全文获取类型
收费全文 | 154490篇 |
免费 | 1460篇 |
国内免费 | 421篇 |
专业分类
化学 | 85811篇 |
晶体学 | 2432篇 |
力学 | 6133篇 |
综合类 | 3篇 |
数学 | 14675篇 |
物理学 | 47317篇 |
出版年
2021年 | 1076篇 |
2020年 | 1256篇 |
2019年 | 1321篇 |
2018年 | 1686篇 |
2017年 | 1735篇 |
2016年 | 2686篇 |
2015年 | 1700篇 |
2014年 | 2461篇 |
2013年 | 6284篇 |
2012年 | 5075篇 |
2011年 | 6245篇 |
2010年 | 4357篇 |
2009年 | 4393篇 |
2008年 | 6007篇 |
2007年 | 5800篇 |
2006年 | 5645篇 |
2005年 | 5235篇 |
2004年 | 4726篇 |
2003年 | 4340篇 |
2002年 | 4203篇 |
2001年 | 5652篇 |
2000年 | 4069篇 |
1999年 | 3180篇 |
1998年 | 2213篇 |
1997年 | 2331篇 |
1996年 | 2194篇 |
1995年 | 2039篇 |
1994年 | 2020篇 |
1993年 | 1846篇 |
1992年 | 2320篇 |
1991年 | 2400篇 |
1990年 | 2219篇 |
1989年 | 2208篇 |
1988年 | 2194篇 |
1987年 | 2189篇 |
1986年 | 2022篇 |
1985年 | 2506篇 |
1984年 | 2497篇 |
1983年 | 2017篇 |
1982年 | 2023篇 |
1981年 | 1851篇 |
1980年 | 1843篇 |
1979年 | 2122篇 |
1978年 | 2188篇 |
1977年 | 2131篇 |
1976年 | 2056篇 |
1975年 | 1921篇 |
1974年 | 1937篇 |
1973年 | 1960篇 |
1972年 | 1290篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Weavers RT 《The Journal of organic chemistry》2001,66(19):6453-6461
Four isomeric alcohols derived from the diterpene lauren-1-ene (1) have been examined by NMR methods (nuclear Overhauser enhancements, coupling constants, and variable-temperature studies) and by molecular mechanics using the MM3 force field to investigate a conformational twisting of the [5.5.5.7]fenestrane ring system. Results have been correlated with a concurrent study of remote functionalization reactions induced by iodobenzene diacetate/iodine under ultrasonic irradiation. Three of the laurenan-2-ols, 6, 7, and 8, lead to the same tetrahydrofuran derivative, 2beta,14beta-epoxylaurenane (9), and evidence for beta-cleavage of the alkoxy radical intermediate is obtained through the isolation of a ring-cleavage product 11 with a rearranged carbon skeleton. The products from the remaining alcohol 3 demonstrate solution dynamics involving the conformational twisting of the laurenane skeleton. 相似文献
993.
[reaction: see text]. Polymer-bound chiral electrophilic selenium reagents have been developed and applied to stereoselective selenenylation reactions of various alkenes. Different cleavage protocols allow further functionalization of the addition products leading to improvements in selenium-based solid-phase chemistry. 相似文献
994.
We report an improved procedure for the preparation of 8-alkyl-N5-deazapterins which allows clean preparation of all ring-methyl substituted compounds, including 5- and 7-methyl substituted compounds. The procedure was also successfully applied to the preparation of N5-deazapterins with improved yield over previous reports. The uv/visible and pKa data confirm the predicted increased basicity of 8-alkyl-N5-deazapterins compared with the N5-deazapterin parents, and indicate that N5-deazapterins protonate on N8 and 8-alkyl-N5-deazapterins protonate on N3. 相似文献
995.
Yu. N. Polivin R. A. Karakhanov T. S. Sheveleva E. A. Ageev A. S. Remizov 《Russian Chemical Bulletin》1991,40(12):2518-2520
The reaction of 2-ethyl-3-tetrahydrofuryl thiocyanate (I) and triphenylacetonitrile (VIII) with methylmagnesium iodide in ether was studied using GC/MS, PMR and13C NMR spectroscopy. The reaction of (I) with CH3MgI gives the product of the homolysis of the C-S bond, namely, bis(2-ethyl-3-tetrahydrofuryl) disulfide (II), in 81% yield. This reaction is assumed to be general in nature since triphenylacetonitrile (VIII) is also homolytically cleaved by the action of CH3MgI to give triphenylmethane.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2886–2888, December, 1991. 相似文献
996.
S. V. Lindeman Yu. T. Struchkov M. N. Élinson S. K. Fedukovich G. I. Nikishin 《Journal of Structural Chemistry》1989,30(3):512-514
A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the USSR. N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 3, pp. 171–173, May–June, 1989. 相似文献
997.
A method was developed for the recovery and determination of cyanide ion in organic sample matrices. To facilitate the solubilization of cyanide ions, cetyltrimethylammonium bromide (CTAB) was added at concentrations above the critical micelle concentration. Sample cyanation reaction products consisted of solvent mixtures of a hydroxynitrile in DMF-toluene or DMF-isopropylacetate (IPAC). Spectrophotometric determination of cyanide ion at 578 nm by the pyridine-barbituric acid method was automated by flow injection analysis. Recovery of cyanide ion from spiked samples was 93.2% in DMF-IPAC solvent matrix and 93.9% in DMF-toluene. Low alkali concentration was observed to favor solubilization of cyanide ion in the micellar solution. 相似文献
998.
N. N. Chipanina A. M. Shulunova T. I. Yushmanova L. I. Volkova V. A. Lopyrev Yu. L. Frolov 《Russian Chemical Bulletin》1987,36(10):2182-2183
Conclusions An IR spectroscopic study showed that the C=0 values in the spectra of N,N-dimethylhydrazides of aromatic acids, RCONHN(Me)2, correlate linearly with a regression equation obtained for the unsubstituted hydrazides of aromatic acids, RCONHNH2. This dependence differs significantly from that for N-methyl-N-aroylhydrazines, in which ,-conjugation in the RC(O)N group is more pronounced.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2353–2355, October, 1987. 相似文献
999.
The total phospholipids of kenaf of variety Opytnyi-1847 and homogeneous fractions of them have been studied. A known law has been confirmed — positions 2 of the main fractions of phospholipids are esterified predominantly with unsaturated fatty acids and in N-aceylated phospholipids the amide-bound acids are more saturated than the O-acyls.Institute of the Chemistry of Plant Substances of the Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 775–777, November–December, 1980. 相似文献
1000.
A method to observe the phosphorescence emissions from the spin sublevels of low emissivity is proposed, and its application to quinoxaline is attempted. The newly observed phosphorescence spectra from the Tx and Ty sublevels differ considerably in vibrational structures from the usually observed phosphorescence from the Tz sublevel. 相似文献