全文获取类型
收费全文 | 4144篇 |
免费 | 56篇 |
国内免费 | 4篇 |
专业分类
化学 | 2769篇 |
晶体学 | 13篇 |
力学 | 82篇 |
数学 | 553篇 |
物理学 | 787篇 |
出版年
2019年 | 23篇 |
2018年 | 31篇 |
2017年 | 32篇 |
2016年 | 44篇 |
2015年 | 46篇 |
2014年 | 49篇 |
2013年 | 134篇 |
2012年 | 115篇 |
2011年 | 164篇 |
2010年 | 108篇 |
2009年 | 98篇 |
2008年 | 156篇 |
2007年 | 167篇 |
2006年 | 144篇 |
2005年 | 157篇 |
2004年 | 161篇 |
2003年 | 122篇 |
2002年 | 122篇 |
2001年 | 78篇 |
2000年 | 107篇 |
1999年 | 82篇 |
1998年 | 58篇 |
1997年 | 63篇 |
1996年 | 61篇 |
1995年 | 62篇 |
1994年 | 61篇 |
1993年 | 94篇 |
1992年 | 71篇 |
1991年 | 56篇 |
1990年 | 36篇 |
1989年 | 42篇 |
1988年 | 52篇 |
1987年 | 48篇 |
1986年 | 64篇 |
1985年 | 75篇 |
1984年 | 51篇 |
1983年 | 43篇 |
1982年 | 47篇 |
1981年 | 61篇 |
1980年 | 54篇 |
1979年 | 67篇 |
1978年 | 49篇 |
1977年 | 53篇 |
1976年 | 51篇 |
1975年 | 38篇 |
1974年 | 47篇 |
1973年 | 39篇 |
1972年 | 32篇 |
1971年 | 25篇 |
1970年 | 27篇 |
排序方式: 共有4204条查询结果,搜索用时 18 毫秒
101.
The distances and orientations among reactant centers in the active site of coenzyme B12-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized in the Co(II)-product radical pair state by using X-band electron paramagnetic resonance (EPR) and two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopies in the disordered solid state. The unpaired electron spin in the product radical is localized on C2. Our approach is based on the orientation-selection created in the EPR spectrum of the biradical by the axial electron-electron dipolar interaction. Simulation of the EPR line shape yielded a best-fit Co(II)-C2 distance of 9.3 A. ESEEM spectroscopy performed at four magnetic field values addressed the hyperfine coupling of the unpaired electron spin on C2 with 2H in the C5' methyl group of 5'-deoxyadenosine and in the beta-2H position at C1 of the radical. Global ESEEM simulations (over the four magnetic fields) were weighted by the orientation dependence of the EPR line shape. A Nelder-Mead direct search fitting algorithm was used to optimize the simulations. The results lead to a partial model of the active site, in which C5' is located a perpendicular distance of 1.6 A from the Co(II)-C2 axis, at distances of 6.3 and 3.5 A from Co(II) and C2, respectively. The van der Waals contact of the C5'-methyl group and C2 indicates that C5' remains close to the radical species during the rearrangement step. The C2-Hs-C5' angle including the strongly coupled hydrogen, Hs, and the C5'-Hs orientation relative to the C1-C2 axis are consistent with a linear hydrogen atom transfer coordinate and an in-line acceptor p-orbital orientation. The trigonal plane of the C2 atom defines sub-spaces within the active site for C5' radical migration and hydrogen atom transfers (side of the plane facing Co(II)) and amine migration (side of the plane facing away from Co(II)). 相似文献
102.
Michael Charwath Prof. Dr. Kurt Utvary Joseph M. Kanamueller 《Monatshefte für Chemie / Chemical Monthly》1977,108(5):1093-1097
Prolonged heating of (CH3NPF3)2 in a sealed tube at 120°C yields (CH3N)4P3F7 and PF5. Under the experimental conditions these compounds react to (CH3N)4P3F6
+PF6
–. Reaction of (CH3NPF3)2 in the presence of PF5 leads to a marked decrease in the formation of the ionic compound. The influence of the fluoride donating saltsMF (M=Li, Na, K, Cs) is investigated.
Mit 1 Abbildung
Früher: Alkyliminophosphorsäuretrihalogenide, 1. und 2. Mitt. 相似文献
Mit 1 Abbildung
Früher: Alkyliminophosphorsäuretrihalogenide, 1. und 2. Mitt. 相似文献
103.
Johansson C 《Talanta》1972,19(11):1349-1354
Fluoride is determined by an integration technique with a flow calorimeter. All samples are absorbed on an anion-exchange resin before being brought into the calorimetric system. The reaction between hydrofluoric add and sodium hydroxide is used in the determinations. The method is applied to samples with between 7 mug and 5 mg of fluoride. The confidence limit is +/-2-5 mug for small samples and +/-25 mug for large samples. The enthalpy of the reaction H(+) + F(-) --> HF((aq)) was found to be 13.77 +/- 0.08 kJ mole . 相似文献
104.
Mats Johansson Eva Malmstrm Anders Hult 《Journal of polymer science. Part A, Polymer chemistry》1993,31(3):619-624
A model two-step synthesis of a saturated hyperbranched hydroxyl-terminated ester has been developed to show a synthesis route. Three different series of hyperbranched esters with different terminations have been synthesized to relate some of their properties to their structures. This route has then been used to synthesize three different allyl ethermaleate functional hyperbranched ester resins in a two-step procedure. The resins have been characterized with respect to rheology, structure, and properties, and the differences are discussed. The allyl ether-maleate functional resins have also been studied with respect to curing performance and final film properties. © 1993 John Wiley & Sons, Inc. 相似文献
105.
