全文获取类型
收费全文 | 2131篇 |
免费 | 80篇 |
国内免费 | 22篇 |
专业分类
化学 | 1427篇 |
晶体学 | 14篇 |
力学 | 35篇 |
数学 | 209篇 |
物理学 | 548篇 |
出版年
2024年 | 5篇 |
2023年 | 17篇 |
2022年 | 54篇 |
2021年 | 60篇 |
2020年 | 50篇 |
2019年 | 44篇 |
2018年 | 35篇 |
2017年 | 37篇 |
2016年 | 92篇 |
2015年 | 54篇 |
2014年 | 73篇 |
2013年 | 167篇 |
2012年 | 144篇 |
2011年 | 166篇 |
2010年 | 89篇 |
2009年 | 82篇 |
2008年 | 121篇 |
2007年 | 125篇 |
2006年 | 115篇 |
2005年 | 112篇 |
2004年 | 96篇 |
2003年 | 87篇 |
2002年 | 77篇 |
2001年 | 39篇 |
2000年 | 41篇 |
1999年 | 26篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 20篇 |
1995年 | 13篇 |
1994年 | 16篇 |
1993年 | 30篇 |
1992年 | 17篇 |
1991年 | 5篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 4篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有2233条查询结果,搜索用时 0 毫秒
901.
Joel?R.?Kimmel Oh?Kyu?Yoon Ignacio?A.?Zuleta Oliver?Trapp Richard?N.?ZareEmail author 《Journal of the American Society for Mass Spectrometry》2005,16(7):1117-1130
Hadamard transform (HT) time-of-flight mass spectrometry (TOFMS) is a multiplexing technique that offers high duty cycle for
the mass analysis of continuous ion sources. The multiplexing advantage is maximized when spectral noise is independent of
signal intensity. For conditions in which shot noise predominates, the variance in each peak is a function of the population
of all measured species. We develop expressions for the performance of a HT-TOF mass spectrometer based on Poissonian statistics
for the arrival times of ions at the detector. These expressions and complementary probabilistic simulations are used to estimate
the magnitude of the baseline noise as a function of mass spectral features and acquisition conditions. Experiment validates
the predictions that noise depends on the total number of ions in the acquired spectrum, and the achieved signal-to-noise
ratio for a given species depends on its relative population. We find that for HT-TOFMS experiments encoded with an n-order binary off-on sequence that contains N=2
n
−1 elements, the peak height precision, which is the peak intensity divided by its standard deviation, is greater than that
of an equivalent conventional TOF experiment by a factor of √N/2 times the square root of the fractional abundance of the
peak of interest. Thus, HT-TOFMS is superior to conventional TOF for all species whose fractional abundance F
i exceeds 2/N, which for a typical N value of 2047 corresponds to F
i > 0.001. HT-TOF mass spectra collected at 2500 per second demonstrates the method’s capability of monitoring transient processes
not possible by conventional means. 相似文献
902.
903.
Kim CK Lee KA Sohn CK Sung DD Oh HK Lee I 《The journal of physical chemistry. A》2005,109(12):2978-2983
The Hammett rho+ and rho- values have been determined by varying substituent Y' for a given Y in the benzhydryl cation and anion formation (YH4C6-CH-C6H4Y' where C is a cationic or an anionic center) at the RHF/3-21G, RHF/6-31G, RHF/6-31+G, and B3LYP/6-31+G levels. The failure of RHF theory in accounting for the stabilization by delocalization leads to the smaller magnitudes of rho+ and rho- with electron-donating and -withdrawing substituents, Y, respectively, than the corresponding DFT values. The effects of solvent (benzene, dichloroethane, and acetonitrile) on the rho values were calculated by applying the conductor polarizable continuum model method to the DFT results. Finally, the cross-interaction constants (rho(YY)') and their variation with solvent were determined. As the polarity (dielectric constant, epsilon) of the solvent is increased, the magnitude of rho+ and rho- decreased, whereas that of rho(YY)' increased. Satisfactory correlations were obtained between rho values (rho+, rho- and rho(YY)') and the Kirkwood function f(k) (= epsilon - 1/2epsilon + 1). The rho(YY)' values are negative with a magnitude greater for the anionic (rho(YY)'-) than the cationic (rho(YY)'+) system. 相似文献
904.
A reaction method is described for selective reductive cleavage of 2-(phenylthio)pyrimidines using Pd(OAc)2 and Et3SiH to produce 2-(H)pyrimidines. The reaction proceeds efficiently with a wide range of 2-(phenylthio)pyrimidines. Considering the ready availability of 2-(arylthio)pyrimidines derived from oxidative CS cross coupling of 3,4-dihydropyrimidin-1H-2-thiones (DHPMs), this method unambiguously provides a shortcut to the preparation of 2-(H)pyrimidines with unprecedented diversity. 相似文献
905.
M. Baubillier I. J. Bloodworth A. Burns J. N. Carney G. F. Cox U. Dore J. B. Kinson K. Knudson F. Levy P. J. Negus B. Y. Oh E. Quercigh J. M. Scarr G. A. Smith O. Villalobos Baillie M. F. Votruba J. Whitmore R. Zitoun 《Zeitschrift fur Physik C Particles and Fields》1983,17(4):309-317
A partial wave analysis of theK \(\bar K\) system produced by 8.25 GeV/cK ? mesons in the reaction \(K^ - p \to K\bar K\Lambda ^{ 0} \) has been performed, taking into account the information provided by the Λ0 decay. Thef′ region is dominated byD 0 (?) andD 1 (+) waves. We see no evidence for the production of new 0++ states in the mass region 1.05 to 1.75 GeV. 相似文献
906.
