全文获取类型
收费全文 | 4179篇 |
免费 | 442篇 |
国内免费 | 565篇 |
专业分类
化学 | 3979篇 |
晶体学 | 17篇 |
力学 | 2篇 |
综合类 | 8篇 |
数学 | 62篇 |
物理学 | 1118篇 |
出版年
2024年 | 4篇 |
2023年 | 24篇 |
2022年 | 36篇 |
2021年 | 47篇 |
2020年 | 85篇 |
2019年 | 89篇 |
2018年 | 73篇 |
2017年 | 58篇 |
2016年 | 155篇 |
2015年 | 164篇 |
2014年 | 170篇 |
2013年 | 327篇 |
2012年 | 221篇 |
2011年 | 212篇 |
2010年 | 187篇 |
2009年 | 276篇 |
2008年 | 329篇 |
2007年 | 318篇 |
2006年 | 348篇 |
2005年 | 294篇 |
2004年 | 288篇 |
2003年 | 308篇 |
2002年 | 191篇 |
2001年 | 157篇 |
2000年 | 161篇 |
1999年 | 167篇 |
1998年 | 136篇 |
1997年 | 88篇 |
1996年 | 92篇 |
1995年 | 59篇 |
1994年 | 36篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 18篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有5186条查询结果,搜索用时 62 毫秒
61.
The understanding of the electronic effects of the diastereoselective addition of a nucleophile to a polar substituted aldehyde or ketone is not complete, with several theories competing to explain the data. For numerous hydride reductions of 3‐X‐2‐butanones (X = F, Cl, Br), the selectivity for the major syn isomer is significantly and consistently higher for X = Br than for X = F. This result is rationalized as a shift in mechanism from Cornforth (X = F) to Felkin–Anh (X = Br). The experimental data is well modeled by ab initio calculations for the addition to these ketones by BH3, but not by other nucleophiles such as LiH or LiAlH4. The energetic ordering of the BH3 transition states largely follows the trends for the ground state ketones. Here, consistent with electrostatic arguments, the anti orientation of the C―X and C?O bonds is always lower in energy than the syn arrangement. The gauche conformer is intermediate between these two, becoming gradually lower in energy as X increases in size. The hyperconjugative interaction invoked by the Felkin–Anh model provides only a modest stabilization of the relevant transition states as judged by NBO analysis. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
62.
Inside Back Cover: A Terminal,Fluxional η4‐Benzene Complex with a Thermally Accessible Triplet State is the Primary Photoproduct in the Intercyclobutadiene Haptotropism of (CpCo)phenylenes (Chem. Eur. J. 12/2015) 下载免费PDF全文
63.
The structural and electronic properties of crystalline nitrates have been investigated within the framework of density functional theory including van der Waals interactions. Pressure behavior of nitrates has been investigated using semiempirical dispersion correction scheme DFT-D. The optimizations of the crystal structures have been done with full relaxation of the atomic positions and lattice parameters under the experimentally determined crystal symmetries. The pressure dependences of geometrical parameters, band gaps, densities of states, charge distributions, overlap populations and atomic charges are computed. The predicted results agree well with the available experimental data. 相似文献
64.
65.
G.P. Cousland R.A. Mole M.M. Elcombe X.Y. Cui A.E. Smith C.M. Stampfl A.P.J. Stampfl 《Journal of Physics and Chemistry of Solids》2014
A combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO2 about the X-point. Compared to pure cubic ZrO2, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO2 at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO2, thus providing support for the structural model considered. 相似文献
66.
Dr. Julie Jung M. Ashraful Islam Dr. Vincent L. Pecoraro Dr. Talal Mallah Dr. Claude Berthon Dr. Hélène Bolvin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(66):15112-15122
Two series of lanthanide complexes have been chosen to analyze trends in the magnetic properties and crystal field parameters (CFPs) along the two series: The highly symmetric LnZn16(picHA)16 series (Ln=Tb, Dy, Ho, Er, Yb; picHA=picolinohydroxamic acid) and the [Ln(dpa)3](C3H5N2)3 ⋅ 3H2O series (Ln=Ce–Yb; dpa=2,6-dipicolinic acid) with approximate three-fold symmetry. The first series presents a compressed coordination sphere of eight oxygen atoms whereas in the second series, the coordination sphere consists of an elongated coordination sphere formed of six oxygen atoms. The CFPs have been deduced from ab initio calculations using two methods: The AILFT (ab initio ligand field theory) method, in which the parameters are determined at the orbital level, and the ITO (irreducible tensor operator) decomposition, in which the problems are treated at the many-electron level. It has been found that the CFPs are transferable from one derivative to another, within a given series, as a first approximation. The sign of the second-order parameter differs in the two series, reflecting the different environments. It has been found that the use of the strength parameter S allows for an easy comparison between complexes. Furthermore, in both series, the parameters have been found to decrease in magnitude along the series, and this decrease is attributed to covalent effects. 相似文献
67.
