全文获取类型
收费全文 | 1685篇 |
免费 | 13篇 |
国内免费 | 22篇 |
专业分类
化学 | 855篇 |
晶体学 | 11篇 |
力学 | 37篇 |
数学 | 476篇 |
物理学 | 341篇 |
出版年
2016年 | 10篇 |
2015年 | 10篇 |
2014年 | 15篇 |
2013年 | 73篇 |
2012年 | 63篇 |
2011年 | 58篇 |
2010年 | 41篇 |
2009年 | 32篇 |
2008年 | 58篇 |
2007年 | 62篇 |
2006年 | 77篇 |
2005年 | 61篇 |
2004年 | 56篇 |
2003年 | 43篇 |
2002年 | 57篇 |
2001年 | 33篇 |
2000年 | 26篇 |
1999年 | 26篇 |
1998年 | 27篇 |
1997年 | 25篇 |
1996年 | 37篇 |
1995年 | 34篇 |
1994年 | 31篇 |
1993年 | 17篇 |
1992年 | 29篇 |
1991年 | 17篇 |
1990年 | 26篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 14篇 |
1985年 | 30篇 |
1984年 | 20篇 |
1983年 | 17篇 |
1982年 | 27篇 |
1981年 | 48篇 |
1980年 | 35篇 |
1979年 | 30篇 |
1978年 | 24篇 |
1977年 | 42篇 |
1976年 | 37篇 |
1975年 | 33篇 |
1974年 | 39篇 |
1973年 | 45篇 |
1972年 | 22篇 |
1970年 | 10篇 |
1969年 | 11篇 |
1968年 | 24篇 |
1967年 | 11篇 |
1966年 | 10篇 |
排序方式: 共有1720条查询结果,搜索用时 203 毫秒
11.
Gregory N. Coppin Andrew Hudson Richard A. Jackson Nigel P.C. Simmons 《Journal of organometallic chemistry》1977,131(3):371-375
The formation of the compound RSnX(acac)2 (acac = 2,4-pentanedionato) by reaction of bis(2,4-pentanedionato)tin(II) on a halide RX with R = CH3, C2H5, C4H9, C6H5, CH2I, (C6H5)3SnCH2, (C2H5)3SnCH2 and X = I, Br has been studied by polarography. At 25°C, it is in fact an equilibrium whose constant has been measured. The intermediate formation of the ion-pair [RSn(acac)2+X?] has allowed us to explain the experimental results. 相似文献
12.
13.
14.
Michael J. Costanzo Harold R. AlmondJr. A. Diane Gauthier Bruce E. Maryanoff 《Tetrahedron: Asymmetry》1994,5(12)
The conformational preferences for 2,3-O-isopropylidene-α-
-sorbopyranose derivatives 3–6 were determined by using 1H NMR data and empirical force field calculations. Proton NMR studies of 3–6 indicate that a twist-boat (or skew) conformation (3S0) prevails over possible chair forms for each compound. Force-field calculations (MM2, MNDO, AM1) on a model 2,3-O-isopropylidene-α-
-sorbopyranose system (18) indicate that the 3S0 conformation is among the low-energy structures. X-Ray crystallographic analysis of α-
-sorbopyranose sulfamate 3, a compound with potent anticonvulsant activity, demonstrates that the 3S0 skew conformation is manifested in the solid state, as well. 相似文献
15.
Background
Kynureninase is a key enzyme on the kynurenine pathway of tryptophan metabolism. One of the end products of the pathway is the neurotoxin quinolinic acid which appears to be responsible for neuronal cell death in a number of important neurological diseases. This makes kynureninase a possible therapeutic target for diseases such as Huntington's, Alzheimer's and AIDS related dementia, and the development of potent inhibitors an important research aim. 相似文献16.
A new algorithm is presented for performing molecular dynamics simulations of peptides with fixed geometry, with the aim of simulating conformational changes and of exploring conformational space. The principle of the method is to expand the potential energy as a Taylor's series in the coordinates around the current point, retaining the force and its first two derivatives, and obtain a series solution of the resulting differential equations using a method due to Lyapunov. By choosing the time step so that the second term in the series is small compared to the first, the true solution can in principle be approximated to any desired degree of accuracy. The algorithm has been used to solve numerically Lagrange's equations of motion for N-acetyl alanine amide and N-acetyl methionide amide, regarded as fixed at their C-termini, under the influence of the ECEPP/2 potential energy function, and time steps of 15–30 fsec have been achieved with little variation in the total energy. Possible directions for future development are discussed. 相似文献
17.
Li S Marthandan N Bowerman D Garner HR Kodadek T 《Chemical communications (Cambridge, England)》2005,(5):581-583
We report here a strategy for the photolithographic synthesis of diverse, spatially addressable arrays of cyclic peptides which employs a differential deprotection strategy for the combinatorial addition of side chains to a pre-fabricated cyclic core. 相似文献
18.
