全文获取类型
收费全文 | 52037篇 |
免费 | 10229篇 |
国内免费 | 1436篇 |
专业分类
化学 | 47641篇 |
晶体学 | 357篇 |
力学 | 635篇 |
综合类 | 1篇 |
数学 | 4378篇 |
物理学 | 3566篇 |
无线电 | 7124篇 |
出版年
2021年 | 190篇 |
2020年 | 1817篇 |
2019年 | 3241篇 |
2018年 | 1486篇 |
2017年 | 1042篇 |
2016年 | 4019篇 |
2015年 | 4142篇 |
2014年 | 4134篇 |
2013年 | 4903篇 |
2012年 | 3554篇 |
2011年 | 2763篇 |
2010年 | 3304篇 |
2009年 | 3304篇 |
2008年 | 2895篇 |
2007年 | 2248篇 |
2006年 | 1883篇 |
2005年 | 2057篇 |
2004年 | 1774篇 |
2003年 | 1649篇 |
2002年 | 2283篇 |
2001年 | 1553篇 |
2000年 | 1471篇 |
1999年 | 499篇 |
1998年 | 207篇 |
1997年 | 214篇 |
1996年 | 189篇 |
1995年 | 179篇 |
1994年 | 175篇 |
1993年 | 202篇 |
1992年 | 196篇 |
1991年 | 182篇 |
1990年 | 167篇 |
1989年 | 186篇 |
1988年 | 190篇 |
1987年 | 179篇 |
1986年 | 176篇 |
1985年 | 277篇 |
1984年 | 257篇 |
1983年 | 164篇 |
1982年 | 203篇 |
1981年 | 201篇 |
1980年 | 251篇 |
1979年 | 237篇 |
1978年 | 233篇 |
1977年 | 265篇 |
1976年 | 218篇 |
1975年 | 194篇 |
1974年 | 155篇 |
1973年 | 201篇 |
1971年 | 107篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
161.
Lead tetraacetate (LTA) oxidation of the allylic alcohols 1, 10, 14 and 19 leads to the formation of the epoxides 2, 11, 15 and 20 , products of a novel internal addition reaction of the electron deficient alcohol oxygen to the allylic double bond. In some cases ( 10, 14 ) the formation of a new type of acetoxylated enolethers ( 12, 16 ) is observed. The LTA oxidation of the allylic dienols 21 and 29 gives rise to the formation of the epoxyacetates 25 and 33 , products of a similar internal addition reaction. Furthermore, a variety of cyclization products ( 22, 23, 24, 26, 30, 31, 32 and 34 ) has been isolated whose formation requires an isomerisation of the allylic trans double bond to a cis one. 相似文献
162.
Joachim Pump Eugene G. Rochow Ulrich Wannagat 《Monatshefte für Chemie / Chemical Monthly》1963,94(3):588-598
Zusammenfassung Durch Umsetzung von Disilbercyanamid mit Organyl- und Alkoxy-halogensilanen wurden 9 bisher unbekannte zweifach silylsubstituierte Carbodiimide der Stoffgruppen (R3SiN)2C, [(RO)3SiN]2C, (R2RSiN)2C und R3SiNCNSiR3 dargestellt und in ihrer Struktur über14N- und1H-kernmagnetische Resonanzmessungen, IR- und Raman-Spektren eindeutig als Carbodiimidderivate aufgeklärt.20. Mitt.:U. Wannagat undH. Kuckertz, Angew. Chem.75, 95 (1963).Zugleich 2. Mitt. über silylsubstituierte Carbodiimide; 1. Mitt.J. Pump undU. Wannagat, Ann. Chem.652, 21 (1962); Angew. Chem.74, 117 (1962); Österr. Chemiker-Ztg.62, 319 (1961). 相似文献
163.
164.
Urs P. Wild Tae-Kyu Ha Guido A. Raggio Hans U. Keller Peter O. Brunner 《Helvetica chimica acta》1975,58(3):696-706
An ab initio SCF calculation of 42 points of the energy hypersurface of the fluoronium ion is presented using a contracted F(5s/3p), H(2s) gaussian basis set. In its equilibrium structure a bond length of 1.812 a.u. and a HFH bond angle of 127.2° are predicted. The calculated vibrational frequencies for H2F+, HDF+, and D2F+ are in good agreement with the experimental data. 相似文献
165.
Hans Melissen 《Geometriae Dedicata》1994,50(1):15-25
In 1969 Pirl provided the densest packings ofn equal circles in a circle forn 10. We will prove the optimality for the packings that were conjectured forn=11. The proof is based on elementary combinatorial and analytical techniques. 相似文献
166.
167.
Alkenoyl and functionalized alkanoyl dodecanoyl peroxides are prepared in 70 to 97 % yield and photolyzed at ?78° C. Thereby 4- to 10-alkenoyl and 4-alkanoyl peroxides afford good yields (56 – 68 %) of unsymmetrical coupling products. Similarly α- to σ-haloalkanoyl, cholanoyl or 3- and 4-carboxyalkanoyl peroxides can be coupled (40 – 70 %). The α-chiral diacyl peroxide ls undergoes the photochemical coupling reaction with 80 % retention of its configuration. The photolysis of diacyl peroxides at ?78° C proves to be a favorable supplement of the Kolbe-electrolysis in cases, where the electrolysis fails or produces low yields. 相似文献
168.
Gerhard Talsky Lothar Mayring Hans Kreuzer 《Angewandte Chemie (International ed. in English)》1978,17(11):785-799
Derivative spectrophotometry has gained increasing importance in the past two years and is currently experiencing vigorous development. Following and introduction the present article provides a review of this extremely effective method. In particular, the advantages of higher-order derivative spectrophotometry (HODS method, n > 2) are discussed on the basis of practical examples from a wide range of analytical fields. The results are achieved with the aid of a newly developed analog computer unit, whereby for the first time readily reproducible, low-noise, on-line spectra can be obtained up to the 7th order and even, in favorable cases, up to the 9th order. In practice it has proved valuable to work with spectra of the 3rd to 5th order; but even higher derivations could be profitable for the separation of strongly superposed signals or for “fingerprinting”. 相似文献
169.
Isolation of (?)-dunnione from the leaves of Calceolaria integrifolia, MURR . (fam. Scrophulariaceae) From the leaves of Calceolaria integrifolia, a plant often used in horticulture. partially racemic (?)-dunnione (1) has been isolated. This seems to be the first record of its occurrence outside the family Gesneriaceae, where it previously has been found in two species as the (+)-enantiomer. 相似文献
170.
The preparation and the properties of the hexacyanopalladium(IV) acid (H3O)2[Pd(CN)6] and of a number of metal hexacyanopalladates(IV) are described. UV, IR and Raman spectra are reported, force constants have been calculated and the bonding properties are discussed. K2[Pd(CN)6] crystallizes in a hexagonal lattice; a = 7.42, c = 6.65 Å, Z = 1. 相似文献