全文获取类型
收费全文 | 999篇 |
免费 | 18篇 |
国内免费 | 6篇 |
专业分类
化学 | 670篇 |
晶体学 | 35篇 |
力学 | 24篇 |
数学 | 185篇 |
物理学 | 109篇 |
出版年
2023年 | 4篇 |
2022年 | 14篇 |
2021年 | 18篇 |
2020年 | 13篇 |
2019年 | 17篇 |
2018年 | 10篇 |
2017年 | 20篇 |
2016年 | 23篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 55篇 |
2012年 | 43篇 |
2011年 | 62篇 |
2010年 | 41篇 |
2009年 | 41篇 |
2008年 | 52篇 |
2007年 | 56篇 |
2006年 | 57篇 |
2005年 | 51篇 |
2004年 | 55篇 |
2003年 | 43篇 |
2002年 | 41篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 11篇 |
1996年 | 19篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1977年 | 3篇 |
1976年 | 6篇 |
1918年 | 2篇 |
1914年 | 2篇 |
1913年 | 2篇 |
1907年 | 2篇 |
排序方式: 共有1023条查询结果,搜索用时 15 毫秒
111.
112.
Matczak-Jon E Videnova-Adrabińska V Burzyńska A Kafarski P Lis T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(8):2357-2372
The crystal structures of 3-methylpiperid-1-ylmethane-1,1-diphosphonic (2), 4-methylpiperid-1-ylmethane-1,1-diphosphonic (3), 2-ethylpiperid-1-ylmethane-1,1-diphosphonic (4), and 2-methylpiperid-1-ylmethane-1,1-diphosphonic (5) acids have been determined and are discussed with respect to their molecular organization and crystal-packing preferences. The chair conformation, predominant also in solution, favors equatorial positioning of the bulky substituents of the heterocyclic N and C atoms. The molecular geometry also provides access to intramolecular hydrogen-bond formation between the axial protons located on the nitrogen atoms, as well as the carbon atoms closest to it, and phosphonic/phosphonate oxygen atoms. The molecules preferably arrange in monolayers, observed in all crystals with an exception of 3. The layers are held in place in the third direction through van der Waals interactions. The analysis of two-dimensional hydrogen-bonded networks is concentrated on revealing how the substituent's topology of the molecule affects the solid-state organization in well-defined structures and is aimed at unraveling the consequences and the possible conformational changes by stepwise network disruption upon crystal dissolution in water. The solution NMR studies are focused on revealing the role that the topochemistry of the substituent plays for the stereodynamics in 2-5. It is demonstrated that in contrast to piperid-1-ylmethane-1,1-diphosphonic acid (1), in which the ring inversion/rotation around the C-N bond concerted with the N-H...O hydrogen-bond breaking/formation process leads to a mixture of two interconverting conformers, the concerted N-H...O breaking/rotation/N-H...O formation process in 2 and 3 allows for a predominance of one conformer in solution. However, placement of a substituent at 2-position in the ring hampers the rotation around the C-N bond; this makes 4 and 5 significantly less flexible relative to compounds 1-3. In addition, both compounds 4 and 5 are proved to exist as a mixture of two conformers, the equilibrium of which in acidic solution is shifted towards the conformer found in solid state. In alkaline solutions of 4 and 5, the equilibrium is shifted towards the conformer that is forced by the flipping of the heterocyclic ring. These results correlate well with recently documented differences in the biological potency of this group of compounds. 相似文献
113.
Tadeusz Bałaban 《Communications in Mathematical Physics》1982,86(4):555-594
This is the second part of the paper entitled, “(Higgs)2,3 Quantum Fields in a Finite Volume.” The proof of an upper bound for vacuum energy is completed with the exception of some technical estimates. 相似文献
114.
115.
Wojciech P. Oziminski Tadeusz M. Krygowski 《Journal of Physical Organic Chemistry》2010,23(6):551-556
The geometries, natural charges, and resonance structures of 11 monosubstituted benzene derivatives were analyzed at the B3LYP/6‐311++G(d,p) and HF/6‐311++G(d, p) levels of theory. The following angular substituents were chosen: OCH3, CH2CH3, OH, SH, NHCH3, NHNH2, N?O, CH?CH2, N?CH2, N?NH, and CHO. The analysis of resonance structures was performed by using two different methodologies: harmonic oscillator stabilization energies (HOSE) and natural resonance theory (NRT). Also, the natural bond orbital (NBO) donor–acceptor stabilization energies for different resonance structures were calculated. We found that for all the substituents, the purely geometric resonance stabilization parameter (HOSE) is linearly correlated with quantum chemically derived resonance structure weight (NRT) of a given structure. Also, the calculations provide qualitative support for the earlier assumption of a through space angular group induced bond alternation (AGIBA) effect. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
116.
