全文获取类型
收费全文 | 677篇 |
免费 | 6篇 |
专业分类
化学 | 491篇 |
晶体学 | 12篇 |
力学 | 26篇 |
数学 | 39篇 |
物理学 | 115篇 |
出版年
2021年 | 7篇 |
2018年 | 7篇 |
2016年 | 7篇 |
2013年 | 29篇 |
2012年 | 25篇 |
2011年 | 25篇 |
2010年 | 13篇 |
2009年 | 13篇 |
2008年 | 25篇 |
2007年 | 33篇 |
2006年 | 20篇 |
2005年 | 19篇 |
2004年 | 18篇 |
2003年 | 18篇 |
2002年 | 28篇 |
2001年 | 11篇 |
2000年 | 16篇 |
1999年 | 12篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 22篇 |
1991年 | 13篇 |
1990年 | 9篇 |
1988年 | 14篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 12篇 |
1973年 | 8篇 |
1972年 | 10篇 |
1971年 | 12篇 |
1970年 | 6篇 |
1969年 | 5篇 |
1960年 | 5篇 |
1958年 | 9篇 |
1954年 | 5篇 |
排序方式: 共有683条查询结果,搜索用时 15 毫秒
151.
This paper presents a method for calculating power transmitted to the hands of operators who use vibrating hand tools. Results that relate to a comprehensive multidisciplined NIOSH field study of several hundred chipper and grinder workers who used pneumatic hand tools are presented. The results of this study indicated that the power in the frequency range of 6·3 Hz to 1000 Hz transmitted to the hand ranged from 1·08 × 103 to 7·23 × 103 J/s for the chisel and from 8·52 × 10?1 to 1·57 × 102 J/s for the handle of chipping hammers. For pneumatic grinders the power transmitted to the hands of the tool operators was in the range of 6·58 × 10?3 to 2·35 × 10?3 J/s over the same frequency range. 相似文献
152.
153.
154.
155.
156.
The reaction of 2,6-dimethyl- and 2,6-diphenyl-4-dicyanomethylene-4H-pyran with hindered primary amines such as isopropylamine and cyclohexylamine gave 1-alkyl-2-amino-3-cyano-6-rnethyl(or phenyl)-4-acetonylidene (or phenacylidene)-1,4-dihydropyridine derivatives. The 6-methyl-4-acetonylidene examples underwent a facile thermal rearrangement to give 1-alkyl-2,6-dimethyl-4-dicy anomethy lene-1,4-dihydropy ridines. Several reactions of the acylidene derivatives are described. 相似文献
157.
The reactions of 4-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)-2,6-diphenyl-4H-pyran ( 1 ) with primary amines gave the corresponding 1-substituted 1,4-dihydropyridine derivatives. The related benzo derivative of 1 (12) and primary amines gave 3-substituted 3,4-dihydro-2-phenyl-5H-[1]benzopyrano[3,4-c] pyridine-4,5-dione derivatives. With secondary amines, 12 gave 2-phenyl-4H,5H-pyrano[3,4-c] [1]benzopyrane-4,5-dione, and with isopropylamine, N,N-dimethylhydra-zine, and methanolic potassium hydroxide, 12 gave 4-phenacylcoumarin. Some reaction intermediates were isolated which indicate probable reaction paths. The reactions with amines were extended to a naphtho derivative of 1 (19) and to a thia homolog of 12 (24). 相似文献
158.
Aboelella NW York JT Reynolds AM Fujita K Kinsinger CR Cramer CJ Riordan CG Tolman WB 《Chemical communications (Cambridge, England)》2004,(15):1716-1717
Two highly reactive heterodinuclear bis(mu-oxo) complexes were prepared by combining mononuclear peroxo species with reduced metal precursors at -80 degrees C and were identified by UV-vis, EPR/NMR, and resonance Raman spectroscopy, with corroboration in the case of the CuPd system from density functional calculations. 相似文献
159.
J. A. Vanallan C. C. Petropoulos G. A. Reynolds D. P. Maier 《Journal of heterocyclic chemistry》1970,7(6):1363-1368
4-Dicyanomethylene-2-phenyl-4H-benzo[b]pyran, the corresponding thiapyran, and 4-dicyanomethylene-2,6-diphenyl-4H-pyran react with malononilrile under basic conditions to give 5-amino-4-cyano-2-phenylbenzo[b]pyrano[4,3,2-de] [1,6]naphthyridine, the corresponding thia-pyranonaphthyridine derivative, and 5-amino-6-cyano-2,8-diphenylpyrano[4,3,2-de] [1,6]naphthyridine, respectively. Certain other related reactions were studied in the course of investigating the above reactions. Hydrazine hydrate and 4-dicyanomethylene-2-phenyl-4H-benzo[b]pyran gave 5-(2-hydroxyphenyl)-3-phenylpyrazole, and phenylhydrazine gave 5-(2-hydroxyphenyl)-1,3-diphenylpyrazole. 相似文献
160.
John D Reynolds 《Tetrahedron letters》2004,45(14):2955-2959
The chemistry of strained dihydroanthraquinones was investigated for the purpose of developing syntheses for highly strained anthraquinones. The reaction of (1,4-dihydro-9,10-dioxo-anthracen-1-yl)-acetates with triethylamine under aerobic conditions was found to be dependent on the degree of substitution on the acetate. Dihydroanthraquinones bearing dimethylacetates underwent elimination of enolate exclusively, while those without acetate substitution dehydrogenated as expected. Furthermore, oxidation of the cyclohexadiene ring using iodine failed due to a competitive iodolactonization reaction. The desired strained anthraquinones could be prepared, in low yield, by treatment of the resulting lactones with acidic ethanol. 相似文献