全文获取类型
收费全文 | 2203篇 |
免费 | 63篇 |
国内免费 | 17篇 |
专业分类
化学 | 1750篇 |
晶体学 | 17篇 |
力学 | 44篇 |
数学 | 240篇 |
物理学 | 232篇 |
出版年
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 20篇 |
2020年 | 31篇 |
2019年 | 22篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 47篇 |
2015年 | 49篇 |
2014年 | 59篇 |
2013年 | 97篇 |
2012年 | 129篇 |
2011年 | 196篇 |
2010年 | 68篇 |
2009年 | 72篇 |
2008年 | 117篇 |
2007年 | 157篇 |
2006年 | 172篇 |
2005年 | 154篇 |
2004年 | 116篇 |
2003年 | 78篇 |
2002年 | 109篇 |
2001年 | 29篇 |
2000年 | 25篇 |
1999年 | 20篇 |
1998年 | 25篇 |
1997年 | 25篇 |
1996年 | 33篇 |
1995年 | 25篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 14篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 9篇 |
1987年 | 16篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 22篇 |
1981年 | 20篇 |
1980年 | 23篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 9篇 |
排序方式: 共有2283条查询结果,搜索用时 0 毫秒
31.
RuCl2(PPH3)3 has been attached to a phosphinated polymer support (phosphinated polystyrene crosslinked with 2% divinylbenzene) and the reagent converted to the polymer supported analogue of RuClH(PPH3)3 in the presence of base. The polymer supported catalyst efficiently hydrogenates terminal olefins under ambient conditions. Hydrogenation of 1-hexene has revealed that the reaction rate is proportional to [Ru], [H2] and [olefin]/(1 + [olefin]). The polymer support environment allows for selectivity in olefin hydrogenation and under suitable reaction conditions short chain terminal olefins are hydrogenated more rapidly than long chain terminal olefins. The extent of metal loading on the polymer and the reaction solvent composition also influence the reaction selectivity and these effects are discussed. 相似文献
32.
Alex Berlin Neil Gilkes Douglas Kilburn Vera Maximenko Renata Bura Alexander Markov Anton Skomarovsky Alexander Gusakov Arkady Sinitsyn Oleg Okunev Irina Solovieva John N. Saddler 《Applied biochemistry and biotechnology》2006,130(1-3):528-545
Seven cellulase preparations from Penicillium and Trichoderma spp. were evaluated for their ability to hydrolyze the cellulose fraction of hardwoods (yellow poplar and red maple) pretreated by organosolv extraction, as well as model cellulosic substrates such as filter paper. There was no significant correlation among hydrolytic performance on pretreated hardwood, based on glucose release, and filter paper activity. However, performance on pretreated hardwood showed significant correlations to the levels of endogenous β-glucosidase and xylanase activities in the cellulase preparation. Accordingly, differences in performance were reduced or eliminated following supplementation with a crude β-glucosidase preparation containing both activities. These results complement a previous investigation using softwoods pretreated by either organosolv extraction or steam explosion. Cellulase preparations that performed best on hardwood also showed superior performance on the softwood substrates. 相似文献
33.
Mokoena Emma M. Datye Abhaya K. Coville Neil J. 《Journal of Sol-Gel Science and Technology》2003,28(3):307-317
DL-tartaric acid was used as a template for the formation of silica nanotubes and spheres by the sol-gel method from tetraethylorthosilicate (TEOS) as silica source. The reactions were carried out in ethanol/water mixtures in the presence of aqueous ammonia, between 0°C and 75°C, using both stirred and non-stirred conditions. TEM and SEM images show that the yield and microstructure of the silica is influenced by the synthetic conditions (temperature, ammonia (aq) concentration, gelation time, solvent mixture). It was observed that the chiral form of the tartaric acid used and the diffusion of TEOS to the template determines the eventual silica structure. 相似文献
34.
Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements 总被引:1,自引:0,他引:1
Miranda N Daublain P Horner JH Newcomb M 《Journal of the American Chemical Society》2003,125(18):5260-5261
Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions. 相似文献
35.
James N. Sturgis C. Neil Hunter Robert A. Niederman 《Photochemistry and photobiology》1988,48(2):243-247
Abstract— Membranes from Rhodobacter (formerly Rhodopseudomonas) sphaeroides mutant strains that lack one or more of the bacteriochlorophyll a (BChl)-protein complexes were used to obtain spectra and molar extinction coefficients of the near-IR absorption bands. The strains examined were NF57 which lacks the B875 light-harvesting and reaction center complexes, and M21 which lacks the B800–850light-harvesting complex. The extinction coefficients obtained for the B800, B850 and B875 bands were 226 ± 10, 170 ± 5, and 118 ± 5 m M -1 cm-1 , respectively, in reasonable agreement with values reported for detergent-isolated complexes (Clayton, R. K. and B. J. Clayton, Proc. Natl. Acad. Sci. USA 78 ,5583–5587, 1981). The results also demonstrated that detergent solubilization altered the spectra, causing a band broadening on the blue side of the B875 and B800 peaks. The data obtained from the analysis of the mutant strains were used for deconvolution of the BChl species in membranes of the wild-type. A short BASIC computer program for performing this deconvolution is included. 相似文献
36.
The synthesis and chemical properties of a series of amino acid derivatives containing the thiolsulphonate functionality are described. 相似文献
37.
Xuejun Pan Dan Xie Neil Gilkes David J. Gregg Jack N. Saddler 《Applied biochemistry and biotechnology》2005,124(1-3):1069-1079
Pretreatment of Douglas-fir by steam explosion produces a substrate containing approx 43% lignin. Two strategies were investigated
for reducing the effect of this residual lignin on enzymatic hydrolysis of cellulose: mild alkali extraction and protein addition.
Extraction with cold 1% NaOH reduced the lignin content by only approx 7%, but cellulose to glucose conversion was enhanced
by about 30%. Before alkali extraction, addition of exogenous protein resulted in a significant improvement in cellulose hydrolysis,
but this protein effect was substantially diminished after alkali treatment. Lignin appears to reduce cellulose hydrolysis
by two distinct mechanisms: by forming a physical barrier that prevents enzyme access and by non-productively binding cellulolytic
enzymes. Cold alkali appears to selectively remove a fraction of lignin from steam-exploded Douglas-fir with high affinity
for protein. Corresponding data for mixed softwood pretreated by organosolv extraction indicates that the relative importance
of the two mechanisms by which residual lignin affects hydrolysis is different according to the pre- and post-treatment method
used. 相似文献
38.
39.
Ab initio molecular orbital calculations using the STO3-21G basis set has been carried out for the cluster series Na
n
+
, Na
n
, and Na
n
–
(wheren=2–7). The basis set is shown to be reliable compared with more extensive basis sets at the Hartree-Fock level. Thirty-one optimized structures are reported and discussed, many of which (especially for the anions) have not been considered. The STO3-21G//STO3-21G calculations suggest that for most of the species the optimum geometries are planar. In particular, the optimized structures for the anionic species should provide a starting point for more sophisticated configuration interaction calculations. 相似文献
40.
The stereochemistry observed in organocuprate SN2′ displacements can be rationalized as a stereoelectronic effect arising from “bidentate” binding involving a d orbital of nueleophilic copper and π* and σ* orbitals of the substrate. The extension of this idea to other reactions of organocuprates including additions to acetylenes and enones is discussed. 相似文献