全文获取类型
收费全文 | 2633篇 |
免费 | 40篇 |
国内免费 | 8篇 |
专业分类
化学 | 1595篇 |
晶体学 | 16篇 |
力学 | 107篇 |
数学 | 496篇 |
物理学 | 467篇 |
出版年
2022年 | 36篇 |
2021年 | 49篇 |
2020年 | 47篇 |
2019年 | 28篇 |
2018年 | 29篇 |
2017年 | 24篇 |
2016年 | 73篇 |
2015年 | 40篇 |
2014年 | 50篇 |
2013年 | 129篇 |
2012年 | 132篇 |
2011年 | 142篇 |
2010年 | 88篇 |
2009年 | 82篇 |
2008年 | 145篇 |
2007年 | 144篇 |
2006年 | 131篇 |
2005年 | 125篇 |
2004年 | 104篇 |
2003年 | 98篇 |
2002年 | 81篇 |
2001年 | 42篇 |
2000年 | 35篇 |
1999年 | 34篇 |
1998年 | 27篇 |
1997年 | 38篇 |
1996年 | 57篇 |
1995年 | 28篇 |
1994年 | 38篇 |
1993年 | 28篇 |
1992年 | 35篇 |
1991年 | 28篇 |
1990年 | 22篇 |
1989年 | 28篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 23篇 |
1985年 | 22篇 |
1984年 | 25篇 |
1983年 | 26篇 |
1982年 | 29篇 |
1981年 | 16篇 |
1980年 | 24篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1973年 | 15篇 |
1971年 | 10篇 |
排序方式: 共有2681条查询结果,搜索用时 0 毫秒
991.
Nicolas DesroyRémi Le Roux Phannarath PhansavathLucia Chiummiento Carlo Bonini Jean-Pierre Genêt 《Tetrahedron letters》2003,44(9):1763-1766
The stereocontrolled synthesis of a C15C24 fragment of dolabelides is reported. The C19 and C21 hydroxyl-bearing stereocenters were installed using ruthenium-mediated asymmetric hydrogenations of cyclic hemiketal 4 and β-keto ester 7. The C25C30 portion of dolabelides was prepared as well by ring opening of chiral epoxy alcohol 12 to set up the C27 stereogenic center. 相似文献
992.
Carlo A. Ghilardi Paolo Innocenti Stefano Midollini Annabella Orlandini 《Journal of organometallic chemistry》1982,231(4):C78-C80
Sodium tetrahydroborate reacts with iron(II) tetrafluoroborate and 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) to give the complex (triphos)-FeH(BH4) whose molecular structure, determined from X-ray data consists of monomolecular units in which the iron atom is six-coordinated by the three phosphorus atoms of the ligand, two hydrogen atoms of the BH4 group, and a hydridic hydrogen atom. Variable temperature 31P NMR spectra reveal stereochemical non-rigidity of the complex in solution. 相似文献
993.
Surface heterogeneity study of some reference Cu–Ag alloys using laser‐induced breakdown spectroscopy 下载免费PDF全文
S. A. Sheta G. Di Carlo G. M. Ingo M. A. Harith 《Surface and interface analysis : SIA》2015,47(4):514-522
Investigation of the chemical composition, surface structure, metallurgical features, corrosion mechanism, and surface modification techniques of archeological metallic artifacts from Romans and pre‐Roman times aimed to simulate the most commonly used Cu‐based and Ag‐based alloys. These simulated reference alloys will be used as sacrificial materials to study the most appropriate conservation materials and procedures. In the present work, laser‐induced breakdown spectroscopy (LIBS) is introduced as a new validated surface mapping technique to study the micro‐chemical distribution of elements in binary reference copper–silver alloy samples. Using different techniques for surface and bulk analysis, such as SEM coupled with energy‐dispersive X‐ray spectroscopy and X‐ray diffraction, it has been proven that LIBS is a simple, sensitive, and direct technique in the determination of heterogeneity of the sample's surface. By changing the laser wavelength (λ/nm) and focal length of the used focusing lens (f/cm), different spot sizes can be obtained. It was possible to control the spatial resolution in mapping the investigated samples' surface and to achieve local chemical information. In the present work, Q‐switched neodymium‐doped yttrium aluminum garnet laser has been used at its fundamental wavelength 1064 nm and its second harmonic 532 nm. The studied samples were specially manufactured heterogeneous copper–silver alloys with known grain size as studied via SEM investigation. The obtained LIBS results are in good agreement with those by other analytical techniques and extend the applicability of the surface techniques to study metallic ancient objects. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
994.
