全文获取类型
收费全文 | 212篇 |
免费 | 0篇 |
专业分类
化学 | 152篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 33篇 |
物理学 | 23篇 |
出版年
2021年 | 4篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2013年 | 4篇 |
2012年 | 1篇 |
2011年 | 8篇 |
2010年 | 5篇 |
2009年 | 2篇 |
2008年 | 5篇 |
2007年 | 6篇 |
2006年 | 4篇 |
2005年 | 6篇 |
2004年 | 7篇 |
2003年 | 7篇 |
2002年 | 10篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 3篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1965年 | 1篇 |
1961年 | 2篇 |
排序方式: 共有212条查询结果,搜索用时 15 毫秒
81.
Carotenoid Glycosyl Esters. The Synthesis of Crocetin-di-(β-D -glucosyl) Ester. A New Method for the Selective Esterification of Unprotected β-D -Glucose The naturally occurring crocetin-di-(β-D -glucosyl) ester is easily synthesized by the reaction of crocetin-di-imidazolide or crocetin-di-(1,2,4-triazolide) and unprotected β-D -glucose in pyridine in presence of a base (Scheme 4). Under the described experimental conditions the esterification takes place exclusively at the anomeric C-atom and furthermore produces only the β-anomer. It is the first time that an unprotected carbohydrate has been used for the selective synthesis of glucosyl esters at the anomeric C-atom. This represents the major advantage of this new method. 相似文献
82.
The purpose of this paper is: (i) to construct a space which is semilocally simply connected in the sense of Spanier even though its Spanier group is non-trivial; (ii) to propose a modification of the notion of a Spanier group so that via the modified Spanier group semilocal simple connectivity can be characterized; and (iii) to point out that with just a slightly modified definition of semilocal simple connectivity which is sometimes also used in literature, the classical Spanier group gives the correct characterization within the general class of path-connected topological spaces.While the condition “semilocally simply connected” plays a crucial role in classical covering theory, in generalized covering theory one needs to consider the condition “homotopically Hausdorff” instead. The paper also discusses which implications hold between all of the abovementioned conditions and, via the modified Spanier groups, it also unveils the weakest so far known algebraic characterization for the existence of generalized covering spaces as introduced by Fischer and Zastrow. For most of the implications, the paper also proves the non-reversibility by providing the corresponding examples. Some of them rely on spaces that are newly constructed in this paper. 相似文献
83.
In this paper we consider a diffusion approximation to a classical risk process, where the claims are reinsured by some reinsurance with deductible b ∈ [0,b?], where b = b? means “no reinsurance” and b = 0 means “full reinsurance”. The cedent can choose an adapted reinsurance strategy (b t ) t ≥0, i.?e. the deductible can be changed continuously. In addition, the cedent has to inject fresh capital in order to keep the surplus positive. The problem is to minimise the expected discounted cost over all admissible reinsurance strategies. We find an explicit expression for the value function and the optimal strategy using the Hamilton–Jacobi–Bellman approach. Some examples illustrate the method. 相似文献
84.
A New and Simple Method for the Synthesis of Spirocyclic la-Quinolines The reaction of anilines with (+)-(R)-pulegone in toluene at temperatures between 125 and 150° with 4-toluenesulfonic acid or I2 as catalysts leads to diastereoisomeric mixtures of spiro[cyclohexane-1,2′(1′H)-quinoline] derivatives (see 1–4 , Scheme 1; Table). The diastereoisomers are separated by column chromatography, and the structure of the single isomers is determined by NMR-spectroscopic methods. A reaction mechanism proceeding via several 6π-electrocyclic rearrangements and H-shifts is proposed for the formation of 1H-quinolines 1–4 (Scheme 2). This mechanism is in accordance with the results of the reaction of 2-isopropenylaniline with 3-methylcyclohexanone which leads to a stereoisomeric mixture of 3,4′-dimethylspiro[cyclohexane-1,2′(1H)-quinolines] (Scheme 3). 相似文献
85.
Photochemical nitrogen elimination of 1,2,3-thiadiazoles leads to primary fragments ( 1 , X = S), which are able to cyclize to thiirenes ( 2 , X = S). An exception is found for bicyclic systems like 4 . These results, gained by isotope labelling experiments are related to the oxirene formation of α-oxocarbenes. 相似文献
86.
Structure Elucidation of Elaiophylin: Spectroscopy and Chemical Degradation The structure of the antibiotic elaiophylin (azalomycin B) was elucidated by extended spectroscopic investigations and chemical degradation. Elaiophylin ( 26 ) is a macrodiolide with a 16membered dilactone ring. The synthesis of 7-acetoxy-6-ethyl-3-octanone ( 14 ), the acetyl derivative of an important degradation product, is described. 相似文献
87.
An autonomous fourth order model of vocal fold vibrations is proposed. Each fold is represented by a lower and upper mass, and the aerodynamic forces are derived from a modified Bernoulli equation. The model exhibits many features of normal phonation in a wide parameter region. At the borderlines of this region coexistence of limit cycles, period-doubling and chaos are observed. Implications for an understanding of pathological voices are discussed. 相似文献
88.
89.
Synthesis and Structure Elucidation of Merucathine and Pseudomerucathine. Two Constituents of Catha edulis FORSK . Starting from L-alanine, two constituents of Catha edulis FORSK . (3R,4S)-4-amino-1-phenyl-1-penten-3-ol ((3R,4S)- 4 , merucathine) and (3S,4S)-4-amino-1-phenyl-1-penten-3-ol ((3S,4S)- 4 , pseudomerucathine) were synthesized. 相似文献
90.