全文获取类型
收费全文 | 2466篇 |
免费 | 204篇 |
国内免费 | 239篇 |
专业分类
化学 | 2445篇 |
晶体学 | 35篇 |
力学 | 8篇 |
综合类 | 15篇 |
数学 | 139篇 |
物理学 | 267篇 |
出版年
2024年 | 4篇 |
2023年 | 23篇 |
2022年 | 71篇 |
2021年 | 55篇 |
2020年 | 75篇 |
2019年 | 73篇 |
2018年 | 68篇 |
2017年 | 77篇 |
2016年 | 90篇 |
2015年 | 76篇 |
2014年 | 106篇 |
2013年 | 193篇 |
2012年 | 113篇 |
2011年 | 154篇 |
2010年 | 133篇 |
2009年 | 144篇 |
2008年 | 156篇 |
2007年 | 150篇 |
2006年 | 153篇 |
2005年 | 137篇 |
2004年 | 157篇 |
2003年 | 137篇 |
2002年 | 134篇 |
2001年 | 52篇 |
2000年 | 57篇 |
1999年 | 40篇 |
1998年 | 30篇 |
1997年 | 36篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 22篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1982年 | 8篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有2909条查询结果,搜索用时 15 毫秒
921.
The facile preparation of N(H)-bridged azacalix[4]arenes has been achieved by stepwise nucleophilic aromatic substitutions assisted by hydrogen bonding interactions. The synthesis is uncatalyzed and affords previously unknown tetranitroazacalix[4]arenes. 相似文献
922.
923.
We propose evaluation approaches to multi-item base-stock inventory policies where unidirectional substitutions are allowed. The problems in the paper are in the context of spare parts management and we identify two substitution cases: substitution upon demand arrivals and substitution upon order deliveries. This leads us to three unidirectional substitution policies, for each of which we develop Markovian models. As the number of part types increases, computational effort required to solve the Markovian models increases rapidly. To reduce computation burden, an approximation approach based on the decomposition of multi-dimensional state transition is used for systems with two or more spare part types. Numerical studies show unidirectional substitution improves various system performance measures such as the average inventory level, the average backlogged demand, and the fill rate. The proposed decomposition approach reduces the computation required to compute the performance measures and the approximation errors seems to be quite small. 相似文献
924.
采用以玻色子展开和MJS(ModifiedJancovici-Schiff)代换为基础的微观IBM-2方案,用直接对角化的计算方法,研究108—118Cd偶同位素集体能谱.计算结果与实验能谱作了比较,并对近满壳偶偶核中价质子空穴激发特性及影响能谱结构的因素作了讨论. 相似文献
925.
Toby D. M. Bell Dr. Sheryll Yap Chintan H. Jani Sheshanath V. Bhosale Dr. Johan Hofkens Prof. Frans C. De Schryver Prof. Steven J. Langford Prof. Kenneth P. Ghiggino Prof. 《化学:亚洲杂志》2009,4(10):1542-1550
The synthesis and photophysics of two new aminopropenyl naphthalene diimide (SANDI) dyes are reported. A general and convenient method for the synthesis of the precursor mono‐, di‐, and tetrabrominated 1,4,5,8‐naphthalene tetracarboxylic dianhydrides is described. The two core‐substituted SANDIs exhibit many of the photophysical properties required for fluorescence labeling applications including high photostability and high fluorescence quantum yields (>0.5) in the visible region of the spectrum. The emission wavelength is sensitive to the number of substituents on the NDI core, and the fluorescence decay times are in the range of ~8–12 ns for both compounds in the solvents investigated. Preliminary fluorescence emission data from single molecules of the compounds embedded in poly(methyl methacrylate) films are also reported and show that single molecules have very low yields of photobleaching, particularly the di‐substituted system. Furthermore, only a small proportion (<10 %) of the single molecules studied display fluorescence intermittencies or “blinks” in their photon trajectory. The compounds appear to be excellent candidates for applications at the single molecule level, for example, as FRET labels. 相似文献
926.
Structural modification of unsymmetrically substituted monophthalocyanines by nucleophilic reactions
New 2-nitro-substituted phthalocyanine zinc complexes were synthesized. The nucleophilic substitution of the NO2 group was used for the first time for the structural modification of unsymmetrically substituted monophthalocyanines. The
electronic absorption spectra of phthalocyanines were studied.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2036–2040, September, 2005. 相似文献
927.
The spiro-activated cyclopropane 1 undergoes desymmetrization either with Li-thiophenoxide in the presence of a chiral complexing ligand, or with ion pairs formed from thiophenols and aromatic chiral amines. The latter procedure is more efficient and provides the ring-opened thioether 6 in up to 79% yield and up to 60% ee. 相似文献
928.
Ken Sakakibara 《Tetrahedron letters》2005,46(6):959-962
Addition of a phosphenium dramatically improved the reaction yields in the carboxylation of arenes by formic acid catalyzed by Pd(II). Control experiments revealed that the majority of the phosphenium triflate was converted to a mixed anhydride of phosphonic acid and formic acid (7), which however did not substitute for the phosphenium to improve the reaction yield. 相似文献
929.
Friedrich?HammerschmidtEmail author Biljana?Peric Simov Susanne?Schmidt Sabine?Schneider Ilse?Zolle 《Monatshefte für Chemie / Chemical Monthly》2005,136(2):229-239
Summary. Metomidate (ee 99%) substituted with iodine in the phenyl ring was prepared from (S)-1-(4-iodophenyl)ethanol (ee >98%) obtained by lipase-catalysed resolution and methyl 1H-imidazole- 5-carboxylate. The two fragments were joined highly regioselectively (alkylation only at N-1 of substituted imidazole) with inversion of configuration using the Mitsunobu reaction. Finally, p-iodometomidate was transformed into the p-trimethylstannyl derivative. 相似文献
930.