全文获取类型
收费全文 | 325篇 |
免费 | 26篇 |
国内免费 | 2篇 |
专业分类
化学 | 258篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 33篇 |
物理学 | 51篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 12篇 |
2021年 | 15篇 |
2020年 | 18篇 |
2019年 | 18篇 |
2018年 | 9篇 |
2017年 | 13篇 |
2016年 | 27篇 |
2015年 | 23篇 |
2014年 | 30篇 |
2013年 | 31篇 |
2012年 | 19篇 |
2011年 | 25篇 |
2010年 | 18篇 |
2009年 | 8篇 |
2008年 | 22篇 |
2007年 | 13篇 |
2006年 | 16篇 |
2005年 | 17篇 |
2004年 | 8篇 |
2003年 | 2篇 |
2002年 | 2篇 |
1982年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有353条查询结果,搜索用时 359 毫秒
101.
Tert‐butyl hydroperoxide‐promoted guanylation of amines with benzoylthioureas: Mechanistic insights by HRMS and 1H NMR
下载免费PDF全文
![点击此处可从《Journal of Physical Organic Chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Henrique Esteves Tiago Oliveira Brito Renato Ribeiro‐Viana Ângelo de Fátima Fernando Macedo Jr. 《Journal of Physical Organic Chemistry》2017,30(12)
Herein, we present a mechanistic study on tert‐butyl hydroperoxide‐promoted guanylation of thioureas by monitoring short lifetime intermediates using electrospray ionisation/time‐of‐flight high resolution mass spectrometry (ESI‐Q‐TOF HRMS). Moreover, 1H nuclear magnetic resonance data allowed us to access kinetic parameters for the main species involved which, allied to the HRMS results, furnished valuable insights over previously reported observations. The results suggested an addition/elimination mechanism involving the aminoiminomethanesulphinic acid, RNC(SO2H)NHR′, and the nucleophilic amine as the main pathway to yield the guanidine. Noteworthy, benzoylthiourea consumption rate presented a nonlinear kinetic behaviour, while tBuOOH and the nucleophilic amine consumptions were found to follow second‐order kinetics. 相似文献
102.
103.
The effect of microwave radiation on some thermal,rheological and structural properties of cassava starch 总被引:1,自引:0,他引:1
Tiago André Denck Colman Ivo Mottin Demiate Egon Schnitzler 《Journal of Thermal Analysis and Calorimetry》2014,115(3):2245-2252
Three samples of native or untreated cassava starch were exposed to microwave radiation for periods of 5, 10 or 15 min. The temperature of each sample was measured immediately after each exposure time and the temperature of the samples was around 135 °C. The samples were cooled to room temperature and maintained in a desiccator with anhydrous calcium chloride. All the samples were analysed by thermogravimetry-derivative thermogravimetry, differential scanning calorimetry (DSC), rapid viscoamylographic analysis (RVA), X-ray diffraction powder patterns, non-contact atomic force microscopy and colour characteristics by reflectance spectrophotometry. The thermal behaviour, gelatinisation temperatures, enthalpy and pasting properties were determined. Relative to the time of microwave exposure, the peak viscosity and gelatinisation (RVA and DSC) increased slightly after 5 min, and, after 10 and 15 min, it decreased considerably. The degree of relative crystallinity (%) decreased, while the average roughness increased. The reflectance spectrophotometry showed that microwave action occured quickly and progressively, causing colour changes (mainly with trends to yellow) and very small differences to the starch samples that were heated at controlled temperature in a conventional oven. 相似文献
104.
105.
106.
Hugo Gallardo Fernando R. Bryk André A. Vieira Tiago E. Frizon Gilmar Conte Bruno S. Souza 《Liquid crystals》2013,40(8):839-845
The thermal and optical properties of two series of unsymmetrical liquid crystalline compounds based on an isoxazole ring are described. Of these materials, 3–7 and 8–12 displayed strong absorption at 285 nm and a weak blue fluorescence in solution at around 377 nm, even with the presence of a high conjugated core. The fluorescence quantum yields observed varied from low to moderate (Φ F ?=?1–62%), with large Stokes shifts (60–178 nm). All compounds exhibited liquid crystalline behaviour with a wide mesomorphism temperature range and characteristic phases of calamitic compounds, among them smectic A (SmA), smectic C (SmC) and nematic (N) phases. Their phases were characterised using polarising optical microscopy and differential scanning calorimetry. In addition, with X-ray diffraction experiments, layer spacing of 33.3–40.0 Å were observed for the smectic phases. 相似文献
107.
