全文获取类型
收费全文 | 5792篇 |
免费 | 780篇 |
国内免费 | 282篇 |
专业分类
化学 | 3920篇 |
晶体学 | 47篇 |
力学 | 1207篇 |
综合类 | 47篇 |
数学 | 627篇 |
物理学 | 1006篇 |
出版年
2024年 | 3篇 |
2023年 | 55篇 |
2022年 | 76篇 |
2021年 | 274篇 |
2020年 | 247篇 |
2019年 | 160篇 |
2018年 | 198篇 |
2017年 | 175篇 |
2016年 | 280篇 |
2015年 | 283篇 |
2014年 | 308篇 |
2013年 | 561篇 |
2012年 | 345篇 |
2011年 | 373篇 |
2010年 | 252篇 |
2009年 | 359篇 |
2008年 | 334篇 |
2007年 | 351篇 |
2006年 | 321篇 |
2005年 | 306篇 |
2004年 | 276篇 |
2003年 | 263篇 |
2002年 | 183篇 |
2001年 | 105篇 |
2000年 | 109篇 |
1999年 | 65篇 |
1998年 | 61篇 |
1997年 | 75篇 |
1996年 | 72篇 |
1995年 | 49篇 |
1994年 | 39篇 |
1993年 | 40篇 |
1992年 | 48篇 |
1991年 | 37篇 |
1990年 | 32篇 |
1989年 | 16篇 |
1988年 | 33篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 2篇 |
1982年 | 14篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有6854条查询结果,搜索用时 15 毫秒
11.
12.
Martín Soto Dr. Raquel G. Soengas Prof. Artur M. S. Silva Prof. Vicente Gotor-Fernández Prof. Humberto Rodríguez-Solla 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(57):13104-13108
In this work, the first example of a radical stereodivergent reaction directed towards the stereoselective synthesis of both (R*,R*)- and (R*,S*)-2,2′-biflavanones promoted by samarium diiodide is reported. Control experiments showed that the selectivity of this reaction was exclusively controlled by the temperature. It was possible to generate a variety of 2,2′-biflavanones bearing different substitution patterns at the aromatic ring in good-to-quantitative yields, being both stereoisomers of the desired compounds obtained with total or high control of selectivity. A mechanism that explains both the generation of the corresponding 2,2′-biflavanones and the selectivity is also discussed. The structure and stereochemistry determination of each isomer was unequivocally elucidated by single-crystal X-ray diffraction experiments. 相似文献
13.
Natural products with a variety of pharmacological effects are important sources for commercial drugs, and it is very crucial to develop effective techniques to selectively extract and isolate bioactive natural components from the plants against the background of sustainable development. Ionic liquids (ILs) are a kind of designable material with unique physicochemical properties, including good thermal stability, negligible vapor pressure, good solvation ability, etc. ILs have already been used in pharmaceuticals for extraction, purification, drug delivery, etc. It has been reported that multi-interactions, like hydrogen bonding, hydrophobic interactions, play important roles in the extraction of bioactive components from the plants. In this review, recent progress in the understanding of scientific essence of hydrogen bonding, the special interaction, in ILs was summarized. The extraction of various natural products, one important area in pharmaceutical, by conventional and functional ILs as well as the specific roles of multi-interactions in this process were also reviewed. Moreover, problems existing in bioactive compound extraction by ILs and the future developing trends of this area are given, which might be helpful for scientists, especially beginners, in this field. 相似文献
14.
Dr. Gregory K. Pierens 《Chemphyschem》2021,22(21):2207-2214
The calculation of DFT (density functional theory) chemical shifts have become an important technique for the verification of a proposed structure. An easily calculated metric developed for proton and carbon chemical shifts of natural products and organic compounds, the calculated chemical shift index (CCSI), has been developed, which uses the deviation of each pair of calculated and experimental chemical shifts. The mean absolute deviation (MAD), which is commonly used as the goodness of fit metric for DFT calculated chemical shifts, can conceal large deviations in the calculated data. A classification strategy is also proposed for the CCSI to highlight when further assessment of the NMR data is required. 相似文献
15.
Ayoub Abdelkader Mekkaoui Abderrazak Aberkouks Lahcen Fkhar Mustapha Ait Ali Larbi El Firdoussi Soufiane El Houssame 《应用有机金属化学》2020,34(11):e5917
Various ratios of palladium nanoparticles supported on mesoporous natural phosphate (Pd@NP) were prepared using the wetness impregnation method. The prepared catalysts were characterized by IR, XRD, CV, SEM, EDX, XRF, TEM and BET analysis. The reduction and preparation of the palladium nanoparticles afford a crystallite size of 10.88 nm. The performance of the synthesized catalyst was investigated in the solvent-free dehydroaromatization of α-, β- and γ-himachalene mixture from Cedrus atlantica oil as a model substrate. In order to achieve an efficient and selective catalysis, the catalytic dehydroaromatization of various terpenes such as limonene, limonaketone, carvone, carveol and perillyl alcohol was studied. The Pd@NP catalyst performed a high catalytic activity, selectivity and recyclability in the terpenes dehydroaromatization reaction. 相似文献
16.
