全文获取类型
收费全文 | 679篇 |
免费 | 25篇 |
国内免费 | 3篇 |
专业分类
化学 | 495篇 |
晶体学 | 3篇 |
力学 | 18篇 |
数学 | 61篇 |
物理学 | 130篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 11篇 |
2015年 | 13篇 |
2014年 | 16篇 |
2013年 | 35篇 |
2012年 | 45篇 |
2011年 | 67篇 |
2010年 | 33篇 |
2009年 | 37篇 |
2008年 | 59篇 |
2007年 | 50篇 |
2006年 | 52篇 |
2005年 | 28篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 28篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1998年 | 12篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1985年 | 11篇 |
1984年 | 4篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1968年 | 3篇 |
1940年 | 1篇 |
排序方式: 共有707条查询结果,搜索用时 15 毫秒
71.
We have applied crystal structure prediction methods to understand and predict the formation of a DMSO solvate of the anti-convulsant drug carbamazepine (CBZ), in which the DMSO molecules are disordered. Crystal structure prediction calculations on the 1:1 CBZ:DMSO solvate revealed the generation of two similar low energy structures which differ only in the orientation of the DMSO molecules. Analysis of crystal energy landscapes generated at 0 K suggests the possibility of solvent disorder. A combined computational and experimental study of the changes in the orientation of the DMSO within the crystal structure revealed that the nature of the disorder changes with temperature. At low temperature, the DMSO disorder is static whilst at high temperature the DMSO configurations can interconvert by a 180° rotation of the DMSO molecules within the lattice. This 180° rotation of the DMSO molecules drives a phase change from a high temperature dynamically disordered phase to a low temperature phase with static disorder. Crystallisation of a DMSO solvate of the related molecule epoxycarbamazepine resulted in a different degree of DMSO disorder in the crystal structure, despite the similarity of the carbamazepine and epoxycarbamazepine molecules. We believe consideration of disorder and its contribution to entropy and crystal free energies at temperature other than 0 K is fundamental for the accuracy of future energy rankings in crystal structure prediction calculations of similar solvated structures. 相似文献
72.
H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N-H and O-H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound (1)L(b) levels, (λ(phot) ≤ 284.893 nm) O-H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the (1)L(b)/(1)πσ*((OH)) conical intersection (CI). No evidence was found for a significant N-H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ(phot) ≥ 193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N-H bond fission is deduced to occur at λ(phot) ≤ 271.104 nm, by direct excitation to the (1)πσ*((NH)) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the (1)πσ*((NH)) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O-H bond dissociation when excited on (1)L(b) levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the (1)L(b)/(1)πσ* CI diabats in 5-HI is ~60% greater than the corresponding area in 4-HI and O-H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N-H and/or O-H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O-H bond fission in 5-HI at λ(phot) > 235 nm and that N-H bond fission is the dominant source of H atoms in the wavelength region 255 > λ(phot) > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the -OH substituent, and its effect on the overall electronic structure. 相似文献
73.
We cloak a region from a known incident wave by surrounding the region with three or more devices that cancel out the field in the cloaked region without significantly radiating waves. Since very little waves reach scatterers within the cloaked region, the scattered field is small and the scatterers are for all practical purposes undetectable. The devices are multipolar point sources that can be determined from Green's formula and an addition theorem for Hankel functions. The cloaking devices are exterior to the cloaked region. 相似文献
74.
G. Fäldt U. Tengblad C. Wilkin 《The European Physical Journal A - Hadrons and Nuclei》2006,28(2):245-249
The cross-section for the dd →^4Heππ reaction is estimated near threshold in a two-step model where a pion created in a first interaction produces a second
pion in a subsequent interaction. This approach, which describes well the rates of 2π and η production in the pd →^3Heππ and dd →^4Heη reactions, leads to predictions that are much too low compared to experiment. Alternatives to this and the double-Δ
model will have to be sought to explain these data. 相似文献
75.
Graeme J. Gainsford Cees Lensink Andrew F. Falshaw 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(7):m331-m334
The title compound, [Rh(C8H12)(C34H38O6P2)]BF4·CHCl3, a novel asymmetric hydrogenation catalyst, crystallizes with two independent almost identical cations and anions. Cell‐packing interactions are provided by nonclassical hydrogen bonding between phenyl and chloroform H atoms and fluoro and chloro donors of the BF4− anion and the chloroform solvent molecule. 相似文献
76.
Nathan Hollingsworth Graeme A. Horley Muhammed Mazhar Mary F. Mahon Kieran C. Molloy Peter W. Haycock Christopher P. Myers Gary W. Critchlow 《应用有机金属化学》2006,20(10):687-695
Tin(II) methoxide reacts with N,N′‐dimethylaminoethanol (dmaeH) to yield Sn(dmae)2 ( 1 ) along with small amounts of the hydrolysis product Sn6(O)4(dmae)4 ( 2 ). The geometrically more regular iso‐structural cage Sn6(O)4(OEt)4 ( 3 ) was obtained as the only tractable product isolated from reaction of 2 and Sb(OEt)3, while 1 reacted with CdX2 (X = acac, I) to afford Sn(dmae)2Cd(acac)2 ( 4 ) and Sn(dmae)2CdI2 ( 5 ). The X‐ray structures of 2, 3 and 4 are reported. Decomposition of 4 under aerosol‐assisted chemical vapour deposition conditions leads to amorphous tin oxide films with no detectable cadmium (i.e. ca < 2% cadmium), rather than a stoichiometric Sn:Cd oxide. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
77.
Graeme Cooke Lee M. Daniels James F. Garety Andrew Parkin Vincent M. Rotello Patrice Woisel 《Tetrahedron》2007,63(45):11114-11121
We report the convenient synthesis of a pyrrole-functionalized tetracationic cyclophane, [2]rotaxane, and [2]catenane. X-ray crystallography has confirmed the interlocked structure of the catenane. We have investigated the solution properties of these systems using solution electrochemistry, NMR, and UV-vis spectroscopy. We have also demonstrated that it is possible to immobilize these systems onto a platinum working electrode surface. We have shown that films of the cyclophane have the ability to undergo complexation with a dialkyloxynaphthalene derivative. 相似文献
78.
Graeme Fairweather Heping Ma Weiwei Sun 《Numerical Methods for Partial Differential Equations》2008,24(2):449-464
An orthogonal spline collocation (OSC) spatial discretization is proposed for the solution of the fully coupled stream function‐vorticity formulation of the Navier–Stokes equations in two dimensions. For the time‐stepping, a three‐level leapfrog scheme is employed. This method is algebraically linear, and, at each time step, gives rise to a system of linear equations of the form arising in the OSC approximation of the biharmonic Dirichlet problem and can be solved by a fast direct method. Error estimates in the Hl–norm in space, l = 1,2, are derived for the semi‐discrete method and the fully‐discrete leapfrog scheme which is also shown to be second order accurate in time. Numerical results are presented which confirm the theoretical analysis and exhibit superconvergence phenomena, which provide superconvergent approximations to the components of the velocity. © John Wiley & Sons, Inc. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2008 相似文献
79.
80.
We discuss the quantisation of one-dimensional MIT bags by expanding the fields as a sum of classical modes and truncating the series after the first term. We obtain the lowest states of a bag in a world containing two scalar quark fields. Problems associated with the zero-point oscillations of the field are discussed. 相似文献