全文获取类型
收费全文 | 175篇 |
免费 | 11篇 |
国内免费 | 2篇 |
专业分类
化学 | 152篇 |
晶体学 | 3篇 |
力学 | 1篇 |
数学 | 11篇 |
物理学 | 21篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 8篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 5篇 |
2017年 | 8篇 |
2016年 | 10篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 16篇 |
2012年 | 7篇 |
2011年 | 25篇 |
2010年 | 15篇 |
2009年 | 9篇 |
2008年 | 12篇 |
2007年 | 15篇 |
2006年 | 8篇 |
2005年 | 3篇 |
2004年 | 4篇 |
2002年 | 2篇 |
1999年 | 1篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有188条查询结果,搜索用时 218 毫秒
91.
In this work, the dependence of the ethanol production using Saccharomyces cerevisiae 251TP(3-2) on calcium, sodium, and magnesium ion concentration and interaction effects were studied with the use of a statistical
experimental design. The parameters of the ethanol concentration model proposed on the basis of Box–Wilson experimental design
method were evaluated with the use of the experimental data. Comparison of the predicted values from the model with the experimentally
observed values showed that the model is a good fit. From the analysis of model equation, it was seen that sodium ion concentration
has significant main effects on ethanol production, and there is interactive effect only between calcium and magnesium. With
the use of developed model, maximum ethanol concentration of 3.73% (v/v) was obtained when calcium, sodium, and magnesium concentration were 1,515, 930, and 128 mg/L, respectively, for the 10%
sugar concentration in synthetic molasses. 相似文献
92.
In this study, acrylamide (AAm)/aconitic acid (ACA) copolymers were prepared with two different mol% of aconitic acid 4%, 17% and were irradiated with gamma irradiation at different irradiation doses (4 - 25kGy). The percent yield was assigned by gravimetrical method. The effect of irradiation dose, pH and involved amounts of monomers (AAm/ACA) in hydrogels on swelling properties were investigated. The conversion of monomers to hydrogels was 100% at 25kGy. Poly(acrylamide-co-aconitic acid) P(AAm/ACA) hydrogels have been used for the adsorption of some aqueous solutions of dyes such as Methylene Blue (MB) and Safranine-O (S). The hydrogels were swollen in distilled water at pH 3, 5, 7, 8 and in aqueous solutions of dyes. The initial swelling rates of hydrogels are increased by increasing of pH. The effects of concentration of the aqueous solutions of dye and hydrogel composition on the adsorption were investigated. The adsorption is increased and changed depending on the structure of dye and composition of hydrogel. 相似文献
93.
94.
Determination of citalopram by capillary electrophoresis is described. Compounds were separated at 28 kV in 75 μm i.d. fused silica capillary tubing (total length 85 cm, effective length 65 cm) with 10 mM borate buffer, pH 8.5, containing 10% (v/v) methanol as running buffer. Citalopram and propylparaben (IS) appeared at 3.5 and 5.5 min, respectively. Repeatable linear results were obtained. The limits of detection and quantification were 5.73 × 10−6 and 1.72 × 10−5 M, respectively. When citalopram was determined in a pharmaceutical tablet by capillary electrophoresis and by a UV-spectrophotometric method differences between the results were not significant. The citalopram content of tablets was 100.8 ± 2.95% of the label claim. The amount found in serum was 26.7 ± 0.1% of the free drug, indicating that 73.3% of the drug was bound to protein. 相似文献
95.
Determination of total antioxidant capacity by a new spectrophotometric method based on Ce(IV) reducing capacity measurement 总被引:2,自引:0,他引:2
Dietary antioxidants widely found in fruits and vegetables may serve the task of reducing oxidative damage in humans induced by free radicals and reactive oxygen species under ‘oxidative stress’ conditions. The aim of this work is to develop a simple, low-cost, sensitive, and diversely applicable indirect spectrophotometric method for the determination of total antioxidant capacity of several plants. The method is based on the oxidation of antioxidants with cerium(IV) sulfate in dilute sulfuric acid at room temperature. The Ce(IV) reducing capacity of the sample is measured under carefully adjusted conditions of oxidant concentration and pH such that only antioxidants and not other organic compounds would be oxidized. The spectrophotometric determination of the remaining Ce(IV) was performed after completion of reaction with antioxidants. Quercetin and gallic acid were used as standards for flavonoids and phenolic acids, respectively, and results of antioxidant measurements were reported as trolox equivalents. The developed procedure was successfully applied to the assay of total antioxidant capacity due to simple compounds such as trolox, quercetin, gallic acid, ascorbic acid, catechin, naringin, naringenin, caffeic acid, chlorogenic acid, ferulic acid, and p-coumaric acid, and due to phenolic acids and flavonoids in the arieal parts of nettle (Urtica Dioica L.). Blank correction of significantly absorbing plant extracts at 320 nm could be made with the aid of spectrophotometric titration. Plant selection was made in respect to high antioxidant content, and extraction was made with water. The proposed method was reproducible, and the trolox equivalent antioxidant capacities (TEAC coefficients) of the tested antioxidant compounds were correlated to those found by reference methods such as ABTS and CUPRAC. Since the TEAC coefficients found with the proposed method of naringin-naringenin and rutin-catechin pairs were close to each other, this Ce(IV)-based assay probably caused the simultaneous hydrolysis of flavonoid glycosides to the corresponding aglycones and their subsequent oxidation such that the hydrolysis products exhibed antioxidant capacities roughly proportional the number of -OH groups contained in a molecule. 相似文献
96.