Séan J. Cassady Rishav Choudhary Vivek Boddapati Nicolas H. Pinkowski David F. Davidson Ronald K. Hanson 《国际化学动力学杂志》2020,52(11):725-738
The pyrolysis of propane plays an important role in determining the combustion properties of natural gas mixtures and offers insight into the cracking patterns of larger fuels. This work investigates propane pyrolysis behind reflected shock waves with a multiwavelength laser-absorption speciation technique. Nine laser wavelengths, sensitive to key pyrolysis species, were used to measure absorbance time histories during the decomposition of 2% propane in argon between 1022 and 1467 K, 3.7-4.3 atm. Absorbance models were developed at each diagnostic wavelength to interrogate common initial conditions, and time histories of all major species are reported at 1250, 1290, 1330, 1370, and 1410 K. Nearly complete carbon recovery observed at lower temperatures enabled the inference of hydrogen formation from atomic conservation, while decaying carbon recovery at high temperatures suggests the formation of allene and 1-butene. The results show systematically faster pyrolysis than predicted by kinetic modeling and motivate further study into the kinetics of propane pyrolysis. 相似文献
17.
Dr. Yong Guan Jonathan W. Attard Prof. Dr. Anita E. Mattson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(8):1742-1747
The stereocontrolled construction of biologically relevant chromanones and tetrahydroxanthones has been achieved through the addition of alkynes to benzopyrylium trilfates under the influence of copper bis(oxazoline) catalysis. Excellent levels of enantiocontrol (63–98 % ee) are achieved in the addition of a variety of alkynes to an array of chromenones with a hydrogen in the 2-position. Promising levels of enantiocontrol (54–67 % ee) are achieved in the alkynylation of chromenones with esters in the 2-position, generating tertiary ether stereocenters resembling those frequently found in naturally occurring metabolites. 相似文献
18.
Pierre Thesmar Seemon Coomar Dr. Alessandro Prescimone Prof. Dr. Daniel Häussinger Prof. Dr. Dennis Gillingham Prof. Dr. Olivier Baudoin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(66):15298-15312
This article provides a detailed report of our efforts to synthesize the dithiodiketopiperazine (DTP) natural products (−)-epicoccin G and (−)-rostratin A using a double C(sp3)−H activation strategy. The strategy's viability was first established on a model system lacking the C8/C8’ alcohols. Then, an efficient stereoselective route including an organocatalytic epoxidation was secured to access a key bis-triflate substrate. This bis-triflate served as the functional handles for the key transformation of the synthesis: a double C(sp3)−H activation. The successful double activation opened access to a common intermediate for both natural products in high overall yield and on a multigram scale. After several unsuccessful attempts, this intermediate was efficiently converted to (−)-epicoccin G and to the more challenging (−)-rostratin A via suitable oxidation/reduction and protecting group sequences, and via a final sulfuration that occurred in good yield and high diastereoselectivity. These efforts culminated in the synthesis of (−)-epicoccin G and (−)-rostratin A in high overall yields (19.6 % over 14 steps and 12.7 % over 17 steps, respectively), with the latter being obtained on a 500 mg scale. Toxicity assessments of these natural products and several analogues (including the newly synthesized epicoccin K) in the leukemia cell line K562 confirmed the importance of the disulfide bridge for activity and identified dianhydrorostratin A as a 20x more potent analogue. 相似文献
19.
20.
针对流固耦合传热问题,本文提出了一种基于浸没边界-简化热格子玻尔兹曼方法(immersed boundary method-simplified thermal lattice Boltzmann method,IB-STLBM)的耦合模型.不同于传统的格子玻尔兹曼方法使用分布函数演化流场和温度场,简化热格子玻尔兹曼方法(simplified thermal lattice Boltzmann method,STLBM)的演化过程不需要依赖分布函数,只涉及平衡态分布函数和非平衡态分布函数,能够直接演化宏观量,极大减小了计算过程中所占用的虚拟内存,简化了边界条件的实现方式,同时具有较高的稳定性.传统的浸没边界法对流场的计算采用欧拉网格,对固体边界采用拉格朗日网格,认为固体边界是对流场产生某种体积力.在应用浸没边界法时,汲取介观的思想,把固体的介入看作是对流场的干扰,打破了固体附近流体介观微团颗粒原始的平衡状态,这种干扰可以看作是在耦合边界上产生的一个非平衡项,可用非平衡态分布函数来表示.基于此,在模型中浸没边界法与简化热格子玻尔兹曼方法更紧密联系在一起,更大程度发挥二者的优点,整个计算过程更加简单直观,符合物理特性.通过对热圆柱绕流和内含热颗粒的封闭方腔自然对流问题的模拟以及对其结果的分析,验证了该算法在求解流固耦合传热问题的有效性和可行性. 相似文献