全文获取类型
收费全文 | 349篇 |
免费 | 16篇 |
专业分类
化学 | 260篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 31篇 |
物理学 | 45篇 |
无线电 | 17篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 12篇 |
2015年 | 13篇 |
2014年 | 8篇 |
2013年 | 18篇 |
2012年 | 21篇 |
2011年 | 16篇 |
2010年 | 9篇 |
2009年 | 8篇 |
2008年 | 22篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 17篇 |
2004年 | 5篇 |
2003年 | 14篇 |
2002年 | 11篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1941年 | 2篇 |
1934年 | 2篇 |
排序方式: 共有365条查询结果,搜索用时 180 毫秒
141.
Ferretti S Lee SK MacCraith BD Oliva AG Richardson DJ Russell DA Sapsford KE Vidal M 《The Analyst》2000,125(11):1993-1999
Nitrite is an important human health and environmental analyte. As such, the European Union (EU) has imposed a limit for nitrite in potable water of 0.1 mg l-1 (2.18 microM). In order to develop an optical biosensing system for the determination of nitrite ions in environmental waters, cytochrome cd1 nitrite reductase has been extracted and purified from the bacterium Paracoccus pantotrophus. The protein has been spectroscopically characterised in solution and important kinetic parameters of nitrite reduction of the cytochrome cd1 enzyme, i.e., Km, Vmax and kcat have been determined. The influence of pH on the activity of the cytochrome cd1 has been investigated and the results suggest that this enzyme can be used for the determination of nitrite in the pH range 6-9. Biosensing experiments with the cytochrome cd1 in solution suggested that the decrease in intensity of the absorption band associated with the d1 haem (which is the nitrite binding site), at 460 nm, with increasing nitrite concentrations would enable the measurement of this analyte with the optimum limit of detection. The cytochrome cd1 has been encapsulated in a bulk sol-gel monolith with no structural changes observed and retention of enzymatic activity. The detection of nitrite ions in the range 0.075-1.250 microM was achieved, with a limit of detection of 0.075 microM. In order to increase the speed of response, a sol-gel sandwich thin film structure was formulated with the cytochrome cd1. This structure enabled the determination of nitrite concentrations within ca. 5 min. The sol-gel sandwich entrapped cytochrome cd1 enzyme was found to be stable for several months when the films were stored at 4 degrees C. 相似文献
142.
Ma José Negro Ignacio Ballesteros Paloma Manzanares José Miguel Oliva Felicia Sáez Mercedes Ballesteros 《Applied biochemistry and biotechnology》2006,132(1-3):922-932
The use of stalks instead of tubers as a source of carbohydrates for ethanol production has been investigated. The inulin
present in the stalks of Jerusalem artichoke was extracted with water and the effect of solid-liquid ratio, temperature, and
acid addition was studied and optimized in order to attain a high-fructose fermentable extract. The maximum extraction efficiency
(corresponding to 35 g/L) of soluble sugars was obtained at 1/6 solidliquid ratio.
Fermentations of hydrolyzed extracts by baker's yeast and direct fermentation by an inulinease activity yeast were also performed
and the potential to use this feedstock for bioethanol production assessed. The results show that the carbohydrates derived
from Jerusalem artichoke stalks can be converted efficiently to ethanol by acidic hydrolysis followed by fermentation with
Saccharomyces cerevisiae or by direct fermentation of inulin using Kluyveromyces marxianus strains. In this last case about 30 h to complete fermentation was required in comparison with 8–9 h obtained in experiments
with S. cerevisiae growth on acid extracted juices. 相似文献
143.
144.
Alvarez-Ibarra C Csákÿ AG Gómez de la Oliva C 《The Journal of organic chemistry》2002,67(9):2789-2797
alpha,beta-Didehydroglutamates have been diastereoselectively transformed into 6-oxoperhydropyridazine-3-carboxylic acid derivatives (OPCAs), which constitute a new class of cyclic amino acid derivatives. Acylation at N-1 renders dipeptides which show considerable conformational rigidity. Semiempirical calculations suggest that OPCAs might force peptide turns with different amplitudes depending on the substitution pattern and relative stereochemistry of the substituents of the pyridazinone ring. 相似文献
145.
Ballesteros I. Oliva J. M. Carrasco J. Cabañas A. Navarro A. A. Ballesteros M. 《Applied biochemistry and biotechnology》1998,(1):369-381
In this work, the effect of the addition of different concentrations of Tween-80 and three different zeolite-like products
on enzymatic hydrolysis, ethanol fermentation, and simultaneous saccharification and fermentation (SSF) process has been investigated.
