全文获取类型
收费全文 | 35359篇 |
免费 | 1279篇 |
国内免费 | 259篇 |
专业分类
化学 | 25026篇 |
晶体学 | 280篇 |
力学 | 699篇 |
综合类 | 1篇 |
数学 | 5999篇 |
物理学 | 4892篇 |
出版年
2023年 | 161篇 |
2022年 | 178篇 |
2021年 | 431篇 |
2020年 | 617篇 |
2019年 | 606篇 |
2018年 | 420篇 |
2017年 | 395篇 |
2016年 | 950篇 |
2015年 | 834篇 |
2014年 | 1012篇 |
2013年 | 1726篇 |
2012年 | 2204篇 |
2011年 | 2488篇 |
2010年 | 1350篇 |
2009年 | 1193篇 |
2008年 | 2300篇 |
2007年 | 2115篇 |
2006年 | 2134篇 |
2005年 | 1991篇 |
2004年 | 1706篇 |
2003年 | 1401篇 |
2002年 | 1352篇 |
2001年 | 498篇 |
2000年 | 490篇 |
1999年 | 421篇 |
1998年 | 410篇 |
1997年 | 433篇 |
1996年 | 469篇 |
1995年 | 340篇 |
1994年 | 387篇 |
1993年 | 352篇 |
1992年 | 323篇 |
1991年 | 276篇 |
1990年 | 247篇 |
1989年 | 215篇 |
1988年 | 215篇 |
1987年 | 231篇 |
1986年 | 184篇 |
1985年 | 390篇 |
1984年 | 348篇 |
1983年 | 287篇 |
1982年 | 344篇 |
1981年 | 286篇 |
1980年 | 295篇 |
1979年 | 248篇 |
1978年 | 239篇 |
1977年 | 226篇 |
1976年 | 215篇 |
1975年 | 201篇 |
1974年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 520 毫秒
991.
Ben Yahia H Gaudin E Darriet J Banks M Kremer RK Villesuzanne A Whangbo MH 《Inorganic chemistry》2005,44(9):3087-3093
A new magnetic oxide, CuMnVO4, was prepared, and its crystal structure was determined by single-crystal X-ray diffraction. The magnetic properties of CuMnVO4 were characterized by magnetic susceptibility and specific heat measurements, and the spin exchange interactions of CuMnVO4 were analyzed on the basis of spin-polarized electronic band structure calculations. CuMnVO4 contains MnO4 chains made up of edge-sharing MnO6 octahedra containing high-spin Mn2+ cations. Our work shows that CuMnVO4 undergoes a three-dimensional antiferromagnetic transition at approximately 20 K. Both the intrachain and interchain spin exchanges are antiferromagnetic, and the interchain spin exchange is not negligible compared to the intrachain spin exchange. 相似文献
992.
Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield. 相似文献
993.
Lewis WK Lindsay CM Bemish RJ Miller RE 《Journal of the American Chemical Society》2005,127(19):7235-7242
Electron impact ionization of a helium atom in a helium nanodroplet is followed by rapid charge migration, which can ultimately result in the localization of the charge on an atomic or molecular solute. This process is studied here for the cases of hydrogen cyanide, acetylene, and cyanoacetylene in helium, using a new experimental method we call optically selected mass spectrometry (OSMS). The method combines infrared laser spectroscopy with mass spectrometry to separate the contributions to the overall droplet beam mass spectrum from the various species present under a given set of conditions. This is done by vibrationally exciting a specific species that exists in a subset of the droplets (for example, the droplets containing a single HCN molecule). The resulting helium evaporation leads to a concomitant reduction in the ionization cross sections for these droplets. This method is used to study the charge migration in helium and reveals that the probability of charge transfer to a solvated molecule does not approach unity for small droplets and depends on the identity of the solvated molecule. The experimental results are explained quantitatively by considering the effect of the electrostatic potential (between the charge and the embedded molecule) on the trajectory of the migrating charge. 相似文献
994.
In this study, mobility and structure of water molecules in Aerosol OT (bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with water content w0 = 5 and Na+, K+, Cs+ counterions have been explored with molecular dynamics (MD) simulations. Using the Faeder/Ladanyi model (J. Phys. Chem. B, 2000, 104, 1033) of the reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function, FS(Q,t), for water hydrogen atoms that could be measured in a quasielastic neutron scattering experiment. Separate intermediate scattering functions FRS(Q,t) and FCMS(Q,t) were determined for rotational and translational motion. We find that the decay of FCMS(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior arises from decreased water mobility for molecules close to the interface and from confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay, which is consistent with relatively rapid restricted rotation and slower rotational relaxation over the full angular range. Rotational relaxation is anisotropic, with the O-H bond short-time rotational mobility considerably higher than that of the molecular dipole. This behavior is related to the decreased density of water-water hydrogen bonds in the vicinity of the interface compared to core or bulk water. We find that the interfacial mobility of water molecules is quite different for the three counterion types, but that the core mobility exhibits weak counterion dependence. Differences in interfacial mobility are strongly correlated with structural features, especially ion-water coordination, and the extent of disruption by the counterions of the water hydrogen bond network. 相似文献
995.
