
综合类 | 7篇 |
数学 | 15742篇 |
物理学 | 49018篇 |
出版年
2020年 | 1154篇 |
2019年 | 1213篇 |
2018年 | 1505篇 |
2017年 | 1466篇 |
2016年 | 2520篇 |
2015年 | 1604篇 |
2014年 | 2529篇 |
2013年 | 6480篇 |
2012年 | 5114篇 |
2011年 | 6472篇 |
2010年 | 4397篇 |
2009年 | 4396篇 |
2008年 | 5853篇 |
2007年 | 5938篇 |
2006年 | 5596篇 |
2005年 | 5157篇 |
2004年 | 4557篇 |
2003年 | 3979篇 |
2002年 | 4025篇 |
2001年 | 4402篇 |
2000年 | 3384篇 |
1999年 | 2614篇 |
1998年 | 2275篇 |
1997年 | 2227篇 |
1996年 | 2032篇 |
1995年 | 1938篇 |
1994年 | 1922篇 |
1993年 | 1767篇 |
1992年 | 2022篇 |
1991年 | 2151篇 |
1990年 | 1950篇 |
1989年 | 1944篇 |
1988年 | 1878篇 |
1987年 | 1778篇 |
1986年 | 1720篇 |
1985年 | 2243篇 |
1984年 | 2374篇 |
1983年 | 1935篇 |
1982年 | 2123篇 |
1981年 | 2004篇 |
1980年 | 1935篇 |
1979年 | 2085篇 |
1978年 | 2245篇 |
1977年 | 2105篇 |
1976年 | 2156篇 |
1975年 | 2044篇 |
1974年 | 2099篇 |
1973年 | 2089篇 |
1972年 | 1372篇 |
1971年 | 1227篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
A. S. Atavin A. N. Mirskova É. F. Zorina T. S. Proskurina 《Chemistry of Heterocyclic Compounds》1973,9(12):1456-1458
The photochemical chlorination of 2-trichloromethyl-1,3-dioxolane proceeds only at the carbon atoms in the 4 and 5 positions. The structures of the resulting 2-trichloromethyl-4-chloro-and 2-trichloromethyl-4,5-dichloro-1,3-dioxolanes were proved by means of the IR spectra and chemical transformations.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1611–1613, December, 1973. 相似文献
962.
M. F. Shostakovskii A. S. Atavin M. A. Ignat'eva A. V. Gusarov B. A. Trofimov 《Russian Chemical Bulletin》1969,18(5):1082-1084
Conclusions Vinyloxyethyltrialkylammonium salts possessing strong hypertensive activity were prepared by reacting 2-(dialkylamino)ethyl vinyl ethers with alkyl iodides and 2-halogenoethyl vinyl ethers with tertiary amines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1178–1180, May, 1969. 相似文献
963.
The dipole strengths of the 4A2 → 4T1(F), 4T1(P), d-electron transitions of the cobalt(II) tetrahalides are found to decrease with an increase in temperature, in agreement with a dynamic ligand-polarisation model for the absorption mechanism. The reduction in dipole strength is accounted for by a decrease in the coulombic coupling between the quadrupole moment of the d-electron transition of the metal ion and the transient electric dipole moment induced in the ligands, due to the anharmonic increase in the mean bond length and the progressively larger mean-square amplitudes of the bending modes of the complex ion in its ground electronic state as the temperature is raised. 相似文献
964.
A liquid chromatographic analysis with electrochemical detection of 5-methoxytryptamine, 5-methoxytryptophol, 5-methoxyindoleacetic acid and melatonin is described. Optimal elution conditions were determined by studying several variables: pH, buffer salt, counter ion and organic modifier. Measurement of 5-methoxyindoles in the pineal gland and plasma of hamsters has been performed after extraction. This method is specific and sensitive, and enables detection of 5-methoxyindoles in a pool of two hamster pineal glands. This is also the first time that these three 5-methoxyindoles have been measured simultaneously in plasma. 相似文献
965.
Photolysis of (η5-C5H5)2V(CH3)2 results in the formation of methane and ethane. The percentage of ethane detected in the evolved gas can vary from a trace to 70%. The percentage of ethane formed is dependent on the solvent, the concentration of (η5-C5H5)2V(CH3)2, time and temperature. The results of oxidation and reduction reactions involving (η5-C5H5)2V(CH3)2 are also presented. 相似文献
966.
Zusammenfassung Je nach den Reaktionsbedingungen entstehen verschiedene Produkte bei der Reduktion des 4,10-(-Hydroximino-trimethylen)-phenthiazins. Durch Vergleich mit Verbindungen bekannter Struktur wurde der Beweis erbracht, daß die von uns schon früher für das Produkt der Natriumamalgamreduktion angenommene Struktur richtig ist.Mit 2 Abbildungen 相似文献
967.
Quantum chemical study of the appearance of the “α-effect” in35Cl NQR spectra of chloromethylsilanes
We consider the connection between the electron distribution in ClCH2MX1X2X3 molecules (M = C, Si) calculated by the CNDO/2 method and the experimental35Cl NQR frequencies. We show that the ratio of the NQR frequencies for chloromethylsilanes and their organic analogs can be explained on the basis of ideas concerning the totality of induction and conjugation effects influencing the population of the orbital of the chlorine atom which participates in formation of the C-Cl bond.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 2, pp. 241–244, March–April, 1990. 相似文献
968.
A new speetrofluorimetric method for determination of ultratraces of lead is based on solvent extraction into chloroform of the ion-pair formed between the positively-charged cryptate of lead with cryptand 2.2.2 and the eosinate anion. The detection limit for lead is 1 ng ml , and the linear working range is from the detection limit to 250 ng ml . The relative standard deviation is 3.7% at the 100 ng ml level. The method is highly selective for the extraction and determination of lead in the presence of other cations, and has been tested for direct determination of lead contamination in soft drinks. Aggregation of the extracted ion-pair in the organic phase has been demonstrated in fundamental extraction studies. 相似文献
969.
The 1-(3,4-methylenedioxyphenyl)-3-butanamines (HMDAs) are prepared via reductive amination of the corresponding ketone with a series of low molecular weight alkylamines. These amines are homologues of the N-substituted 3,4-methylenedioxyamphetamines (MDAs). Compounds of the HMDA series have UV absorption properties similar to the MDAs because both series contain the same 3,4-methylenedioxyphenyl chromophore. The HMDAs are separated via reversed-phase liquid chromatographic methods using a C18 stationary phase and an acidic aqueous acetonitrile mobile phase. The mass spectra of these potential designer drugs are very similar to the spectra of the MDA homologues having the same N-substituent. 相似文献
970.