A new route to bilanes and porphyrins bearing four distinct meso substituents has been studied to elucidate the scope and gain entry to previously inaccessible compounds. The route entails (i) synthesis of a 1-bromo-19-acylbilane by acid-catalyzed condensation of a 1-acyldipyrromethane and a 9-bromodipyrromethane-1-carbinol and (ii) intramolecular cyclization of the 1-bromo-19-acylbilane in the presence of a metal salt (MgBr2, 3 mol equiv) and a non-nucleophilic base (DBU, 10 mol equiv) in a noncoordinating solvent (toluene) at 115 degrees C exposed to air to afford the corresponding magnesium(II) porphyrin. In this study, two sets of bilanes were initially prepared to explore substituent effects. In the first set, all bilanes vary only in the nature of the substituent at the 10-position. In the second set, all bilanes vary only in the nature of the substituent attached to the acyl unit (the 20-position). The substituents examined at the 10- and 20-positions include alkyl, aryl (electron-rich, electron-deficient, hindered), heteroaryl, ester, or no substituent (-H). The bilanes were obtained in 35-87% yield, and the target porphyrins in up to 60% yield. Further study of the scope focused on bilanes and porphyrins bearing three heterocyclic substituents (o-, m-, p-pyridyl) or four alkyl groups (ethyl, propyl, butyl, pentyl), in which case microwave irradiation was used for the porphyrin-forming step. Altogether, 17 bilanes and 19 porphyrins were prepared and characterized. In summary, the new route provides access to meso-substituted bilanes and porphyrins for which access is limited via other methods. 相似文献
97.
Physcia semipinnata was exposed to UV-A (352nm) and visible light (210, 800 and 2000mW/cm2) for 30min, 1, 2, 24, and 48h to seek the alterations in the PSII photosynthetic quantum yield, in response to radiation. Chlorophyll a fluorescence did not influence exposure to light, 210, 800 and 2000mW/cm2. Significant alterations of the photosynthetic quantum yield ratio occurred in response to increase in UV-A exposure time. The photosynthetic quantum yield ratio decreased in P. semipinnata following exposure to UV-A for 24 and 48h. The thalli of P. semipinnata treated with 1mM polyamine were not influenced during the exposure to UV-A for 24 and 48h. It was also found that exogenously spd added samples had higher chla content than spm and put added samples. In this study, we showed that lipid peroxidation levels between UV-A-treated samples and exogenously polyamine treated samples that were previously exposed to UV-A for 24 and 48h were significantly decreased. This result is the first record to indicate that external polyamines might have some protective role on photosystem II and membrane against UV-A stress. 相似文献
98.
İsmet Kaya İbrahim Erdem Orta Eyüp Özdemir Dilek Şenol 《Journal of the Iranian Chemical Society》2018,15(1):35-46
In this study, Schiff bases containing azomethine (–HC=N–) compounds were synthesized from the condensation reaction of 4-aminophenol and 2-aminophenol and 5-methyl-2-furaldehyde. Orange-colored Schiff bases, 4-[(5-methyl- 2-furyl) methyleneamino] phenol (4MFMAP) and 2-[(5-methyl-2-furyl) methyleneamino] phenol (2MFMAP) turned to poly-4-[(5-methyl-2-furyl) methyleneamino] phenol (P-4MFMAP) and poly-2-[(5-methyl-2-furyl) methyleneamino] phenol (P-2MFMAP) polyazomethine derivatives by NaOCl oxidant in alkaline aqueous medium. In addition, poly-2-[(5-methyl-2-furyl) methyleneamino] phenol cobalt (II) and copper (II) complexes were synthesized. FTIR and NMR measurements for structural analysis of the synthesized compounds, UV–Vis analysis for optical properties, electrical conductivity measurements, and thermal analysis with the TG–DTA techniques were determined. The yield of P-4MFMAP and P-2MFMAP was found to be 84.6% and 78.6 as follows [4MFMAP] = 0.066, [KOH] = 0.198 [NaOCl] = 0.066 mol L?1 at 60 °C for 3 h and [2MFMAP] = 0.066, [KOH] = 0.132 [NaOCl] = 0.066 mol L?1 at 70 °C for 3 h, respectively. 相似文献
99.
A new synthetic method is reported for the N-phosphorylation of pyrroles via intramolecular migration of a phosphonate diester group. The reaction proceeds under mild conditions and produces N-phosphorylated pyrrole derivatives in moderate to good yields. 相似文献
100.
Akın Baysal Feyyaz Durap Bahattin Gümgüm Leyla T. Yıldırım Dinçer Ülkü Ayse Dilek Boğa Saim Özkar 《Helvetica chimica acta》2007,90(6):1211-1217
The reaction of 1,5‐dihydro‐2H‐cyclopenta[1,2‐b:5,4‐b′]dipyridin‐2‐one ( 3 ) with an alkylamine (butylamine, hexylamine or ethylenediamine) yields, quite unexpectedly and in the absence of catalyst, the novel compound 1,5‐dihydro‐2H‐cyclopenta[1,2‐b:5,4‐b′]dipyridin‐2‐imine ( 4 ) as the sole, analytically pure, solid product, which was fully characterized. The structure of 4 was unequivocally solved by single‐crystal X‐ray‐diffraction analysis. The compound crystallizes in a monoclinic cell (space group P 21/c), with two molecules in the asymmetric unit, held together by intermolecular H‐bonds. Compound 4 could be interesting as a bi‐ or even tridentate ligand, and exhibits a strong fluorescence upon excitation at 310 nm. A mechanism, based on the observed C? N bond cleavage, is proposed. 相似文献