Harold C. Beachell Jean C. Peterson 《Journal of polymer science. Part A, Polymer chemistry》1969,7(8):2021-2029
A linear polyurethane of high molecular weight was prepared in solution by the polyaddition of equimolar amounts of ethylene glycol and methylene bis(4-phenyl isocyanate). The polymer was fractionated by using a direct sequential extraction procedure, with a solvent–nonsolvent system consisting of N,N′-dimethylformamide (DMF) and acetone (A). The resulting fractions were characterized by viscosity and lightscattering measurements. The relationship between the intrinsic viscosity and molecular weight was found in DMF at 25°C. to be [η] = 3.64 × 10?4M0.71. The unperturbed polymer chain dimensions were determined from intrinsic viscosity measurements carried out under experimentally determined theta conditions. 相似文献
19.
Keikichi Uno Mutsuko Ohara Harold G. Cassidy 《Journal of polymer science. Part A, Polymer chemistry》1968,6(10):2729-2740
Ten vinylhydroquinone and one vinyl resorcinol derivatives are compared, particularly with respect to NMR spectra and copolymerizability with styrene. They are vinylhydroquinone dimethyl ether (I), vinyl-O,O′-bis(1-ethoxyethyl)hydroquinone (II), vinylhydroquinone di(2-pentyl)ether (III), 4-vinyl resorcinol bismethoxymethyl ether (IV), 2-vinyl-5-methylhydroquinone dimethyl ether (V), 2-vinyl-5-methyl-O,O′-bis(1-ethoxyethyl)hydroquinone (VI), 2-vinyl-6-methylhydroquinone dimethyl ether (VII), 2-vinyl-5-tert-butylhydroquinone dimethyl ether (VIII), 2-vinyl-5-chlorohydroquinone dimethyl ether (IX), 2-vinyl-3,6-dimethylhydroquinone dimethyl ether (X), and 2-vinyl-3,5,6-trimethylhydroquinone dimethyl ether (XI). All the vinyl protons have almost the same coupling constants. Though subtle distinctions are found among all the spectra, they can in general be put into two groups on the basis of the chemical shifts. Let the hydrogen on carbon-1 of the vinyl group be A, the hydrogen cis to A be B the hydrogen trans to A be C, then in the first group, (I) through (IX), the chemical shifts (τ) are (A) 3.02 ± 0.08, (C) 4.41 ± 0.05, and (B) 4.87 ± 0.07, and in the second group, (X) and (XI), they are (A) 3.30 ± 0.03, (C) 4.49 ± 0.01, and (B) 4.59 ± 0.03. It is supposed that in (X) and (XI) the vinyl group is out of the plane of the ring, because of the two ortho substituents, and this conformation is reflected in the NMR data. Ultraviolet spectra are consonant with this interpretation, since the λmax of (X) and (XI) correspond closely with those of nonvinyl reference compounds, while those of (II), (V), and (VIII) are shifted to longer wavelengths. When these compounds are copolymerized separately with styrene, the behaviors are classifiable into the following three groups, where r1 and r2 are monomer reactivity ratios with styrene as the first monomer: (i) r1 < 1 and r2 < 1 for compounds (II) and (III) and the reference compound O,O′-dibenzoylvinylhydroquinone, (ii) r1 < 1 and r2 > 1 for compounds (I), (V), (VII), (VIII), (IX), and (iii) r1 > 1 and r2 = 0 for compounds (X) and (XI). These behaviors are correlated with the effect of electronegativity of groups on the stability of the radical at the growing end of the chain and with the simultaneous effects of steric hindrance. 相似文献
20.
Christian Margot DanaP. Simmons Daniel Reichlin David Skuy 《Helvetica chimica acta》2004,87(10):2662-2684
Only one out of the four possible trans isomers of the important perfumery alcohol Norlimbanol® ( 1 ) possesses a very strong amber‐woody smell, the isomer 1A with (1′R,3S,6′S) absolute configuration. Its enantiomer 1B is almost odorless and devoid of amber‐woody character, whereas the diastereoisomers 1C and 1D are considerably weaker and perceptible only by the most‐sensitive persons. The same is true for a whole series of perceptual analogs of 1 , including β‐alkoxy alcohols. These ethers belong to two structural classes: [(2,2,6‐trimethylcyclohexyl)oxy]‐ (see 3, 4 , and 16 ) or {[2‐(tert‐butyl)cyclohexyl]oxy}alkan‐2‐ol derivatives (see 19 and 20 ; Table). A superimposition model allowing for good overlap of the respective hydroxylated side chains offers a tentative explanation for the shared perceptual characteristics of the two classes (Fig. 5). The lipophilic cyclohexane moieties present only a minimal overlap in this model, suggesting that quite larger molecules might possess the same smell. (S)‐Configured β‐alkoxy alcohols can conveniently be obtained on a larger scale by enantioselective reduction of the corresponding ketones (Scheme 9). 相似文献