全文获取类型
收费全文 | 30820篇 |
免费 | 1180篇 |
国内免费 | 244篇 |
专业分类
化学 | 20574篇 |
晶体学 | 275篇 |
力学 | 1064篇 |
综合类 | 11篇 |
数学 | 2875篇 |
物理学 | 7445篇 |
出版年
2023年 | 159篇 |
2022年 | 178篇 |
2021年 | 534篇 |
2020年 | 505篇 |
2019年 | 510篇 |
2018年 | 391篇 |
2017年 | 376篇 |
2016年 | 836篇 |
2015年 | 728篇 |
2014年 | 925篇 |
2013年 | 1707篇 |
2012年 | 2200篇 |
2011年 | 2437篇 |
2010年 | 1474篇 |
2009年 | 1283篇 |
2008年 | 1994篇 |
2007年 | 1874篇 |
2006年 | 1771篇 |
2005年 | 1662篇 |
2004年 | 1440篇 |
2003年 | 1141篇 |
2002年 | 1105篇 |
2001年 | 761篇 |
2000年 | 679篇 |
1999年 | 396篇 |
1998年 | 294篇 |
1997年 | 332篇 |
1996年 | 376篇 |
1995年 | 314篇 |
1994年 | 329篇 |
1993年 | 328篇 |
1992年 | 307篇 |
1991年 | 236篇 |
1990年 | 182篇 |
1989年 | 163篇 |
1988年 | 169篇 |
1987年 | 150篇 |
1986年 | 115篇 |
1985年 | 193篇 |
1984年 | 137篇 |
1983年 | 108篇 |
1982年 | 150篇 |
1981年 | 116篇 |
1979年 | 102篇 |
1978年 | 111篇 |
1977年 | 116篇 |
1976年 | 120篇 |
1975年 | 122篇 |
1974年 | 100篇 |
1973年 | 116篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The electrocyclic reactions of [16]annulene have been investigated by ab initio and DFT calculations. Among the six conformers of [16]annulene identified, 3, with Cs symmetry, is taken as the starting reactant for the cyclization reactions of [16]annulene, even though it is 31.4 kJ mol(-1) less stable than the most stable conformer, 1. The pathways of the electrocyclic reactions from reactant 3 to two tricyclic products, 5 and 6, have been found. All pathways identified are stepwise, i.e., the two ring closure processes occur one after the other. Among the pathways found, the ones with the lowest overall barrier for reactions 3 --> 5 and 3 --> 6 have the same rate-determining step and hence the same overall barrier, 131.0 kJ mol(-1). Thus, based on the barriers calculated, it is not possible to determine whether 5 or 6 is the dominant product in the cyclization reaction of 3. 相似文献
992.
Humeres E Debacher NA Franco JD Lee BS Martendal A 《The Journal of organic chemistry》2002,67(11):3662-3667
The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in 20% aqueous ethanol at 25 degrees C, mu = 1.0 (KCl, for pH > 0). The pH-rate profiles showed a dumbell shape with a plateau where the observed first-order rate constant k(obs) was equal to k(o), the rate constant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants of the dithiocarbamic acids (pK(a)) and their conjugate acids (pK(+)) were calculated from the pH-rate profiles. Comparatively, k(o) was more than 10(4)-fold faster than alkyldithiocarbamates (alkDTCs) with similar pK(N) (the acid dissociation constant of the parent amine). It was observed that the values of pK(a) and pK(+)were 5 and 8 units of pK, respectively, higher than the expected values from the pK(N) of alkylDTCs. The higher values were attributed to the inhibition of the delocalization of the nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect of the thiocarbonyl group. Comparison of the activation parameters showed that the rate acceleration was due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of a multiproton transition state, and it was consistent with an S to N proton transfer through a water molecule. There are two hydrogens contributing to a secondary SIE, and there are also two protons that are being transferred at the transition state to form a zwitterion followed by fast C-N bond cleavage. The mechanism could also be a concerted asynchronic process where the N-protonation is more advanced than the C-N bond breakdown. The kinetic barrier is similar to the torsional barrier of thioamides, suggesting that the driving force to reach the transition state is the needed torsion of the C-N bond that inhibits the resonance with the thiocarbonyl group and the aromatic moiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamically favorable. 相似文献
993.
Highly ordered mesoporous silica can be regenerated from a mesoporous carbon CMK-3 that is a negative replica of mesoporous silica SBA-15, indicating reversible replication between carbon and inorganic materials. 相似文献
994.
Virginia Lea Miller Wei-li Lee Nai-Phuan Ong 《Journal of solid state chemistry》2005,178(5):1508-1512
We report the synthesis and elementary properties of the Co7Se8−xSx (x=0-8) and Ni7Se8−xSx (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni7Se8−xSx system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co7Se8−xSx system, as well as Ni7Se8 and Ni7SeS7, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/molCoK2 and ∼7 mJ/molNiK2 for Co7Se8−xSx and Ni7Se8−xSx, respectively. 相似文献
995.
