全文获取类型
收费全文 | 302篇 |
免费 | 7篇 |
学科分类
数理化 | 309篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 9篇 |
2014年 | 16篇 |
2013年 | 32篇 |
2012年 | 18篇 |
2011年 | 23篇 |
2010年 | 11篇 |
2009年 | 10篇 |
2008年 | 13篇 |
2007年 | 16篇 |
2006年 | 10篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 11篇 |
2002年 | 9篇 |
2001年 | 7篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有309条查询结果,搜索用时 15 毫秒
91.
Dwairath Dhar Ian Beadham Srinivasan Chandrasekaran 《Journal of Chemical Sciences》2003,115(5-6):365-372
New organic-inorganic hybrid catalysts were synthesized by covalent grafting of proline and benzylpenicillin derivatives into
mesoporous MCM-41. These catalysts were extensively characterized using FT-IR,13C CP MAS solid stateNMR, XRD and TEM techniques. These were used as catalysts for direct, asymmetric aldol reaction between
acetone and activated aromatic aldehydes. In the reaction of 4-nitro and 4-fluoro benzaldehyde, the aldol products were obtained
in 36% and 59%ee respectively. The catalysts were reusable with neither significant drop in enantioselectivity nor loss of mesostructure.
An attempt was made to substantiate the proposed ’enamine’ mechanism for direct aldol reaction by trapping the intermediate
between proline-MCM-41 and acetone.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
92.
Ernest L. Eliel Kwang-ting Liu Subramanian Chandrasekaran 《Magnetic resonance in chemistry : MRC》1983,21(3):179-182
17O chemical shifts of axial hydroxyl groups in cyclohexanols are upfield of those of corresponding equatorial groups, but in 5-hydroxy-1,3-dioxanes the opposite is observed: the axial OH resonates downfield of the equatorial OH. The situation is the same in the corresponding methyl ethers and is, thus, not a result of intramolecular hydrogen bonding in the axial 5-hydroxy-1,3-dioxane, but appears to parallel the effect on 13C and 19F shifts observed in corresponding equatorial and axial 5-methyl- and 5-fluoro-1,3-dioxanes, which has been attributed to an upfield shifting effect of the antiperiplanar γ-located heteroatoms. Surprisingly, the reciprocal effect is not seen in the ring 17O shifts of the 5-hydroxy-1,3-dioxanes. A δ compression shift is seen in the 17O spectrum of trans-3,3,5-trimethylcyclohexanol (syn-axial OH and CH3), analogous to the effect earlier reported in 13C spectra. Conversion of four of the alcohols to methyl ethers produces a large upfield effect on the 17O shift, larger in the cyclohexanols than in the 1,3-dioxane-5-ols. Similar upfield shifts have been recorded in the literature; their extent depends on whether the alcohols are primary, secondary or tertiary. 相似文献
93.
Degeneracy checking in linear programming is NP-complete. So is the problem of checking whether there exists a basic feasible solution with a specified objective value. 相似文献
94.
The standard rate constants for the reduction of europium in perchlorate and chloride media and the formal rate constants in sulphate, acetate and lactate solutions are reported. The rate increases in the order of increasing polarisability of the anion or the increasing stability of the complex,viz., perchlorate < sulphate > lactate ≈ acetate. The effect of methanol on the polarography of europium has been studied. In perchlorate and chloride media, the half-wave potential shifts to negative values and then to positive values as the methanol content is increased. In acetate solutions, the shift is continuously to negative potentials. This is attributed to the effect of complex formation in acetate media and ion-pairing in perchlorate and chloride media. The effect of viscosity on the diffusion current was found to obey the Stokes-Einstein equation. 相似文献
95.
This article presents our work on the σ-Ferrier ring-expansion of carbohydrate derived vinylcyclopropanes (VCPs) under electrophilic conditions mediated by chloramine-T and a phase-transfer catalyst. The present work serves as the first example on the studies of the reactivity of carbohydrate VCPs towards the synthesis of densely functionalized oxepane analogues. The work elaborates on a reasonable mechanism for the product formation and our observations on the diastereoselectivity based on control experiments and gas-phase calculations. 相似文献
96.
