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排序方式: 共有83条查询结果,搜索用时 31 毫秒
1.
Gurmessa Getahun Tadesse Singh Girija Shankar 《Research on Chemical Intermediates》2017,43(11):6447-6504
Research on Chemical Intermediates - Metal–carbenoids from α-diazocarbonyl compounds are well-known reactive intermediates with a long history of useful applications in synthetic organic... 相似文献
2.
In this paper a new method is described for determining the triplet distribution function g3 from computer simulations of dense fluids. The method involves expressing g3 in terms of hyperspherical coordinates (?,θ,φ) and then expanding g3 in spherical harmonics of the angular variables θ and φ. We first test the convergence of the spherical harmonic expansion by applying the method to the superposition approximation for g3. We then show how the expansion coefficients may be obtained from an analysis of computer simulation data. Lastly, we report new results for g3 obtained by applying the method to data from a molecular dynamics simultion of the Lennard-Jones fluid. 相似文献
3.
Ashebir Fiseha Mekuria Tadesse Tesfaye Bekele Belete Bedemo 《Chemistry of Natural Compounds》2010,46(5):692-695
Vernonia galamensis is a new potential industrial oil seed crop for semiarid areas with very high content of naturally epoxidized vernolic acid
(80% of the oil). Chemical investigation of the seeds of Vernonia galamensis afforded two compounds, which are vernolic acid derivatives. Their structures were established by various spectroscopic techniques:
IR, UV, NMR, and 2D NMR spectra (COSY, HSQC, and HMBC), and by comparison with literature data for the known compound methylvernolate.
The cis-(12S,13R)-(3-methylpentyl) vernolate (1) and the cis-(12S,13R)-(2,3-propanediol) vernolate (2) were isolated and characterized. 相似文献
4.
Mahelet Aweke Tadesse Carlo Galli Patrizia Gentili 《Journal of Physical Organic Chemistry》2011,24(7):529-538
The aminoxyl radical 6‐trifluoromethyl‐benzotriazol‐N‐oxyl (TFNO) has been generated from the parent hydroxylamine 6‐CF3‐1‐hydroxy‐benzotriazole (TFBT) by one‐electron oxidation with a CeIV salt and characterized by spectrophotometry and cyclic voltammetry (CV). Rate constants of H‐abstraction (kH) by TFNO from a number of H‐donor benzylic substrates have been determined spectrophotometrically in MeCN solution at 25 °C. A radical H‐atom transfer (HAT) route of oxidation is substantiated for TFNO by several pieces of evidence. The kinetic data also testify the relevance of stereoelectronic effects upon the HAT reactivity of TFNO. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
5.
Continuous and pulsed ultrasound-assisted extractions of antioxidants from pomegranate peel 总被引:1,自引:0,他引:1
There is a great demand for developing efficient extraction methods in order to reduce extraction time and increase the yield and activity of functional antioxidants. The yields, activities, and extraction kinetics of antioxidants from dry peel of pomegranate marc were studied using ultrasound-assisted extraction in continuous and pulsed modes and the results were compared with conventional extraction (CE) at a temperature of 25±2°C and water/peel ratio of 50/1, w/w. The studied factors were intensity level and treatment time for continuous ultrasound-assisted extraction (CUAE), and intensity level, number of pulse repetition, and pulse duration and interval for pulsed ultrasound-assisted extraction (PUAE). The results showed that all factors significantly affected the antioxidant yield, but only treatment time had a significant effect on the antioxidant activity. Compared to CE, PUAE at intensity level of 59.2 W/cm(2), and the 5 and 5s of pulse duration and interval increased the antioxidant yield by 22% and reduced the extraction time by 87%. Similarly, CUAE at the same intensity level increased the antioxidant yield by 24% and reduced the extraction time by 90%. Since PUAE had 50% energy saving compared to CUAE, we recommend using PUAE for the extraction with antioxidant yield of 14.5% and DPPH scavenging activity of 5.8 g/g. A second-order kinetic model was successfully developed for describing the mechanism of ultrasound-assisted extractions under PUAE and CUAE. This research clearly demonstrated the superiority of PUAE for producing antioxidants from peel of pomegranate marc. 相似文献
6.