First-principles [density-functional theory (DFT)] molecular-dynamic simulations of the Berry pseudorotation mechanism in SF4 were performed using the atom-centered density-matrix propagation method. The reaction was monitored by following the chemical shieldings of the fluorine atoms, computed on snapshots along the trajectories. In particular we compared the results obtained using a standard functional based on the generalized gradient approximation with those issuing from its hybrid Hartree-Fock-DFT counterpart using a number of basis sets. Our results show that both the basis set and the functional choice rule the quality of the molecular properties monitored as well as the trajectory over the potential-energy surface. 相似文献
995.
Malavasi L Mozzati MC Ritter C Pomjakushin V Tealdi C Azzoni CB Flor G 《The journal of physical chemistry. B》2006,110(35):17430-17436
In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La(0.5)Sr(1.5)MnO(4) and B-site lightly doped samples, i.e. La(0.5)Sr(1.5)Mn(0.95)B(0.05)O(4), where B = Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La(0.5)Sr(1.5)MnO(4) manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La(0.5)Sr(1.5)MnO(4). No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature ( approximately 22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the e(g) electrons along with the overall cation size. 相似文献
996.
Franois Diederich Ulrich Jonas Volker Gramlich Andreas Herrmann Helmut Ringsdorf Carlo Thilgen 《Helvetica chimica acta》1993,76(7):2445-2453
A fullerene derivative 1 of benzo[18]crown-6 was obtained by Diels-Alder addition of fullerene[60](C60) to the ortho-quinodimethane prepared in situ from 4,5-bis(bromomethyl)benzo[18]crown-6 ( 3 ) with Bu4NI in toluene. Extraction experiments show that the complexation of K+ ions strongly increases the solubility of 1 in protic solvents like MeOH. Using Langmuir-Blodgett techniques, monolayers of the highly amphiphilic fullerene-derived crown ether 1 and its K+ ion complex were prepared. An X-ray crystal structure was obtained from a benzene clathrate of comparison compound 2 , synthesized by Diels-Alder reaction of C60 with the ortho-quinodimethane derived from 1,2-bis(bromomethyl)-4,5-dimethoxybenzene ( 4 ). Both the fullerene molecule 2 and the benzene molecule are fully ordered in a crystal packing which is stabilized by intermolecular van-der-Waals contacts between the benzene ring and the C-spheres, intermolecular C…?C contacts between the C60 moieties, and intermolecular O…?C contacts between the O-atoms of the veratrole moieties and fullerene C-atoms. 相似文献
997.
Giuseppe Marcotrigiano Ledi Menabue Gian Carlo Pellacani 《Transition Metal Chemistry》1978,3(1):108-112
Summary Tetrahalometallates of the type (Etmorphl1)21MX4 (M = Co11, Ni11 or Zn11 and with X = Cl, Br or l; M = CuII with X = CI or Br) and mixed tetrahalocuprates, Etmorphll)2[CUXmY4–m] (X = Cl; N' = 13r; m = 1,2,3) of theN-ethylmorpholinium cation were prepared and investigated by means of spectroscopic and magnetic measurements. While the cobalt(II), nickel(II) and zinc(II) complexes appear to be essentially, tetrahedral, the copper(II) complexes are discussed on the basis of a distorted ('flattened') tetrahedral symmetry. The electronic spectra of the complexes are assigned on this basis. The far i.r. spectra of the complexes show bands which are unambiguously assignable to the metal-halogen stretching modes. The effects of the counter cation on the geometry around the metal ion, compared with that of the morpholinium and piperidinium cations, are discussed in relation to the pKa of the amine. 相似文献
998.