Nizkorodov SA Ziemkiewicz M Nesbitt DJ Knight AE 《The Journal of chemical physics》2005,122(19):194316
Spectroscopy and predissociation dynamics of (H2O)2 and Ar-H2O are investigated with vibrationally mediated dissociation (VMD) techniques, wherein upsilon(OH) = 2 overtones of the complexes are selectively prepared with direct infrared pumping, followed by 193 nm photolysis of the excited H2O molecules. As a function of relative laser timing, the photolysis breaks H2O into OH and H fragments either (i) directly inside the complex or (ii) after the complex undergoes vibrational predissociation, with the nascent quantum state distribution of the OH photofragment probed via laser-induced fluorescence. This capability provides the first rotationally resolved spectroscopic analysis of (H2O)2 in the first overtone region and vibrational predissociation dynamics of water dimer and Ar-water clusters. The sensitivity of the VMD approach permits several upsilon(OH) = 2 overtone bands to be observed, the spectroscopic assignment of which is discussed in the context of recent anharmonic theoretical calculations. 相似文献
996.
Thaddeus Prusik Tatiana Kolubayev Michael J. Morelli Henry C. Brenner 《Photochemistry and photobiology》1980,31(4):315-321
Abstract— The luminescence of acridine dyes intercalated in DNA was studied as a function of the concurrent binding of metal ions to DNA, in an effort to deduce specific site interactions of the dyes. Two dyes, proflavine (PF) and acridine orange (AO), and two metal ions, silver and mercuric, were used. Both ions quench the fluorescence of the dyes in aqueous solution at room temperature. The metal ions have a different effect on the fluorescence of these dyes when they are intercalated between the base pairs of DNA. The fluorescence of AO is decreased when silver is bound, while the fluorescence of PF is enhanced. Since Ag+ initially binds to GC sites in DNA, which quench the PF fluorescence, it ostensibly 'turns off' the quenching by DNA at these sites, and this effect is greater than the quenching effect of the silver ion itself. Hg2+ ion initially binds to AT sites in DNA. Since both dyes fluoresce from AT sites, Hg2 + is expected to quench their fluorescence. This behavior is observed at low r (metal ion/base). At higher r values, however, where Hg2 + is expected to begin binding to GC sites, the fluorescence of PF is enhanced. These quenching turn-off effects are tentatively interpreted in terms of a change in the structure of the dye/DNA complex which occurs when a metal ion binds at the intercalation site. At 77 K. no fluorescence enhancement is observed when metal ions bind; Ag+ quenches the fluorescence and enhances the phosphorescence of both dyes. Qualitatively similar results are obtained with Hg2 + . 相似文献
997.
Nucleosides and Nucleotides. Part 16. The Behaviour of 1-(2′-Deoxy-β-D -ribofuranosyl)-2(1H)-pyrimidinone-5′-triphosphate, 1-(2′-Deoxy-β-D -ribofuranosyl-2(1H))-pyridinone-5′-triphosphate and 4-Amino-1-(2′-desoxy-β-D -ribofuranosyl)-2(1H)-pyridinone-5′-triphosphate towards DNA Polymerase The behaviour of nucleotide base analogs in the DNA synthesis in vitro was studied. The investigated nucleoside-5′-triphosphates 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyrimidinone-5′-triphosphate (pppMd), 1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppIId) and 4-amino-1-(2′-deoxy-β-D -ribofuranosyl)-2(1 H)-pyridinone-5′-triphosphate (pppZd) can be considered to be analogs of 2′-deoxy-cytidine-5′-triphosphate. However, their ability to undergo base pairing to the complementary guanine is decreased. When pppMd, pppIId or pppZd are substituted for pppCd in the enzymatic synthesis of DNA by DNA polymerase no incorporation of these analogs is observed. They exhibit only a weak inhibition of the DNA synthesis. The mode of the inhibition is uncompetitive which shows that these nucleotide analogs cannot serve as substrates for the DNA polymerase. 相似文献
998.
Michael Aizenman FranÇois Delyon Bernard Souillard 《Journal of statistical physics》1980,23(3):267-280
We rigorously prove that the probabilityP
n
that the origin of ad-dimensional lattice belongs to a cluster of exactlyn sites satisfiesP
n
> exp(–n
(d–1)/d
) whenever percolation occurs. This holds for the usual (noninteracting) percolation models for any concentrationp > p
c
, as well as for the equilibrium states of lattice spin systems with quite general interactions. Such a lower bound applies also if no percolation occurs, but if it appears in some other phase of the system. 相似文献
999.
Michael Aizenman 《Communications in Mathematical Physics》1980,73(1):83-94
It is shown that any Gibbs state of the two dimensional ferromagnetic Ising system is of the form ++(1–)–, with some [0, 1]. This excludes the possibility of a locally stable phase coexistence and of translation symmetry breaking, which are known to occur in higher dimensions. Use is made in the proof of the stochastic aspects of the geometry of the interface lines.Supported in part by the U.S. National Science Foundation under the grant PHY — 7825390 相似文献
1000.
Michael Renardy 《Communications in Mathematical Physics》1980,76(3):277-287
We discuss bounded solutions of the equation $$r^2 \left( {\frac{{\partial ^2 u}}{{\partial r^2 }} + \frac{{\partial ^2 u}}{{\partial t^2 }}} \right) = u^3 - u$$ in the halfspacer>0. All solutions depending only ont/r are characterized topologically. Then we prove the existence of infinite dimensional manifolds oft-periodic as well as nonperiodic solutions which are small in a suitable norm. 相似文献