全文获取类型
收费全文 | 4617篇 |
免费 | 120篇 |
国内免费 | 27篇 |
专业分类
化学 | 2920篇 |
晶体学 | 37篇 |
力学 | 155篇 |
数学 | 577篇 |
物理学 | 1075篇 |
出版年
2022年 | 35篇 |
2021年 | 45篇 |
2020年 | 45篇 |
2019年 | 51篇 |
2018年 | 42篇 |
2017年 | 45篇 |
2016年 | 90篇 |
2015年 | 99篇 |
2014年 | 122篇 |
2013年 | 194篇 |
2012年 | 269篇 |
2011年 | 317篇 |
2010年 | 191篇 |
2009年 | 189篇 |
2008年 | 259篇 |
2007年 | 235篇 |
2006年 | 263篇 |
2005年 | 224篇 |
2004年 | 192篇 |
2003年 | 154篇 |
2002年 | 144篇 |
2001年 | 92篇 |
2000年 | 88篇 |
1999年 | 58篇 |
1998年 | 52篇 |
1997年 | 47篇 |
1996年 | 63篇 |
1995年 | 63篇 |
1994年 | 53篇 |
1993年 | 54篇 |
1992年 | 64篇 |
1991年 | 44篇 |
1990年 | 34篇 |
1989年 | 63篇 |
1988年 | 50篇 |
1987年 | 30篇 |
1986年 | 30篇 |
1985年 | 37篇 |
1984年 | 38篇 |
1983年 | 41篇 |
1982年 | 46篇 |
1981年 | 48篇 |
1980年 | 39篇 |
1979年 | 35篇 |
1978年 | 41篇 |
1977年 | 43篇 |
1976年 | 40篇 |
1975年 | 35篇 |
1974年 | 43篇 |
1973年 | 48篇 |
排序方式: 共有4764条查询结果,搜索用时 15 毫秒
51.
Regioselective ortho lithiation of 2-, 3-, and 4-halopyridines is achieved with lithium diisopropylamide (?78°, tetrahydrofuran) to afford, upon quenching with electrophilic reagents, 2,3- and 3,4-disubstituted pyridines in good to excellent yield. 相似文献
52.
Under the agency of the potent lithiating agent, n-butyllithium in TMEDA, an array of organosilanes was found to undergo 1, n-silyl rearrangements via carbanionic intermediates. Unambiguous 1, 2-, 1, 3- and 1, 4-silyl shifts were uncovered in 1-trimethylsilyl-1, 1, 2-triphenylethane, 1, 1-bis(trimethylsilyl)-1-phenylalkanes and 1, 2-bis(trimethylsilyl)-1, 2-diphenylethane, respectively. Cross-over and competition experiments established that these rearrangements generally are intramolecular and occur with decreasing ease in the order, 1, 2 > 1, 3 > 1, 4. In other compounds, such as 1, 1-bis(trimethylsilyl)-1, 2-diphenylethane, 1, n-bis(trimethylsilyl)benzenes and triphenyl(trimethylsilyl)methane, competing 1, n-silyl shifts occurred. Attack of the organolithium intermediates on solvent and silicon—lithium exchange were significant side reactions in some instances. 1-Trimethylgermyl-1, 1, 2-triphenylgermane underwent no discernible rearrangement but rather gave the product expected from germanium—lithium exchange. By conducting time and competition studies, it was shown that lithiation is the product-determining step in these rearrangements and that dual pathways, namely 1, 3-versus consecutive 1, 2- 1, 4-pathways, are operative in certain rearrangements. 相似文献
53.
Hong-Bing Tsai Yu-Der Lee 《Journal of polymer science. Part A, Polymer chemistry》1987,25(8):2195-2206
The kinetics of interfacial polycondensation of bisphenol A with isophthaloyl chloride and terephthaloyl chloride in dichloromethane with triethylbenzylammonium chloride (TEBAC) as the catalyst was investigated via measurements of bisphenolate concentration by UV. The reaction was found to be second order with respect to bisphenolate. The dependence of the rate constant on stirring speed, amount of TEBAC, and reaction temperature was studied. The rate constant was increased with an increase of stirring speed, quantity of TEBAC added, as well as the reaction temperature. The activation energy was found to be 7.7 kcal/mol at a stirring speed of 700 rpm in the presence of 0.160 of TEBAC. The role of TEBAC was found to be interesting. It did not alter the equilibrium (the partition coefficient remained the same in the presence of TEBAC), but it did enhance the transfer rate of bisphenolate. 相似文献
54.