This paper is focused on a spectroscopic study of some ancient pottery shreds from an archaeological site Manaveli village, Puducherry, India. Analytical characterization using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and differential scanning colorimetric coupled with thermo gravimetric analysis were carried out on red and black ware and red ware recently excavated from the above site. The experimental results of FT-IR and XRD are similar and allowed us to identify the mineralogical composition of pottery samples. In addition, TGA was applied in order to study the dehydration of hydroscopic water and decomposition of carboxyl group in the powdered pottery samples during heating. Moreover, this paper proves that all the above spectroscopic techniques are very useful analytical tool for the examination of ancient pottery, which is also suitable for the identification of its firing temperature and firing atmosphere. 相似文献
97.
Nagarajan Usharani Gladstone Christopher Jayakumar Swarna Vinodh Kanth Jonnalagadda Raghava Rao Bangaru Chandrasekaran 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(8):666-673
The present study explains the molecular level interaction of valeraldehyde with collagen. Valeraldehyde is a monoaldehyde, which involves crosslinking with protein through covalent linkages. The role of valeraldehyde as a crosslinking agent for collagen stabilization was studied. Molecular modeling approaches was used to understand the interaction of collagen like peptide with valeraldehyde, which mimic the aldehyde tanning processes involved in protein stabilization. Crosslinking efficiency of valeraldehyde was found to increase with an increase in concentration due to the higher availability of aldehydic groups involved in crosslinking with collagen. Valeraldehyde interacted collagen membrane showed an increase in thermal stability by 25°C at pH 8. In the presence of valeraldehyde, collagen fibrils nucleation center was shifted from a lower to a higher range. Shift in the nucleation center was observed in the reduction of gelling time. Water accessibility in valeraldehyde interacted collagen membrane was reduced due to a higher crosslinking rate in the collagen. Modified collagen membrane by valeraldehyde at incubation of about 96 h showed higher resistance to collagenolytic activity of 81%. The amino groups reacting appear to be involved in crosslinking with valeraldehyde. Several interaction sites were identified and the docking energy obtained was ?5.539 kcal/mol. The participation of the aldehyde group with amino groups in collagen was observed, which plays a dominant role in the stabilization of peptide by valeraldehyde. It was found that complexes exhibit covalent bonding, hydrogen bonding and electrostatic interaction in the process of stabilization. 相似文献
98.
We report in this paper the different modes of interaction of coumarin 7 and coumarin 314 with the cavity-containing molecule C-hexylpyrogallol[4]arene. The stoichiometry and the binding constant of coumarin 7 for its binding with C-hexylpyrogallol[4]arene are reported. The analysis is done with UV–Vis absorption, steady-state and time-resolved fluorescence, and 2D ROESY spectroscopy. The fluorescence quenching of the two coumarins is analyzed and the Stern–Volmer constants are reported. The mode of binding of coumarin 7 with the host molecule is optimized with 2D ROESY. Coumarin 314 does not form an inclusion complex and this result is supported by the spectral data. The ground and the excited state pK a values for the coumarins, in the presence of C-hexylpyrogallol[4]arene, are reported. 相似文献
99.
Chandrasekaran Ramasamy Jesús Palma del vel Marc Anderson 《Journal of Solid State Electrochemistry》2014,18(8):2217-2223
A gel electrolyte of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-polyethylene oxide (PEO) in an organic solvent mixture has been prepared and examined for supercapacitor applications by using activated carbon electrodes. The solvent was a mixture of propylene carbonate, dimethyl carbonate, and ethylene carbonate at equal molar ratio, and also, a propylene carbonate-based gel was used for a comparison. The polymer-salt interaction was viewed by infrared spectral study. The cells have been characterized in a two-electrode type using linear sweep voltammetry, cyclic voltammetry, galvanostatic cycling, and impedance techniques at 22 °C. The voltammograms evidence symmetry and reversibility of the cells. The ternary gel has shown better electrochemical performances. Moreover, the cell operative potential window was found to be stable at 2.5 V with high specific capacitance and also a good efficiency at low charge rate. The typical obtained specific capacitance, real power, and energy density values are 24 F g?1, 0.52 kW kg?1, and 18.7 Wh kg?1, respectively, which may be viewable for a compact capacitor. 相似文献
100.
Graph matching problem appears frequently in the applications of computer vision and machine learning. In this work, based on the spectral matching with affine constraint (SMAC) formulation, we present a new formulation, named bounded SMAC (BSMAC), for the graph matching problem by adding an upper‐bound constraint on the solution norm. We demonstrate the existence of a unique solution with BSMAC, whereas SMAC needs not to have any meaningful solution in general. We develop an effective numerical method to solve the BSMAC formulation as an optimization problem. Numerical experiments are presented to verify feasibility and to show the performance of the proposed numerical method. 相似文献