全文获取类型
收费全文 | 370篇 |
免费 | 25篇 |
国内免费 | 8篇 |
专业分类
化学 | 231篇 |
晶体学 | 3篇 |
力学 | 12篇 |
数学 | 30篇 |
物理学 | 127篇 |
出版年
2023年 | 1篇 |
2021年 | 1篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 19篇 |
2013年 | 23篇 |
2012年 | 45篇 |
2011年 | 37篇 |
2010年 | 33篇 |
2009年 | 35篇 |
2008年 | 41篇 |
2007年 | 32篇 |
2006年 | 34篇 |
2005年 | 10篇 |
2004年 | 5篇 |
2003年 | 7篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 11篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1988年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有403条查询结果,搜索用时 609 毫秒
91.
MavroudisA. Demertzis ParasNath Yadav Dimitra Kovala‐Demertzi 《Helvetica chimica acta》2006,89(9):1959-1970
Two novel, stable PdII complexes, compounds 3 and 4 , of two 3‐hydroxypyridine‐2‐carbaldehyde thiosemicarbazones, 1 and 2 , resp., were prepared from Li2PdCl4. The single‐crystal X‐ray structure of complex 3 (= [Pd( 2 )Cl]) shows that the ligand monoanion coordinates in a planar conformation to the metal via the pyridyl N‐, the imine N‐, and the thiolato S‐atoms. Intermolecular H‐bonds, π–π, and CH ? ? ? π interactions lead to a two‐dimensional supramolecular assembly. The electronic, IR, UV/VIS, and NMR spectroscopic data of the two complexes are reported, together with their electrochemical properties. A sophisticated experimental procedure was used to determine the multiple dissociation constants of the ligands 1 and 2 by UV/VIS titration. 相似文献
92.
Zhiwei Miao Jianfeng Zhang Zhanwei Cui Bin Wang Ruyu Chen 《Helvetica chimica acta》2007,90(10):1932-1939
A study on the synthesis of the novel N‐(cyclic phosphonate)‐substituted phosphoramidothioates, i.e., O,O‐diethyl N‐[(trans‐4‐aryl‐5,5‐dimethyl‐2‐oxido‐2λ5‐1,3,2‐dioxaphosphorinan‐2‐yl)methyl]phosphoramidothioates 4a – l , from O,O‐diethyl phosphoramidothioate ( 1 ), a benzaldehyde or ketone 2 , and a 1,3,2‐dioxaphosphorinane 2‐oxide 3 was carried out (Scheme 1 and Table 1). Some of their stereoisomers were isolated, and their structure was established. The presence of acetyl chloride was essential for this reaction and accelerated the process of intramolecular dehydration of intermediate 5 forming the corresponding Schiff base 7 (Scheme 2). 相似文献
93.
EmmanuelA. Meyer Nicola Donati Marine Guillot W.Bernd Schweizer Franois Diederich Bernhard Stengl Ruth Brenk Klaus Reuter Gerhard Klebe 《Helvetica chimica acta》2006,89(4):573-597
This paper describes the rational design, synthesis, and biological evaluation of a new generation of inhibitors of the bacterial enzyme tRNA‐guanine transglycosylase (TGT), which has been identified as a new target in the fight against bacillary dysentery (Shigellosis). The enzyme catalyzes the exchange of guanine in the anticodon wobble position of tRNA by the modified base preQ1, a guanine derivative, according to a ping‐pong mechanism involving a covalent TGT‐tRNA intermediate (Fig. 2). Based on computer modeling (Fig. 3), lin‐benzoguanine (6‐aminoimidazol[4,5‐g]quinazolin‐8(7H)‐one ( 2 )) was selected as an extended central scaffold, to form up to seven in‐plane intermolecular H‐bonds with the protein while sandwiching between Tyr106 and Met260. Versatile synthetic protocols were developed for the synthesis of 2 , and derivatives with phenyl, benzyl, and 2‐phenylethyl side chains (i.e., 16, 17a , and 12a, 12b, 13, 17 , resp.) to reach into the lipophilic pocket lined by Val282, Val45, and Leu68 (Schemes 1–3). To account for the limited solubility of the new ligands and in consequence of a recently developed detailed understanding of the mechanism of TGT catalysis (Fig. 2), the enzyme kinetic assay was completely redesigned, providing competitive (Kic) and uncompetitive (Kiu) inhibition constants with respect to tRNA binding by TGT. The modifications of the various parameters in the new assay are described in detail. Binding affinities of the new inhibitors were found to be in the single‐digit micromolar range (Kic values, Fig. 8). Decoration of the lin‐benzoguanine scaffold with lipophilic residues only gave a modest improvement in biological activity which was explained on structural grounds with the help of four crystal structures (Fig. 10) obtained by soaking the protein with inhibitors 2 and 12a – 12c . Both biochemical and biostructural analyses reported in this paper provide a fertile basis for the development of more potent future generations of TGT inhibitors. 相似文献
94.