Milena M. Andrade Aneli M. Barbosa Matheus R. Bofinger Robert F. H. Dekker Josana M. Messias Carmen L. B. Guedes Tiago Zaminelli Bruno H. de Oliveira Valéria M. G. de Lima Luiz H. Dall’Antonia 《Applied biochemistry and biotechnology》2013,170(7):1792-1806
The effects of soybean and castorbean meals were evaluated separately, and in combinations at different ratios, as substrates for lipase production by Botryosphaeria ribis EC-01 in submerged fermentation using only distilled water. The addition of glycerol analytical grade (AG) and glycerol crude (CG) to soybean and castorbean meals separately and in combination, were also examined for lipase production. Glycerol-AG increased enzyme production, whereas glycerol-CG decreased it. A 24 factorial design was developed to determine the best concentrations of soybean meal, castorbean meal, glycerol-AG, and KH2PO4 to optimize lipase production by B. ribis EC-01. Soybean meal and glycerol-AG had a significant effect on lipase production, whereas castorbean meal did not. A second treatment (22 factorial design central composite) was developed, and optimal lipase production (4,820 U/g of dry solids content (ds)) was obtained when B. ribis EC-01 was grown on 0.5 % (w/v) soybean meal and 5.2 % (v/v) glycerol in distilled water, which was in agreement with the predicted value (4,892 U/g ds) calculated by the model. The unitary cost of lipase production determined under the optimized conditions developed ranged from US$0.42 to 0.44 based on nutrient costs. The fungal lipase was immobilized onto Celite and showed high thermal stability and was used for transesterification of soybean oil in methanol (1:3) resulting in 36 % of fatty acyl alkyl ester content. The apparent K m and V max were determined and were 1.86 mM and 14.29 μmol min?1 mg?1, respectively. 相似文献
108.
Fernando C. Moraes Tiago A. Silva Ivana Cesarino Marcos R. V. Lanza Sergio A. S. Machado 《Electroanalysis》2013,25(9):2092-2099
A sensitive way to determine levofloxacin using a sensor based on vertical aligned carbon nanotubes is described. The morphology and the electrochemical performance of the electrodes were characterised by atomic force microscopy, cyclic voltammetry and square wave voltammetry. A scan‐rate study and electrochemical impedance spectroscopy showed that the levofloxacin oxidation product is adsorbed on the electrode surface. Differential pulse voltammetry in phosphate‐buffer solution allowed the development of a method to determine levofloxacin levels in the range of 1.0–10.0 µmol L?1, with a detection limit of 75.2 nmol L?1. The proposed sensor was successfully applied in the determination of levofloxacin in urine, and the obtained results were in full agreement with those from the HPLC procedure. 相似文献
109.
Tiago Silva José Teixeira Prof. Fernando Remião Prof. Fernanda Borges 《Angewandte Chemie (International ed. in English)》2013,52(4):1110-1121
Recent years have seen a significant increase in published data supporting the positive effects of statins on neurodegenerative diseases, in particular on Alzheimer’s disease. Statins show neuroprotective activity by a combination of different cellular and systemic mechanisms that are based on the inhibition of the biosynthesis of cholesterol and isoprenoid by‐products. The promising results obtained in vivo and in epidemiological studies are generally not in accordance with those of placebo‐controlled randomized clinical trials. Nevertheless, these results make statins valuable assets for disease prevention rather than therapeutic agents for use when disease symptoms are already displayed. Thus, the modulation of midlife cholesterol and/or statin administration prior to the appearance of dementia or cognitive impairment may have a better long‐term outcome. 相似文献
110.
Garcia CC Angeli JP Freitas FP Gomes OF de Oliveira TF Loureiro AP Di Mascio P Medeiros MH 《Journal of the American Chemical Society》2011,133(24):9140-9143
Acetaldehyde is an environmentally widespread genotoxic aldehyde present in tobacco smoke, vehicle exhaust and several food products. Endogenously, acetaldehyde is produced by the metabolic oxidation of ethanol by hepatic NAD-dependent alcohol dehydrogenase and during threonine catabolism. The formation of DNA adducts has been regarded as a critical factor in the mechanisms of acetaldehyde mutagenicity and carcinogenesis. Acetaldehyde reacts with 2'-deoxyguanosine in DNA to form primarily N(2)-ethylidene-2'-deoxyguanosine. The subsequent reaction of N(2)-ethylidenedGuo with another molecule of acetaldehyde gives rise to 1,N(2)-propano-2'-deoxyguanosine (1,N(2)-propanodGuo), an adduct also found as a product of the crotonaldehyde reaction with dGuo. However, adducts resulting from the reaction of more than one molecule of acetaldehyde in vivo are still controversial. In this study, the unequivocal formation of 1,N(2)-propanodGuo by acetaldehyde was assessed in human cells via treatment with [(13)C(2)]-acetaldehyde. Detection of labeled 1,N(2)-propanodGuo was performed by HPLC/MS/MS. Upon acetaldehyde exposure (703 μM), increased levels of both 1,N(2)-etheno-2'-deoxyguanosine (1,N(2)-εdGuo), which is produced from α,β-unsaturated aldehydes formed during the lipid peroxidation process, and 1,N(2)-propanodGuo were observed. The unequivocal formation of 1,N(2)-propanodGuo in cells exposed to this aldehyde can be used to elucidate the mechanisms associated with acetaldehyde exposure and cancer risk. 相似文献