The roles of the rel+ gene and of 4-thiouridine in killing and photoprotection of Escherichia coli by near-ultraviolet radiation 总被引:3,自引:0,他引:3
Abstract— Photoprotection is a reduction in response to far-UV (190–300. nm) radiation in cells that have been previously exposed to longer wavelengths. It has been proposed that photoprotection operates by means of a growth delay that permits more time for dark repair. Growth delay in Escherichia coli utilizes 4-thiouridine (4Srd) in transfer RNA as a chromophore and it requires the rel+ gene, which exerts a stringent control upon RNA synthesis. Mutants that were either rel or 4Srd? were isolated from E. coli B, utilizing a near-UV-induced growth-delay selection technique. The rel mutants, which undergo little growth delay after near-UV irradiation, show only 50% as much photoprotection as wild types, while 4Srd? mutants show no photoprotection at all. Thus, photoprotection appears to utilize 4Srd as its sole chromophore in E. coli B and B/r, and no more than 50% of photoprotection in these strains can be a result of near-UV-induced growth delay. 相似文献
55.
The interaction between trans-1,2-diaminocyclohexane-N, N, N', N'-tetraacetic acid (H1DCTA or H4Z) and Mn(II) and Ni(II) ions has been investigated. The formation constants of the hydrogen chelates (MHZ?), and of the normal chelates (MZ2?), have been measured at 20, 30 and 40°C, and an ionic strength of 0.10 (KNO3). The former was done by a titration technique and the latter by a mercury indicator electrode method. Enthalpy and entropy changes characterizing the formation of the normal chelates have been calculated at 25°C. 相似文献
56.
A high-performance liquid chromatographic method for the separation and determination of 18 alpha-glycyrrhetinic acid and 18 beta-glycyrrhetinic acid has been developed. Indomethacin was used as an internal standard. The drugs were separated on a reversed-phase column and detected by UV detection at a wavelength of 254 nm. Methanol-water-perchloric acid-ammonia (80:20:0.4:0.4, v/v) was used as the mobile phase at pH of 7.0-7.5. The detection limit of both compounds was 0.1 microgram/ml in rat plasma. The method was applied to pharmacokinetic studies of glycyrrhetinic acids in rats. The results suggest that the pharmacokinetics appeared to be non-linear in nature. 相似文献
57.
Serdar S. Celebi Gow-Jen Tsai George T. Tsao 《Applied biochemistry and biotechnology》1991,27(2):163-171
Immobilization of glucoamylase (EC 3.2.1.3) on Celite R649 bio-catalyst carrier for hydrolysis of maltose and maltodextrin has been investigated in both packed bed and recirculated batch reactors. The kinetics parameters on the hydrolysis of maltose were estimated from the packed bed reactor. It is found that this immobilized enzyme is as efficient as the soluble enzyme in catalyzing hydrolysis of maltose. However, it is less efficient than the soluble enzyme in hydrolyzing 30% (w/v) maltodextrin, giving a maximum dextrose equivalent (DE) value of 96.0% instead of 98.2%. 相似文献
58.
Photocycloadditlon of (+)-isoplperltenone and cyclobutene-1-carboxylic acid gives an adduct which upon reduction with NaCNBH3 followed by thermolysis yields an isomer of isoaristolactone in an overall yield of 26%. 相似文献
59.
Bernard Gordon Jacqueline S. Butler Ian R. Harrison 《Journal of polymer science. Part A, Polymer chemistry》1987,25(8):2139-2142
Catalysts bound to polymers in the form of crosslinked beads have been demonstrated to have a number of advantages over homogeneous catalysts. However, there are several problems that exist due to the polymer support being in the form of a bead. The rate of reaction depends on the presence of solvents that adequately swell the bead in order to allow access to the catalytic sites. Differences in polarity and reactant size can inhibit diffusion into the bead. Recently a new system has been developed whereby tris(triphenyl phosphine) chlororhodium (I) (Wilkinson's catalyst) is bound to the surface of polyethylene single crystals. Polyethylene single crystals have a very high surface to volume ratio allowing for greater ease of reaction compared to a bead system. In a previous paper we showed that there is a dramatic increase in catalytic activity and that the reaction rate increased as the polarity of solvent was increased, even in ethanol where the homogeneous catalyst is not soluble and the polystyrene bead support would not swell. In this letter we are describing the activity of hydrogenation of olefins contained in both large and/or polar molecules. The results demonstrate the advantages of supported catalysts on polyethylene single crystals rather than on polymer beads. 相似文献
60.
A.R. László Bursics Martin Murray F. Gordon A. Stone 《Journal of organometallic chemistry》1976,111(1):31-42
Bis(cyclopentadienyl)mercury readily undergoes Diels—Alder reactions with RCCR (R = CO2Me or CF3), CF3CFCFCF3, CF3CFCF2, (CF3)2CC(CN)2, C2(CN)4 and PhO to give stable adducts characterised by1H, 19F and 13C NMR, spectroscopy. Similar reactions of CF3CCCF3 and CF3CFCFCF3 with the cyclopentadiene derivatives Me3MC5H5 and (Me3M)2C5H4 (M = Si, Sn) are also described. 相似文献