Réka Mokrai Jamie Barrett Dr. David C. Apperley Dr. Andrei S. Batsanov Dr. Zoltán Benkő Dr. Dominikus Heift 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(16):4017-4024
To study pnictogen bonding involving bismuth, flexible accordion-like molecular complexes of the composition [P(C6H4-o-CH2SCH3)3BiX3], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single-crystal X-ray diffraction analyses, DFT and ab initio computations. Significantly, 209Bi–31P through-space coupling (J=2560 Hz) is observed in solid-state 31P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction. 相似文献
68.
A. D. Dinga Wonanke Jayne L. Ferguson Christopher M. Fitchett Deborah L. Crittenden 《化学:亚洲杂志》2019,14(8):1293-1303
Photochemical oxidative cyclodehydrogenation reactions are a versatile class of aromatic ring‐forming reactions. They are tolerant to functional group substitution and heteroatom inclusion, so can be used to form a diverse range of extended polyaromatic systems by fusing existing ring substituents. However, despite their undoubted synthetic utility, there are no existing models—computational or heuristic—that predict the outcome of photocyclisation reactions across all possible classes of reactants. This can be traced back to the fact that “negative” results are rarely published in the synthetic literature and the lack of a general conceptual framework for understanding how photoexcitation affects reactivity. In this work, we address both of these issues. We present experimental data for a series of aromatically substituted pyrroles and indoles, and show that quantifying induced atomic forces upon photoexcitation provides a powerful predictive model for determining whether a given reactant will photoplanarise and hence proceed to photocyclised product under appropriate reaction conditions. The propensity of a molecule to photoplanarise is related to localised changes in charge distribution around the putative forming ring upon photoexcitation. This is promoted by asymmetry in molecular structures and/or charge distributions, inclusion of heteroatoms and ethylene bridging and well‐separated or isolated photocyclisation sites. 相似文献
69.
《Comptes Rendus Physique》2014,15(1):70-81
We analyse the transport properties in approximants of quasicrystals α-AlMnSi, 1/1-AlCuFe and for the complex metallic phase λ-AlMn. These phases present strong analogies in their local atomic structures and are related to existing quasicrystalline phases. Experimentally, they present unusual transport properties with low conductivities and a mix of metallic-like and insulating-like characteristics. We compute the band structure and the quantum diffusion in the perfect structure without disorder and introduce simple approximations that allow us to treat the effect of disorder. Our results demonstrate that the standard Bloch–Boltzmann theory is not applicable to these intermetallic phases. Indeed their dispersion relations are flat, indicating small band velocities, and corrections to quantum diffusion, which are not taken into account in the semi-classical Bloch–Boltzmann scheme, become dominant. We call this regime the small velocity regime. A simple relaxation time approximation to treat the effect of disorder allows us to reproduce the main experimental facts on conductivity qualitatively and even quantitatively. 相似文献
70.
Murat Durandurdu 《哲学杂志》2018,98(8):633-645
We generate an amorphous MgCu model using the rapid solidification of the melt through a first-principles molecular dynamics approach within a generalised gradient approximation and reveal, for the first time, its structural features and mechanical properties in details. The liquid and glassy MgCu are found to acquire slightly distinct local structures. Yet in both forms of MgCu, most Cu atoms have a tendency to form the ideal and defective icosahedrons while Mg atoms are arranged in complex configurations. The mean coordination number of Cu and Mg at 300 K is 11.31 and 13.73, respectively. The short-range order of MgCu glass is projected to be different than the known crystalline MgCu and Mg2Cu phases. The mechanical properties of MgCu glass and the CsCl-type MgCu crystal are computed and compared. On the basis of the enthalpy analyses, a possible pressure-induced crystallisation of the MgCu glass into a CsCl-type structure is proposed to occur at around 11 GPa. 相似文献