全文获取类型
收费全文 | 2384篇 |
免费 | 72篇 |
国内免费 | 5篇 |
专业分类
化学 | 1473篇 |
晶体学 | 5篇 |
力学 | 77篇 |
数学 | 476篇 |
物理学 | 430篇 |
出版年
2023年 | 29篇 |
2022年 | 54篇 |
2021年 | 62篇 |
2020年 | 55篇 |
2019年 | 63篇 |
2018年 | 35篇 |
2017年 | 32篇 |
2016年 | 97篇 |
2015年 | 73篇 |
2014年 | 83篇 |
2013年 | 122篇 |
2012年 | 195篇 |
2011年 | 174篇 |
2010年 | 93篇 |
2009年 | 103篇 |
2008年 | 139篇 |
2007年 | 145篇 |
2006年 | 127篇 |
2005年 | 109篇 |
2004年 | 116篇 |
2003年 | 88篇 |
2002年 | 95篇 |
2001年 | 41篇 |
2000年 | 24篇 |
1999年 | 22篇 |
1998年 | 24篇 |
1997年 | 16篇 |
1996年 | 26篇 |
1995年 | 15篇 |
1994年 | 22篇 |
1993年 | 20篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1973年 | 3篇 |
1966年 | 4篇 |
1894年 | 3篇 |
1877年 | 4篇 |
排序方式: 共有2461条查询结果,搜索用时 15 毫秒
991.
992.
[reaction: see text]. An intramolecular cyclization strategy for effecting a biomimetic synthesis of the core structure of the fungal secondary metabolites phomoidrides A and B is described. The cyclization substrate 20 is prepared in eight steps from dibromide 10. Treatment of 20 with triethylamine in acetonitrile results in a rapid cyclization to give 21 and 22 in good yield. 相似文献
993.
Fabio E. Penotti David L. Cooper Peter B. Karadakov Robert Ponec 《International journal of quantum chemistry》2020,120(11):e26183
Motivated by the particularly short metal-metal distance that has been predicted for the D3h [BeH3Be]+ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D3h [MH3M]+ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding. 相似文献
994.
Emanuela FranchiGiovanni Ingrosso Fabio MarchettiCalogero Pinzino 《Tetrahedron》2003,59(27):5003-5018
A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies. 相似文献
995.
A thorough investigation of the adsorption and diffusion of Mg, O, and O(2) on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born-Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O(2) molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O(2) atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed. 相似文献
996.
Reaction of [2.2]paracyclophane with K/Na alloy in THF gives a p-xylylenyl dianion together with its dimer and trimer which are relatively stable at low temperatures; at ambient temperatures further polymerization takes place. 相似文献
997.
Marangoni R Marroni F Gioffré D Ghetti F Colombetti G 《Photochemistry and photobiology》2004,80(3):408-411
We determined the biological weighting function (BWF) of the effect of UV radiation on phototaxis of the freshwater, histophagous ciliate Ophryoglena flava. Dose-effect curves were measured by exposing the cells to 12 different irradiation regimens obtained with two different levels of UV-B radiation and by using six filters with cutoff wavelengths ranging from 280 to 335 nm. The results show that there are significant damages to phototaxis at the doses used and that the effect increases when the cutoff is shifted toward short wavelengths. The data were used to calculate the BWF of phototaxis impairment by applying a nonlinear fit procedure. The BWF thus obtained decays exponentially with increasing wavelength in agreement with similar findings reported in the literature for other systems. 相似文献
998.
Effendy Marchetti F Pettinari C Pettinari R Skelton BW White AH 《Inorganic chemistry》2003,42(1):112-117
1:1 AgX:tz(2)(CH(2)) (X = NO(3), NO(2), ClO(4)), 3:4 (X = O(3)SCF(3) (=OTf), O(2)CCF(3) (= tfa)), and 2:1 adducts (X = BrO(3)) have been synthesized and characterized in the solid state and in solution by analyses, spectral (IR, far-IR, (1)H and (13)C NMR, ESI MS) data, and conductivity measurements. The crystal structures of the 1:1 AgNO(3):tz(2)(CH(2)) and AgNO(2):tz(2)(CH(2)) adducts determined by X-ray studies show that tz(2)(CH(2)) coordinates to silver through the exodentate nitrogen atoms at the 4-positions of the triazole rings, yielding neutral polymers, while the ionic Ag(OTf):tz(2)(CH(2)) (3:4) adduct has a three-dimensional polymeric cation. The NMR and ESI MS data suggest that tz(2)(CH(2)) is only weakly coordinating, adducts between Ag(I) and CH(3)CN being more prevalent in acetonitrile solution. 相似文献
999.
The optimized spatial structures of the small clusters (with N up to 33) formed by an increasing number of (4)He atoms, which act as a microsolvent surrounding the OH(+) ionic molecular dopant, are obtained using a sum-of-potentials scheme corrected by three-body (3B) effects. The most stable structures are generated using the type of genetic algorithm described herein, and the sequential formation of regular shell structures is analyzed in detail. Possible quantum corrections for both the solvent distributions and the stable energetics are analyzed and discussed. 相似文献
1000.
Bikiel DE Di Salvo F González Lebrero MC Doctorovich F Estrin DA 《Inorganic chemistry》2005,44(15):5286-5292
The nature of the solute species present in ethereal solutions of LiAlH(4) is of crucial importance for understanding the mechanisms for the reduction of ketones and other functional groups by LiAlH(4). We have employed a combination of theoretical and experimental techniques to investigate the structure of LiAlH(4) in ethereal solutions. Using complexation agents, we measured the IR spectra of LiAlH(4) and AlH(4)(-) in tetrahydrofuran (THF). Hybrid quantum-classical (QM-MM) simulations have also been carried out to compute the IR spectra of associated and dissociated LiAlH(4) species and the free-energy profile for the dissociation process in solution. Our experimental estimate of the dissociation constant in THF is 0.021 +/- 0.002, while the predicted computational value corresponding to a model dimethyl ether solvent is 0.001. The free-energy profile shows only one minimum corresponding to a contact ion pair at a Li-Al separation distance of 3.0 A. These results are consistent with the fact that LiAlH(4) is essentially associated in ethereal solutions forming contact ion pairs. 相似文献