Shvartsburg AA Bryskiewicz T Purves RW Tang K Guevremont R Smith RD 《The journal of physical chemistry. B》2006,110(43):21966-21980
Approaches to separation and characterization of ions based on their mobilities in gases date back to the 1960s. Conventional ion mobility spectrometry (IMS) measures the absolute mobility, and field asymmetric waveform IMS (FAIMS) exploits the difference between mobilities at high and low electric fields. However, in all previous IMS and FAIMS experiments ions experienced an essentially free rotation; thus the separation was based on the orientationally averaged cross-sections Omega(avg) between ions and buffer gas molecules. Virtually all large ions are permanent electric dipoles that will be oriented by a sufficiently strong electric field. Under typical FAIMS conditions this will occur for dipole moments >400 D, found for many macroions including most proteins above approximately 30 kDa. Mobilities of aligned dipoles depend on directional cross-sections Omega(dir) (rather than Omega(avg)), which should have a major effect on FAIMS separation parameters. Here we report the FAIMS behavior of electrospray-ionization-generated ions for 10 proteins up to approximately 70 kDa. Those above 29 kDa exhibit a strong increase of mobility at high field, which is consistent with predicted ion dipole alignment. This effect expands the useful FAIMS separation power by an order of magnitude, allowing separation of up to approximately 10(2) distinct protein conformers and potentially revealing information about Omega(dir) and ion dipole moment that is of utility for structural characterization. Possible approaches to extending dipole alignment to smaller ions are discussed. 相似文献
117.
We analyze a model of two-leg Hubbard ladders weakly coupled by interladder tunneling. At half filling a semimetallic state with small Fermi pockets is induced beyond a threshold tunneling strength. The sign changes in the single electron Green's function relevant for the Luttinger sum rule now take place at surfaces with both zeros and infinities with important consequences for the interpretation of angle-resolved photoemission spectroscopy experiments. Residual interactions between electron and holelike quasiparticles cause a transition to long range order at low temperatures. The theory can be extended to small doping leading to superconducting order. 相似文献
118.
Lisiecki R Solarz P Dominiak-Dzik G Ryba-Romanowski W Łukasiewicz T 《Optics letters》2010,35(23):3940-3942
Optical spectra and luminescence decay curves were measured for thulium-doped YVO(4), GdVO(4), and LuVO(4) single crystals as a function of temperature in the 300-670 K temperature region. In spite of structural similarity, the three systems studied display significantly different transition intensities and nonradiative relaxation rates. It was found, in particular, that the peak value of the pump band absorption intensity diminishes by about 30% for Tm:YVO(4) and Tm:GdVO(4), and the effective emission cross section for the laser transition of Tm(3+) diminishes by a factor of 2 roughly when temperature increases from 300 to 500 K. An unusually small quantum efficiency of the upper laser level in Tm:LuVO(4) has been derived from the analysis of luminescence decay curves. 相似文献
119.
We consider the critical behavior at the threshold of black-hole formation for the five-dimensional vacuum Einstein equations satisfying the cohomogeneity-two triaxial Bianchi type-IX ansatz. Exploiting a discrete symmetry present in this model we predict the existence of a codimension-two attractor. This prediction is confirmed numerically and the codimension-two attractor is identified as a discretely self-similar solution with two unstable modes. 相似文献
120.
Miroslaw Zimnoch Dorota Jelen Michal Galkowski Tadeusz Kuc Jaroslaw Necki Lukasz Chmura 《Isotopes in environmental and health studies》2013,49(3):421-433
Regular measurements of atmospheric CO 2 mixing ratios and their carbon isotope composition (13C/12C and 14C/12C ratios) performed between 2005 and 2009 at two sites of contrasting characteristics (Krakow and the remote mountain site Kasprowy Wierch) located in southern Poland were used to derive fossil fuel-related and biogenic contributions to the total CO 2 load measured at both sites. Carbon dioxide present in the atmosphere, not coming from fossil fuel and biogenic sources, was considered ‘background’ CO 2. In Krakow, the average contribution of fossil fuel CO 2 was approximately 3.4%. The biogenic component was of the same magnitude. Both components revealed a distinct seasonality, with the fossil fuel component reaching maximum values during winter months and the biogenic component shifted in phase by approximately 6 months. The partitioning of the local CO 2 budget for the Kasprowy Wierch site revealed large differences in the derived components: the fossil fuel component was approximately five times lower than that derived for Krakow, whereas the biogenic component was negative in summer, pointing to the importance of photosynthetic sink associated with extensive forests in the neighbourhood of the station. While the presented study has demonstrated the strength of combined measurements of CO 2 mixing ratios and their carbon isotope signature as efficient tools for elucidating the partitioning of local atmospheric CO 2 loads, it also showed the important role of the land cover and the presence of the soil in the footprint of the measurement location, which control the net biogenic surface CO 2 fluxes. 相似文献