Chueh WC Yang CK Garland CM Lai W Haile SM 《Physical chemistry chemical physics : PCCP》2011,13(14):6442-6451
The impact of hydration on the transport properties of microcrystalline Sm(0.15)Ce(0.85)O(1.925) has been examined. Dense, polycrystalline samples were obtained by conventional ceramic processing and the grain boundary regions were found, by high resolution transmission electron microscopy, to be free of impurity phases. Impedance spectroscopy measurements were performed over the temperature range 250 to 650 °C under dry, H(2)O-saturated, and D(2)O-saturated synthetic air; and over the temperature range 575 to 650 °C under H(2)-H(2)O atmospheres. Under oxidizing conditions humidification by either H(2)O or D(2)O caused a substantial increase in the grain boundary resistivity, while leaving the bulk (or grain interior) properties unchanged. This unusual behavior, which was found to be both reversible and reproducible, is interpreted in terms of the space-charge model, which adequately explains all the features of the measured data. It is found that the space-charge potential increases by 5-7 mV under humidification, in turn, exacerbating oxygen vacancy depletion in the space-charge regions and leading to the observed reduction in grain boundary conductivity. It is proposed that the heightened space-charge potential reflects a change in the relative energetics of vacancy creation in the bulk and at the grain boundary interfaces as a result of water uptake into the grain boundary core. Negligible bulk water uptake is detected under both oxidizing and reducing conditions. 相似文献
7.
8.
采用自悬浮定向流法制备纳米Ni粉体,利用放电等离子烧结技术制备出了直径10 mm、厚2 mm,致密度为96.8 %,显微硬度为4.17 GPa的纳米块体材料。用X射线衍射仪、扫描电子显微镜和显微硬度计分析了烧结块体样品的相组成、晶粒尺寸、微观形貌和显微硬度。研究表明:随烧结温度的升高,块体样品的致密度和晶粒尺寸增大,当烧结温度为650 ℃时,致密度最高,晶粒尺寸为44.8 nm;显微硬度随烧结温度的增高先增大后减小,当烧结温度为550 ℃时,显微硬度最大为4.33 GPa;较高烧结温度下,断口微观形貌的纳米级韧窝出现,显示了韧性断裂的特征。 相似文献
9.
Hui Xu Xiangdong Gu Mahlet G. Tadesse Raji Balasubramanian 《Journal of computational and graphical statistics》2013,22(4):763-772
We present an ensemble tree-based algorithm for variable selection in high-dimensional datasets, in settings where a time-to-event outcome is observed with error. This work is motivated by self-reported outcomes collected in large-scale epidemiologic studies, such as the Women’s Health Initiative. The proposed methods equally apply to imperfect outcomes that arise in other settings such as data extracted from electronic medical records. To evaluate the performance of our proposed algorithm, we present results from simulation studies, considering both continuous and categorical covariates. We illustrate this approach to discover single nucleotide polymorphisms that are associated with incident Type 2 diabetes in the Women’s Health Initiative. A freely available R package icRSF has been developed to implement the proposed methods. Supplementary material for this article is available online. 相似文献
10.
Zhang H Mitsumori S Utsumi N Imai M Garcia-Delgado N Mifsud M Albertshofer K Cheong PH Houk KN Tanaka F Barbas CF 《Journal of the American Chemical Society》2008,130(3):875-886
The development of enantioselective anti-selective Mannich-type reactions of aldehydes and ketones with imines catalyzed by 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives is reported in detail. Both (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid and (R)-3-pyrrolidinecarboxylic acid efficiently catalyzed the reactions of aldehydes with alpha-imino esters under mild conditions and afforded anti-Mannich products with high diastereo- and enantioselectivities (anti/syn up to 99:1, up to >99% ee). For the reactions of ketones with alpha-imino esters, (R)-3-pyrrolidinecarboxylic acid was an efficient catalyst (anti/syn up to >99:1, up to 99% ee). Evaluation of a series of pyrrolidine-based catalysts indicated that the acid group at the beta-position of the pyrrolidine ring of the catalyst played an important role in forwarding the carbon-carbon bond formation and in directing anti-selectivity and enantioselectivity. 相似文献