Three new pteridines, hirudinoidines A – C ( 1 – 3 , resp.), with novel structural features were isolated from Hirudo nipponica Whitman , and their structures were elucidated on the basis of extensive spectroscopic analyses and by comparison of their spectral data with those of related metabolites. The configuration of 1 was unequivocally confirmed by single‐crystal X‐ray diffraction (Fig. 2). 相似文献
95.
The phenyl‐substituted conduritol B 8 was prepared in racemic form in a five‐step sequence starting from 2‐phenyl‐1,4‐benzoquinone ( 10 ) (Scheme 1). The reaction mechanism of the key step 12b → 13 is discussed (Scheme 2). 相似文献
96.
Two spiroheterocyclic 2H‐azirin‐3‐amines, 1f and 1g , were shown to be useful synthons for the dipeptides N‐(4‐aminotetrahydro‐2H‐pyran‐4‐yl)prolinate (Thp‐Pro) and the corresponding thiopyran derivative, Tht‐Pro, respectively. By coupling of 4‐bromobenzoic acid with 1f or 1g and saponification, followed by repeating the coupling and saponification steps, oligopeptides of type 4‐BrBz‐(Thp‐Pro)n‐OMe and 4‐BrBz‐(Tht‐Pro)n‐OMe were prepared, and their conformations were evaluated in solution by NMR techniques and in the crystalline state by X‐ray crystallography. All of these sterically highly congested oligopeptides adopt fairly rigid helical conformations. It is interesting to note that the hexapeptide with Thp forms a 310‐helix, whereas the Tht analog has a β‐bend ribbon spiral confirmation. 相似文献
97.
X荧光光谱仪测定炼钢用铝条试样中的8个元素 总被引:3,自引:0,他引:3
本文报导X荧光光谱仪,通过采用聚四氟乙烯制成孔径10mm的试样盒,测定炼钢用铝条试样,方法简便,快速,准确。 相似文献
98.
99.
2,3‐Dihydrothiophene 1,1‐dioxide (‘2‐sulfolene’) reacted with tosylmethyl isocyanide (TsMIC) in the presence of a base to give the hitherto unknown 3,5‐dihydro‐2H‐thieno[2,3‐c]pyrrole 1,1‐dioxide (‘β′‐sulfolenopyrrole’) from the expected cyclocondensation. A serendipitous formation of this β′‐sulfolenopyrrole was found earlier, when we investigated synthetic routes to a 3,5‐dihydro‐1H‐thieno[3,4‐c]pyrrole 2,2‐dioxide (a ‘β″‐sulfolenopyrrole’) from TsMIC and 2,5‐dihydrothiophene 1,1‐dioxide (‘3‐sulfolene’). Here, we present the synthesis and characterization of β′‐sulfolenopyrrole. The X‐ray crystal‐structure analyses of β′‐sulfolenopyrrole and the isomeric β″‐sulfolenopyrrole are also reported here. This β′‐sulfolenopyrrole is a new type of a functionalized pyrrole, which is likely to be of interest for pharmaceutical purposes. 相似文献
100.
Seyyed Javad Sabounchei Vida Jodaian Sadegh Salehzadeh Sepideh Samiee Alireza Dadrass Mehdi Bayat Hamid Reza Khavasi 《Helvetica chimica acta》2010,93(6):1105-1119
Reactions of phosphonium ylides (4‐MeC6H4)3PCHC(?O)(2‐C4H3S) (tptpy), Ph3PCHC(?O)(2‐C4H3O) (fppy), and (4‐MeC6H4)3PCHC(?O)(4‐BrC6H4) (bbtppy) with HgX2 (X=Cl, Br, and I) in equimolar ratios in MeOH as solvent leads to the binuclear products 1 – 3 (Scheme 1). The bridge‐splitting reaction of the binuclear complex [{HgI2(bbtppy)}2] ( 3c ) by DMSO yields the mononuclear complex [HgI2?(bbtppy) (DMSO)] ( 3d ) (Scheme 2). This bridge‐splitting reaction can also be a method for the synthesis of mononuclear products. C‐Coordination of the ylide and O‐coordination of DMSO are demonstrated by a single‐crystal X‐ray‐analysis of the mononuclear complex 3d . Characterization of the obtained compounds was also performed by means of elemental analysis and IR and 1H‐, 31P‐, and 13C‐NMR spectroscopy. A theoretical study of some HgII complexes with phosphonium ylides is also reported. 相似文献