Hohyun Kim Moon Sun Jang Jung-Ae Lee Dongjin Pyo Hye-Ran Yoon Hee Joo Lee Kyung Ryul Lee 《Chromatographia》2004,60(5-6):335-339
A sensitive and selective method for the determination of sofalcone in human plasma was established by use of protein precipitation and liquid chromatography-tandem mass spectrometry. Plasma samples were transferred into 96-well plate using an automated sample handling system and spiked with 10 L of 2 g mL–1 internal standard solution (d3-sofalcone). 0.5 mL of acetonitrile was added to the 96-well plate and the plasma samples were then vortexed for 30 sec. After centrifugation, the supernatant was transferred into another 96-well plate and completely evaporated at 40 °C under a stream of nitrogen. The dry residue was reconstituted with mobile phase. All sample transfer and protein precipitation was automated through the application of both the PerkinElmer MultiPROBE II HT and TOMTEC Quadra 96 workstation. The limit of quantitation of sofalcone was 2 ng mL–1 using a sample volume of 0.2 mL for the analysis. The reproducibility of the method was evaluated by analyzing five samples at nine quality control (QC) levels over the nominal concentration range from 2 ng mL–1 to 1000 ng mL–1. Validation of the method showed that the assay has good precision and accuracy. Sofalcone and internal standard produced a protonated precursor ion ([M+H]+) at m/z 451 and 454, and both gave a corresponding product ion at m/z 315. The high sample throughput of the method has been successfully applied to a pharmacokinetic study of sofalcone in human plasma. 相似文献
996.
The catalytic efficiency of adenylyl cyclase activity of edema factor (EF) from Bacillus anthracis is enhanced by approximately 1000-fold upon its binding to mammalian protein calmodulin (CaM). A tandem cell-based and protein binding-based screen of a 10,000 member library identified a molecule that inhibits the EF-CaM interaction and therefore the adenylyl cyclase activity. A combination of fluorescence spectroscopy and photolabeling studies showed that the molecule targets the CaM binding region of EF. A series of related compounds were synthesized and evaluated to identify one compound, 4-[4-(4-nitrophenyl)-thiazolylamino]-benzenesulfonamide, that maintained activity against EF but showed minimal toxicity to two cultured cell lines. This compound represents an important reagent to study the role of EF in anthrax pathology and may represent a drug lead against anthrax infection. 相似文献
997.
[reaction: see text] We have developed, on the basis of a chelation-strategy, an efficient copper-catalyzed aziridination protocol with the use of 5-methyl-2-pyridinesulfonamide and PhI(OAc)(2). The reaction proceeds smoothly under mild conditions to give aziridines in moderate to good yields in the absence of external ligands or bases. The coordination-assisted approach offers the additional benefits that efficient deprotection of the N-substituent and selective aziridine ring-opening are effectively achieved. 相似文献
998.
M. H. Lee C. W. Lee K. H. Hong Y. H. Choi B. H. Boo 《Journal of Radioanalytical and Nuclear Chemistry》1996,204(1):135-144
Depth distribution of239,240Pu and137Cs in the soils of South Korea have been studied. The average accumulated depositions were estimated roughly to be 54.8±32.1 Bq·m–2 for239,240Pu, 1.6±1.0 Bq· ·m–2 for238Pu and 1982.8±929.1 Bq·m–2 for137Cs. The activity ratios of239,240Pu/137Cs in soils were found to be in the narrow range of 0.0153 to 0.0364 with a mean value of 0.0230±0.006. The concentrations of239,240Pu and137Cs in soils decrease exponentially with increasing the soil depth. A significant correlation was found between the concentration of239,240Pu and that of137Cs. The activity ratios of239,240Pu/137Cs tend to increase slightly with increasing soil depth. 相似文献
999.
The cyclocondensation of 3-amino-2-iminonaphtho[1,2-d]thiazole ( 1 ) with a series of α-keto mono- and dicarboxylic acid derivatives 5a-i under different conditions was investigated. When the experiments were performed by refluxing in glacial acetic acid, the corresponding cyclized products, 2-substituted 3-oxo-3H-naphtho[1′,2′:4,5]thiazolo[3,2-b][1,2,4]triazines 4 were obtained in fair to good yields. On the other hand, when the reaction was conducted in boiling ethanol, it gave only the open chain condensates 6 , or in rare cases, together with minor amount of 4 . Since the intermediates 6 exist as mixture of both E- and Z-isomers, cyclization through heating was found insufficient. In any event representative compounds 4b,g,i have been evaluated for anti-HIV activity, but none of them were active. 相似文献
1000.
The chiral resolving ability of a novel single-isomer cationic β-cyclodextrin (CD), mono-6A-propylammonium-6A-deoxy-β-cyclodextrin chloride (PrAMCD), as a chiral selector in capillary electrophoresis (CE) is reported in this work for the enantioseparation of hydroxy, carboxylic acids and amphoteric analytes. The effect of chiral selector concentration on the resolution was studied. Good resolutions were achieved for hydroxy acids. Optimum resolutions were obtained even at 3.5 mM CD concentration for carboxylic acids. The electrophoretic method showed good linearity and reproducibility in terms of migration times and peak areas, which should make it suitable for routine analysis. In addition, baseline chiral separation of a six-acid mixture was achieved within 20 min. PrAMCD proved to be an effective chiral selector for acidic analytes. 相似文献