Shape evolution of single-crystalline iron oxide nanocrystals 总被引:1,自引:0,他引:1
Cheon J Kang NJ Lee SM Lee JH Yoon JH Oh SJ 《Journal of the American Chemical Society》2004,126(7):1950-1951
Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature. 相似文献
907.
The first examples of high-dimensional mixed-valence homometallic cyano-bridged copper complexes were synthesized and characterized: net-structured [Cu(CN)(4){Cu(cyclam)}(1.5)](2)(n)()(H(2)O)(5)(n) (1), ladder-type double-chain-structured [Cu(CN)(2){Cu(CN)(2)Cu(cyclam)}](n)()(H(2)O)(n) (2), layer-structured [{Cu(CN)(2)}(2)Cu(cycalm)](n) (3), and hydrogen-bond-based 2-D [Cu(CN)(3)Cu(cyclam)](n)()(CH(3)OH)(n) (4) (cyclam = 1,4,8,11-tetraazacyclotetradecane). (1) Crystallizes in triclinic space group P with a = 8.3589(11) A, b = 13.478(2) A, c = 14.828(2) A, alpha = 66.895(2) degrees , beta = 77.916(3) degrees , gamma = 85.939(3) degrees , and Z = 1; (2) crystallizes in triclinic space group P with a = 8.2305(12) A, b = 9.8861(15) A, c = 13.219(2) A, alpha = 84.863(3) degrees , beta = 75.744(3) degrees , gamma = 89.818(3) degrees , and Z = 2; 3 crystallizes in monoclinic space group P2(1)/c with a = 6.830(2) A, b = 8.482(2) A, c = 17.306(4) A, beta = 98.144(4) degrees , and Z = 2; 4 crystallizes in triclinic space group P with a = 9.470(1) A, b = 10.034(1) A, c = 12.064(1) A, alpha = 67.325(2), beta = 75.593(2), gamma = 70.672(2), and Z = 2. The coordination sphere of Cu(I) sites in the complexes shows diverse structures: tetrahedral [CuC(4)] for (1), tetrahedral [CuC(3)N] and triangular [CuC(2)N] for (2), triangular [CuC(2)N] for (3), and triangular [CuC(3)] for 4. In particular, (1) constitutes the first example of a structurally characterized system containing a bridging tetrahedral [Cu(CN)(4)](3)(-) unit. The diverse structural nature of these complexes is governed by the capping amines and the content of water in the reaction media. The magnetic interactions are negligible in these mixed-valence complexes. 相似文献
908.
Ildiko MeraczTaeboem Oh 《Tetrahedron letters》2003,44(34):6465-6468
Recent interest in ionic liquids has developed various uses for them, including some applications by synthetic chemists. Ionic liquids have joined the potential list of non-traditional solvents for Diels-Alder reactions. We report here our own efforts to examine the rates and selectivities of carbon Diels-Alder reactions. Our investigations show that excellent diastereoselective and enantioselective carbon Diels-Alder reactions can be achieved in imidazolium ionic solvents at room temperature. 相似文献
909.
Madin A O'Donnell CJ Oh T Old DW Overman LE Sharp MJ 《Journal of the American Chemical Society》2005,127(51):18054-18065
Intramolecular Heck reactions of alpha,beta-unsaturated 2-haloanilides derived from azatricyclo[4.4.0.0(2,8)]decanone 5 efficiently install the congested spirooxindole functionality of gelsemine. Depending upon the Heck reaction conditions and the nature of the beta-substituent, either products having the natural or unnatural configuration of the spirooxindole group are formed predominantly. Efforts to elaborate the hydropyran ring of gelsemine from the endo-oriented nitrile substituent of pentacyclic Heck product 18 were unsuccessful. Important steps in the ultimately successful route to (+/-)-gelsemine (1) are as follows: (a) intramolecular Heck reaction of tricyclic beta-methoxy alpha,beta-unsaturated 2-iodoanilide 68 in the presence of silver phosphate to form pentacyclic product 69 having the unnatural configuration of the spirooxindole fragment, (b) formation of hexacyclic aziridine 80 from the reaction of cyanide with intermediate 79 containing an N-methoxycarbonyl-beta-bromoethylamine fragment, (c) introduction of C17 by ring-opening of the aziridinium ion derived from aziridine 80, and (d) base-promoted skeletal rearrangement of pentacyclic equatorial alcohol 82 to form the oxacyclic ring and invert the spirooxindole functional group to provide hexacyclic gelsemine precursor 83. 相似文献
910.
Reactions of morpholine with 4-halocoumarins 1 , 3-halocoumarins 3 , and 2-halo-l,4-naphthoquinones 8 yield two different products, one where halogen is replaced by a nucleophile at the same carbon and the other where the nucleophile is attached to the vicinal carbon away from that bearing the halogen. It is considered that reactions proceeds through nucleophilic vinylic substitution by comparision of the reaction products and deuterium exchange experiments. Plausible mechanisms for these routes are suggested. 相似文献