全文获取类型
收费全文 | 2649篇 |
免费 | 65篇 |
国内免费 | 8篇 |
专业分类
化学 | 1789篇 |
晶体学 | 21篇 |
力学 | 73篇 |
数学 | 568篇 |
物理学 | 271篇 |
出版年
2020年 | 45篇 |
2019年 | 29篇 |
2017年 | 25篇 |
2016年 | 42篇 |
2015年 | 69篇 |
2014年 | 53篇 |
2013年 | 96篇 |
2012年 | 93篇 |
2011年 | 99篇 |
2010年 | 78篇 |
2009年 | 59篇 |
2008年 | 120篇 |
2007年 | 110篇 |
2006年 | 95篇 |
2005年 | 102篇 |
2004年 | 79篇 |
2003年 | 63篇 |
2002年 | 50篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 22篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 36篇 |
1992年 | 31篇 |
1991年 | 20篇 |
1989年 | 25篇 |
1988年 | 32篇 |
1987年 | 30篇 |
1985年 | 37篇 |
1984年 | 46篇 |
1983年 | 26篇 |
1982年 | 23篇 |
1981年 | 32篇 |
1980年 | 39篇 |
1979年 | 27篇 |
1978年 | 23篇 |
1977年 | 30篇 |
1976年 | 24篇 |
1975年 | 33篇 |
1974年 | 29篇 |
1972年 | 21篇 |
1967年 | 20篇 |
1966年 | 20篇 |
1965年 | 19篇 |
1958年 | 22篇 |
1956年 | 27篇 |
排序方式: 共有2722条查询结果,搜索用时 875 毫秒
991.
N-K electron energy-loss near-edge structures for TiN/VN layers: an ab initio and experimental study
Lazar P Redinger J Strobl J Podloucky R Rashkova B Dehm G Kothleitner G Sturm S Kutschej K Mitterer C Scheu C 《Analytical and bioanalytical chemistry》2008,390(6):1447-1453
We study N-K-edge electron energy-loss near-edge structures for well-defined TiN/VN bilayers grown on a MgO(100) substrate
by both calculations and experiments. The structural relaxations and the electronic structure of TiN/VN multilayers are calculated
using the Vienna Ab Initio Simulation Package computer code, which uses density functional theory to describe the electronic
interaction. The effects of the core hole created in the excitation process are included in the calculations. For VN, off-stoichiometric
effects due to nitrogen vacancies are modelled. The partial density of states (PDOS) for the N-K edge of atoms in the vicinity
of the TiN/MgO interface revealed that two new peaks appear between 7 and 9 eV instead of a broad shoulder typical for the
bulk. For the VN/TiN interface, the PDOS is modified only slightly, owing to similar bonding on both sides of the interface,
and is thus very similar to the respective bulk spectra. An experimental spectrum taken at the VN/TiN interface is, however,
well described by an average of the simulated spectra for VN and TiN bulk (interface). Such a finding is characteristic of
an intermixed interface.
相似文献
992.
Josef Dick 《Annali di Matematica Pura ed Applicata》2008,187(3):385-403
The Koksma–Hlawka inequality states that the error of numerical integration by a quasi-Monte Carlo rule is bounded above by
the variation of the function times the star-discrepancy. In practical applications though functions often do not have bounded
variation. Hence here we relax the smoothness assumptions required in the Koksma–Hlawka inequality. We introduce Banach spaces
of functions whose fractional derivative of order is in . We show that if α is an integer and p = 2 then one obtains the usual Sobolev space. Using these fractional Banach spaces we generalize the Koksma–Hlawka inequality
to functions whose partial fractional derivatives are in . Hence we can also obtain an upper bound on the integration error even for certain functions which do not have bounded variation
but satisfy weaker smoothness conditions.
相似文献
993.
The construction of good extensible rank-1 lattices 总被引:1,自引:0,他引:1
Josef Dick Friedrich Pillichshammer Benjamin J. Waterhouse. 《Mathematics of Computation》2008,77(264):2345-2373
It has been shown by Hickernell and Niederreiter that there exist generating vectors for integration lattices which yield small integration errors for for all integers . This paper provides algorithms for the construction of generating vectors which are finitely extensible for for all integers . The proofs which show that our algorithms yield good extensible rank-1 lattices are based on a sieve principle. Particularly fast algorithms are obtained by using the fast component-by-component construction of Nuyens and Cools. Analogous results are presented for generating vectors with small weighted star discrepancy.
994.