Juan Andrés Paul W. Ayers Roberto A. Boto Ramon Carbó-Dorca Henry Chermette Jerzy Cioslowski Julia Contreras-García David L. Cooper Gernot Frenking Carlo Gatti Farnaz Heidar-Zadeh Laurent Joubert Ángel Martín Pendás Eduard Matito István Mayer Alston J. Misquitta Yirong Mo Julien Pilmé Paul L. A. Popelier Martin Rahm Eloy Ramos-Cordoba Pedro Salvador W. H. Eugen Schwarz Shant Shahbazian Bernard Silvi Miquel Solà Krzysztof Szalewicz Vincent Tognetti Frank Weinhold Émilie-Laure Zins 《Journal of computational chemistry》2019,40(26):2248-2283
999.
Carrozzo MM Cannazza G Battisti U Braghiroli D Troisi L Parenti C 《Journal of chromatography. A》2010,1217(52):8136-8145
In this study, configurational and chemical stability of (R,R),(S,S),(R,S),(S,R)-3,6-dimethyl-2,3,5,6-tetrahydro[1,2,4]thiadiazino[6,5,4-hi]indole 1,1-dioxide (1) were investigated by dynamic and stopped-flow HPLC methods. Single epimeric mixtures (R,R),(R,S)-1 and (S,S),(S,R)-1 were obtained combining synthetic and chromatographic strategies. Separation of (R,R)-1 and (R,S)-1 was achieved by chiral chromatography and absolute configuration of eluted epimers has been assigned basing on molecular modelling calculations. Epimerization and hydrolysis of (R,R),(R,S)-1 have been studied by classical off-column, dynamic HPLC and stopped-flow HPLC methods. The influence of different parameters, such as temperature, pH and dielectric constant was evaluated. The data obtained indicate that (R,R),(R,S)-1 undergoes to a rapid epimerization in aqueous solvent and hydrolysis in acidic conditions. Moreover, epimerization and hydrolysis were investigated in presence of an artificial membrane and in physiological buffers (pH 2.2 and 7.0 at 37.5°C) to simulate in vivo conditions. 相似文献
1000.
We report a characterization of the relative stability and structural behavior of various micellar crystals of an athermal model of AB-diblock copolymers in solution. We adopt a previously developed coarse-graining representation of the chains which maps each copolymer on a soft dumbbell. Thanks to this strong reduction of degrees of freedom, we are able to investigate large aggregated systems and for a specific length ratio of the blocks f = M(A)∕(M(A) + M(B)) = 0.6, to locate the order-disorder transition of the system of micelles. Above the transition, mechanical and thermal properties are found to depend on the number of particles per lattice site in the simulation box, and the application of a recent methodology for multiple occupancy crystals [B. M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)] is necessary to correctly define the equilibrium state. Within this scheme we have performed free energy calculations at two reduced density ρ∕ρ? = 4, 5 and for several cubic structures such as fcc, bcc, and A15. At both densities, the bcc symmetry is found to correspond to the minimum of the unconstrained free energy, that is to the stable symmetry among the few considered, while the A15 structure is almost degenerate, indicating that the present system prefers to crystallize in less packed structures. At ρ∕ρ? = 4 close to melting, the Lindemann ratio is fairly high (~0.29) and the concentration of vacancies is roughly 6%. At ρ∕ρ? = 5 the mechanical stability of the stable bcc structure increases and the concentration of vacancies accordingly decreases. The ratio of the corona layer thickness to the core radius is found to be in good agreement with experimental data for poly(styrene-b-isoprene)(22-12) in isoprene selective solvent which is also reported to crystallize in the bcc structure. 相似文献