全文获取类型
收费全文 | 271篇 |
免费 | 31篇 |
国内免费 | 36篇 |
专业分类
化学 | 210篇 |
晶体学 | 3篇 |
力学 | 3篇 |
综合类 | 3篇 |
数学 | 31篇 |
物理学 | 88篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2018年 | 5篇 |
2016年 | 4篇 |
2015年 | 7篇 |
2014年 | 12篇 |
2013年 | 18篇 |
2012年 | 4篇 |
2011年 | 4篇 |
2010年 | 6篇 |
2009年 | 15篇 |
2008年 | 23篇 |
2007年 | 18篇 |
2006年 | 15篇 |
2005年 | 22篇 |
2004年 | 13篇 |
2003年 | 15篇 |
2002年 | 10篇 |
2001年 | 20篇 |
2000年 | 19篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1987年 | 4篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1981年 | 1篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1933年 | 1篇 |
排序方式: 共有338条查询结果,搜索用时 344 毫秒
61.
Bilirubin-IXα bis-dimethylamide and 8- and 12-monodimethylamides, and bilirubin-IIIα and XIIIα bis-dimethylamides have been synthesized and characterized spectroscopically. In deuteriochloroform, as deduced from 1H-nmr N- resonances, the mono-dimethylamides prefer a folded, intramolecularly H-bonded conformation characteristic of the parent bilirubins. The possiblity of similar conformations for the bis-dimethylamides is discussed. 相似文献
62.
Beatriz Veleirinho Manuel F. Rei J. A. Lopes‐DA‐Silva 《Journal of Polymer Science.Polymer Physics》2008,46(5):460-471
This study describes the preparation and characterization of nanofibrous mats obtained by electrospinning poly(ethylene terephthalate) (PET) solutions in trifluoroacetic acid/dichloromethane (TFA/DCM). Special attention was paid to the effect of polymer concentration and solvent properties on the morphology, structure, and mechanical and thermal properties of the electrospun nonwovens. The results show that the spinnable concentration of PET solution in TFA/DCM solvents is above 10 wt %. Mats have nanofibrous morphology with fibers having an average diameter in the range of 200–700 nm (depending on polymer concentration and solvent composition) and an interconnected pore structure. Higher solution concentration favors the formation of uniform fibers without beads and with higher diameter. Morphology and fiber assembly changed with the solvent properties. Solvent mixtures rich in TFA, i.e., those with higher dielectric constant and lower surface tension, originated fibers with small diameter. However, due to the lower volatility, those solvent mixtures also produced more branched and crosslinking fibers, with less morphologic uniformity. Mechanical properties (Young's modulus, ultimate strength, and elongation at break) and thermal properties (glass transition, crystallization, and melting) have been studied for the PET electrospun nanomats and compared with those of the original polymer. Solvent effect on fiber crystallinity was not significant, but a complex effect was observed on the mechanical properties of the electrospun mats, as a consequence of the different structural organization of the fibers within the mat network. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 460–471, 2008 相似文献
63.
(A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (F) 0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (F=0.68, em=493nm, ex=422nm in CHCl3) and UV-Vis absorption (21000 at 424nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (=+15dm3·mol–1·cm–1, =432nm; =–4dm3·mol–1cm–1, =380nm) that correlates with the Exciton Chirality Rule. 相似文献
64.
65.
66.
龙爪芦荟和库拉索芦荟中微量锗的测定与红外光谱区别 总被引:1,自引:0,他引:1
采用吸光光度法对龙爪芦荟和库拉索芦荟中的微量锗进行了测定,并探讨了两种芦荟的红外光谱的区别。结果表明,龙爪芦荟中含锗量在26.1~28.6μg·g-1,回收率为91.6%~97 5%。库拉索芦荟中含锗量在16.8~20.0μg·g-1,回收率为87.1%~94.5%。两种芦荟的FT IR光谱图在2100cm-1处有明显差异,由此可区别两种芦荟。 相似文献
67.
A rare and unusual class of tripyrrolic compounds, violet-colored tripyrrin-1,14-diones, can be prepared easily and in moderately high yields from base (piperidine)-catalyzed condensation of 3-pyrrolin-2-ones with 2,5-diformylpyrroles. Dipyrrinones adopt the all-syn-Z conformation leading to helical, lock-washer like structures, which form dimers that are held together by intermolecular hydrogen bonds in nonpolar solvents and in the crystal. Strong bathochromic spectral shifts of the tripyrrindione ∼480 nm long wavelength UV-visible absorption band are seen with added base: DBU, 615 nm; TFA, 573 nm; and Zn(OAc)2, 586 nm. 相似文献
68.
A rapid and facile synthesis of N-substituted 2,3,3a,4,7,7a-hexahydroisoindole-l,3-dione derivatives via microwave-promoted Beller three-component domino reaction of α,β,-unsaturated aldehydes, amides and N-substituted maleimides was described. 相似文献
69.
The first optically active taurine conjugate of a bilirubin was prepared by reaction of taurine sodium salt with the mixed
anhydride formed from reaction of (βS,β′S)-dimethylmesobilirubin-XIIIα with isobutyl chloroformate. Analysis of the circular dichroism spectra of the conjugate in
water and chloroform indicate a conformational preference for the (M)-helical ridge-tile conformation, thus providing the first spectroscopic evidence on the conformation of ditaurobilirubins. 相似文献
70.
Nicholas T. Salzameda David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2006,44(5):1453-1462
Rarely-seen carboxylic acid to ketone intermolecular hydrogen bonding is found in crystal structures of α-pyrroleglyoxylic
acids. 3,4-Diethyl-1H-pyrrole-2-glyoxylic acid, prepared by saponfication of the reaction product between 3,4-diethylpyrrole and ethyl oxalyl chloride
formed crystals that showed the uncommon carboxyl to ketone intermolecular hydrogen bonding, with the hydrogen-bonded α-pyrroleglyoxylic
acids stacked neatly in layers. This α-ketoacid hydrogen-bonding pattern was repeated in (Z)-2,3,7,8-tetraethyl-10H-dipyrrin-1-one-9-glyoxylic acid, which also engaged in dipyrrinone-to-dipyrrinone intermolecular hydrogen bonding to form
supramolecular ribbons in the crystal. 相似文献