Sloufová I Sisková K Vlcková B Stepánek J 《Physical chemistry chemical physics : PCCP》2008,10(16):2233-2242
Changes in morphology, surface reactivity and surface-enhancement of Raman scattering induced by modification of borate-stabilized Ag nanoparticles by adsorbed chlorides have been explored using TEM, EDX analysis and SERS spectra of probing adsorbate 2,2'-bipyridine (bpy) excited at 514.5 nm and evaluated by factor analysis. At fractional coverages of the parent Ag nanoparticles by adsorbed chlorides <0.6, the Ag colloid/Cl(-)/bpy systems were found to be constituted by fractal aggregates of Ag nanoparticles fairly uniform in size (10 +/- 2 nm) and SERS spectra of Ag(+)-bpy surface species were detected. The latter result was interpreted in terms of the presence of oxidized Ag(+) and/or Ag(n)(+) adsorption sites, which have been encountered also in systems with the chemically untreated Ag nanoparticles. At chloride coverages >0.6, a fusion of fractal aggregates into the compact aggregates of touching and/or interpenetrating Ag nanoparticles has been observed and found to be accompanied by the formation of another surface species, Ag-bpy, as well as by the increase of the overall SERS enhancement of bpy by factor of 40. The same Ag-bpy surface species has been detected under the strongly reducing conditions of reduction of silver nitrate by sodium borohydride in the presence of bpy. The formation of Ag-bpy is thus interpreted in terms of the stabilization of reduced Ag(0) adsorption sites by adsorbed bpy. The formation of reduced adsorption sites on Ag nanoparticle surfaces at chloride coverages >0.6 is discussed in terms of local changes in the work function of Ag. Finally, the SERS spectral detection of Ag-bpy species is proposed as a tool for probing the presence of reduced Ag(0) adsorption sites in systems with chemically modified Ag nanoparticles. 相似文献
995.
Redox and acid-base properties of dispersed oxide nanostructures change markedly as their local structure and electronic properties vary with domain size. These changes give rise to catalytic behavior, site structures, and reaction chemistries often unavailable on bulk crystalline oxides. Turnover rates for redox and acid catalysis vary as oxide domains evolve from isolated monomers to two-dimensional oligomers, and ultimately into clusters with bulk-like properties. These reactivity changes reflect the ability of oxide domains to accept or redistribute electron density in kinetically-relevant reduction steps, in the formation of temporary acid sites via reductive processes, and in the stabilization of cationic transition states. Reduction steps are favored by low-lying empty orbitals prevalent in larger clusters, which also favor electron delocalization, stable anions, and strong Br?nsted acidity. Isomerization of xylenes and alkanes, elimination reactions of alkanols, and oxidation of alkanes to alkenes on V, Mo, Nb, and W oxide domains are used here to demonstrate the remarkable catalytic diversity made available by changes in domain size. The reactive and disordered nature of small catalytic domains introduces significant challenges in their synthesis and their structural and mechanistic characterization, which require in situ probes and detailed kinetic analysis. The local structure and electronic properties of these materials must be probed during catalysis and their catalytic function be related to specific kinetically-relevant steps. Structural uniformity can be imposed on oxide clusters by the use of polyoxometalate clusters with thermodynamically stable and well-defined size and connectivity. These clusters provide the compositional diversity and the structural fidelity required to develop composition-function relations from synergistic use of experiments and theory. In these clusters, the valence and electronegativity of the central atom affects the acid strength of the polyoxometalate clusters and the rate constants for acid catalyzed elementary steps via the specific stabilization of cationic transition states in isomerization and elimination reactions. 相似文献
996.
Hnyk D Wann DA Holub J Bühl M Robertson HE Rankin DW 《Dalton transactions (Cambridge, England : 2003)》2008,(1):96-100
The molecular structure of 1-selena-closo-dodecaborane(11), 1-SeB11H11, has been determined by the concerted use of quantum chemical calculations and gas-phase electron diffraction. The structure has C(5v) symmetry and is distorted from a regular icosahedron mainly through the expansion of the pentagon of boron atoms adjacent to selenium, with r(a3,1)(B-B) = 192.2(2) pm. The Se-B bond length is extremely well determined [r(a3,1) (Se-B) = 212.9(2) pm] and this is reflected by a pronounced peak in the radial-distribution curve. The accuracy of the experimental structure, as well as that calculated at the MP2/962(d) level, has been gauged by comparison of the 11B chemical shifts (calculated at two different gauge-including atomic orbitals (GIAO) levels) with experimental NMR values. The inclusion of electron correlation in the magnetic property calculations (GIAO-MP2) gave superior results to those carried out using GIAO-Hartree-Fock. 相似文献
997.