Testing shelled corn for aflatoxin, Part III: evaluating the performance of aflatoxin sampling plans
Johansson AS Whitaker TB Giesbrecht FG Hagler WM Young JH 《Journal of AOAC International》2000,83(5):1279-1284
The effects of changes in sample size and/or sample acceptance level on the performance of aflatoxin sampling plans for shelled corn were investigated. Six sampling plans were evaluated for a range of sample sizes and sample acceptance levels. For a given sample size, decreasing the sample acceptance level decreases the percentage of lots accepted while increasing the percentage of lots rejected at all aflatoxin concentrations, and decreases the average aflatoxin concentration in lots accepted and lots rejected. For a given sample size where the sample acceptance level decreases relative to a fixed regulatory guideline, the number of false positives increases and the number of false negatives decreases. For a given sample size where the sample acceptance level increases relative to a fixed regulatory guideline, the number of false positives decreases and the number of false negatives increases. For a given sample acceptance level, increasing the sample size increases the percentage of lots accepted at concentrations below the regulatory guideline while increasing the percentage of lots rejected at concentrations above the regulatory guideline, and decreases the average aflatoxin concentration in the lots accepted while increasing the average aflatoxin concentration in the rejected lots. For a given sample acceptance level that equals the regulatory guideline, increasing the sample size decreases misclassification of lots, both false positives and false negatives. 相似文献
106.
Hans-Peter Abicht Peter Lehniger Kurt Issleib 《Journal of organometallic chemistry》1983,250(1):609-616
The complexes of type R2)2 (M = Pd, Pt) are readily deprotonated by n-Buli under various conditions yielding μ-C-bis-dilithiated species. The resulting carbanions are attacked by the electrophiles Mel, Me3SiCl and Ph2PCl to form the corresponding disubstituted derivatives. The reaction with AuCl . PPh3 yields heterobimetallic complexes with two different MC σ-bonds. The compounds obtained are analytically and spectroscopically characterized. 相似文献
107.
Individual hydration water molecules in aqueous protein solutions have been observed using experimental schemes for homonuclear two-dimensional and heteronuclear three-dimensional NMR experiments in H2O solution, which do not require suppression of the solvent line by presaturation. In these experiments, the location of the hydration waters is determined from their nuclear Overhauser effects (NOE s) with individual hydrogen atoms of distinct amino acid residues. In the basic pancreatic trypsin inhibitor (BPTI ), four internal water molecules that had been reported in three different crystal forms were also found to be in the same locations in the solution structure, with lifetimes with respect to exchange of the water protons in excess of 0.3 ns. Additional NOE s with polypeptide protons located on the protein surface may involve either hydration water molecules or hydroxyl protons of amino acid side chains. Their total number is small compared to the number of NOE s expected from the hydration water molecules identified in the crystal structures of BPTI . 相似文献
108.
Improved ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons in sediments 总被引:6,自引:0,他引:6
The aim of this work was to optimize an ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons (PAHs) in sediments and to compare it with the reflux procedure using methanolic potassium hydroxide. Sample extracts were purified with a miniaturized silica gel chromatographic column and analyzed by gas chromatography-mass spectrometry (GC-MS). Ultrasonication using n-hexane-acetone (1:1, v/v) solvent mixture on dried homogenized marine sediment gave better precision (smaller relative standard deviation (RSD) values) and comparable quantities of individual PAH's compared to the reflux procedure. Ultrasonication with the n-hexane-acetone (1:1, v/v) mixture, utilizing four 15 min extraction cycles, was found to be sufficient for extracting PAHs from wet sediments. The optimized ultrasonic extraction procedure extracted aliphatic and aromatic hydrocarbons from the National Institute of Standards and Technology SRM 1941a with recoveries greater than 90%. The major advantages of ultrasonication compared to the reflux method are the lower extraction times, simplicity of the apparatus and extraction procedure. The optimized ultrasonication procedure has been used in our laboratory to extract hydrocarbons from naturally wet sediments from rivers, and coastal and marine areas. 相似文献
109.
[Ph3PNSbPh3Cl] ( 1 ) was prepared by oxidative addition of ClNPPh3 to triphenylstibine in dichloromethane solution. The compound is characterized by IR spectroscopy and by an X‐ray structure determination. 1 crystallizes in the monoclinic space group P21/c with four formula units per unit cell. Lattice dimensions at 193 K: a = 925.3(1), b = 1777.2(1), c = 1825.5(1) pm, β = 94.07(1)°, R1 = 0.0228. 1 forms monomeric molecules with tetrahedrally coordinated phosphorus and trigonal‐bipyramidally coordinated antimony atom, the atoms N and Cl being in axial positions. The bond lengths PN and SbN are 155.0(2) and 198.4(2) pm, respectively, the PNSb angle is 138.6(1)°. 1 reacts with iron trichloride to give the known phosphoraneiminato complex [FeCl2(NPPh3)]2. 相似文献
110.
Johansson A Abrahamsson M Magnuson A Huang P Mårtensson J Styring S Hammarström L Sun L Akermark B 《Inorganic chemistry》2003,42(23):7502-7511
The preparation of donor (D)-photosensitizer (S) arrays, consisting of a manganese complex as D and a ruthenium tris(bipyridyl) complex as S has been pursued. Two new ruthenium complexes containing coordinating sites for one (2a) and two manganese ions (3a) were prepared in order to provide models for the donor side of photosystem II in green plants. The manganese coordinating site consists of bridging and terminal phenolate as well as terminal pyridyl ligands. The corresponding ruthenium-manganese complexes, a manganese monomer 2b and dimer 3b, were obtained. For the dimer 3b, our data suggest that intramolecular electron transfer from manganese to photogenerated ruthenium(III) is fast, k(ET) > 5 x 10(7) s(-)(1). 相似文献