The ability of these products to enhance the effectiveness of the SSF process to ethanol of steam-exploded poplar biomass
using the thermotolerant strainKluyveromyces marxianus EMS-26 has been tested.
Tween-80 (0.4 g/L) increased enzymatic hydrolysis yield by 20% when compared to results obtained in hydrolysis in absence
of the additive. Zeolite-like products (ZESEP-56 and ZECER-56) (2.5 g/L) improved rates of conversion and ethanol yields in
the fermentation of liquid fraction recovered from steam-exploded poplar. The periods required for the completion of fermentation
were approx 10 h in the presence of zeolite-like products and 24 h in the absence of additives. The probable mode of action
is through lowered levels of inhibitory substances because of adsorption by the additive. 相似文献
146.
Theoretical study of diels-alder reactions between substituted derivatives of butadiene and ethylene
The potential surfaces for the butadiene + ethylene and l-hydroxybutadiene+acrolein reactions, have been calculated (MINDO/2). A simplified study of the potential surface for several reactions between monosubstituted derivatives of butadiene and ethylene has also been done (MINDO/3). From the results obtained, the endo-exo ratio, the regioselectivity, the influence of substituents on the reaction rate and the mechanism of the process are discussed. 相似文献
147.
Oliva AI Simón L Muñiz FM Sanz F Ruiz-Valero C Morán JR 《The Journal of organic chemistry》2004,69(20):6883-6885
A combination of a benzoxanthene cleft-type receptor with an electron-rich aromatic ring capable of establishing charge-tranfer interactions provides enantioselective receptors for dinitrobenzoylamino acids. Racemic mixtures of the receptor can be resolved with TLCs impregnated with the guest. The structure of the complexes has been established in an X-ray study. Enantiomeric amino acids provide complexes with different colors. 相似文献
148.
Oliva JM Allan NL Schleyer PV Viñas C Teixidor F 《Journal of the American Chemical Society》2005,127(39):13538-13547
Neutral and especially dianionic 6- and 12-vertex closo ortho-carboranes (o-carboranes) 1,2-R2-1,2-C2BnHn (R = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, as well as e-, CH2-, NH-, O-, SiH2-, PH-, and S- exhibit extremely large variations (over 1 A!) of the cage CC distances, from 1.626 to 2.638 A, at the B3LYP/6-31G//B3LYP/6-31G DFT level. These CC "bond lengths," among the longest ever reported, generally are greater in the icosahedral than those in the corresponding octahedral systems and depend strongly on the substituents. While 1,2-(NH2)2-1,2-C2B10H10 has the longest Cc...Cc separation in neutral species (1.860 A), Cc...Cc distances can be much larger in the corresponding dianions. These range from 1.823 A (R- = e-) to 2.638 A (R- = CH2-) for 1,2-(R-)2-1,2-C2B10H10 and from 1.626 A (R- = SiH2-) to 3.099 A (R- = NH-) for 1,2-(R-)2-1,2-C2B4H4. Remarkably, there is no abrupt discontinuity over the entire range of CC lengths. Consequently, the relationship between the gradual changes in the distances and the nature of the bonding was analyzed by means of the form of the Kohn-Sham orbitals, the Wiberg Cc...Cc bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating Cc...Cc distances. 相似文献
149.
A. Gonzlez-Lafont J. M. Lluch A. Oliva J. Bertrn 《Journal of computational chemistry》1988,9(8):819-826
Monte Carlo statistical thermodynamic computer simulations are reported for several clusters Fe+ (H2O)n at different temperatures and for a dilute aqueous solution of Fe+ at 298 K. The energy of each configuration has been calculated in the pairwise additivity approximation using the MCY potential for the water–water interaction and an ab initio analytical potential built by us for the Fe+–H2O interaction. Energy and structural analysis of the generated configurations lead to the prediction of a coordination number of six for the first hydration shell of the Fe+ ion, both in clusters and in dilute solution. Finally, the variation in the distance to the Fe+ ion of the energy and orientation of water molecules in the solution are discussed. 相似文献
150.
Ballesteros I. Oliva J. M. Ballesteros M. Carrasco J. 《Applied biochemistry and biotechnology》1993,(1):201-211
Different treatments to improve the thermotolerance of fermenting yeasts for simultaneous ethanol saccharification and fermentation
process of cellulosic materials have been examined. Yeasts of the generaSaccharomyces andKluyveromyces were tested for growth and fermentation at progressively higher temperatures in the range of 42–47°C. The best results were
obtained withK. marxianus LG, which was then submitted to different treatments in order to achieve thermotolerant clones. A total of 35 new clones
were obtained that dramatically improved the SSF of 10% Solka-floc substrate at 45°C when compared to the original strain,
some with ethanol concentrations as high as 33 g/L. 相似文献