Bioaffinity magnetic reactor for peptide digestion followed by analysis using bottom-up shotgun proteomics strategy 总被引:1,自引:0,他引:1
Korecká L Jankovicová B Krenková J Hernychová L Slováková M Le-Nell A Chmelik J Foret F Viovy JL Bilková Z 《Journal of separation science》2008,31(3):507-515
We report an efficient and streamlined way to improve the analysis and identification of peptides and proteins in complex mixtures of soluble proteins, cell lysates, etc. By using the shotgun proteomics methodology combined with bioaffinity purification we can remove or minimize the interference contamination of a complex tryptic digest and so avoid the time-consuming separation steps before the final MS analysis. We have proved that by means of enzymatic fragmentation (endoproteinases with Arg-C or/and Lys-C specificity) connected with the isolation of specific peptides we can obtain a simplified peptide mixture for easier identification of the entire protein. A new bioaffinity sorbent was developed for this purpose. Anhydrotrypsin (AHT), an inactive form of trypsin with an affinity for peptides with arginine (Arg) or lysine (Lys) at the C-terminus, was immobilized onto micro/nanoparticles with superparamagnetic properties (silica magnetite particles (SiMAG)-Carboxyl, Chemicell, Germany). This AHT carrier with a determined binding capacity (26.8 nmol/mg of carrier) was tested with a model peptide, human neurotensin, and the resulting MS spectra confirmed the validity of this approach. 相似文献
998.
In the current study, the coordination chemistry of nine-coordinate Ac(III) complexes with 35 monodentate and bidentate ligands was investigated using density functional theory (DFT) in terms of their geometries, charges, reaction energies, and bonding interactions. The energy decomposition analysis with naturals orbitals for chemical valence (EDA-NOCV) and the quantum theory of atoms in molecules (QTAIM) were employed as analysis methods. Trivalent Ac exhibits the highest affinities toward hard acids (such as charged oxophilic donors, fluoride), so its classification as a hard acid is justified. Natural population analysis quantified the involvement of 5f orbitals on Ac to be about 30% of total valence electron natural configuration indicating that Ac is a member of the actinide series. Pearson correlation coefficients were used to study the pairwise correlations among the bond lengths, ΔG reaction energies, charges on Ac and donor atoms, and data from EDA-NOCV and QTAIM. Strong correlations and anticorrelations were found between Voronoi charges on donor atoms with ΔG, EDA-NOCV interaction energies and QTAIM bond critical point densities. 相似文献
999.
Dr. Qin Yang Prof. Julien Bloino Hana Šestáková Dr. Jaroslav Šebestík Dr. Jiří Kessler Dr. Jana Hudecová Dr. Josef Kapitán Prof. Petr Bouř 《Angewandte Chemie (International ed. in English)》2023,62(45):e202312521
Resonance Raman optical activity (RROA) spectra with high sensitivity reveal details on molecular structure, chirality, and excited electronic properties. Despite the difficulty of the measurements, the recorded data for the Co(III) complex with S,S-N,N-ethylenediaminedisuccinic acid are of exceptional quality and, coupled with the theory, spectacularly document the molecular behavior in resonance. This includes a huge enhancement of the chiral scattering, contribution of the antisymmetric polarizabilities to the signal, and the Herzberg-Teller effect significantly shaping the spectra. The chiral component is by about one order of magnitude bigger than for an analogous aluminum complex. The band assignment and intensity profile were confirmed by simulations based on density functional and vibronic theories. The resonance was attributed to the S0→S3 transition, with the strongest signal enhancement of Raman and ROA spectral bands below about 800 cm−1. For higher wavenumbers, other excited electronic states contribute to the scattering in a less resonant way. RROA spectroscopy thus appears as a unique tool to study the structure and electronic states of absorbing molecules in analytical chemistry, biology, and material science. 相似文献
1000.
Josef Tomeček Prof. Stephen T. Liddle Prof. Nikolas Kaltsoyannis 《Chemphyschem》2023,24(18):e202300366
The tri-thorium cluster [{Th(η8-C8H8)(μ3-Cl)2}3{K(THF)2}2]∞ (Nature 2021, 598 , 72–75) was reported to feature intriguing σ-aromatic bonding between the thorium atoms, a mode of metal–metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η8-C8H8)(μ3-Cl)2}3{K(THF)2}2]∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th−Th bonding and Th3 σ-aromaticity. 相似文献