全文获取类型
收费全文 | 442篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
化学 | 293篇 |
晶体学 | 3篇 |
力学 | 7篇 |
数学 | 56篇 |
物理学 | 96篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 10篇 |
2015年 | 8篇 |
2014年 | 11篇 |
2013年 | 27篇 |
2012年 | 14篇 |
2011年 | 21篇 |
2010年 | 17篇 |
2009年 | 18篇 |
2008年 | 19篇 |
2007年 | 19篇 |
2006年 | 13篇 |
2005年 | 14篇 |
2004年 | 16篇 |
2003年 | 8篇 |
2002年 | 9篇 |
2001年 | 15篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 4篇 |
1995年 | 11篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有455条查询结果,搜索用时 15 毫秒
431.
Abraham F. Jalbout Flavio F. Contreras‐Torres L. Adamowicz 《International journal of quantum chemistry》2008,108(2):279-288
In this work we explore the torsional potentials for hydroxyacetone using second order Møller‐Plesset (MP2) and coupled‐cluster (CCSD(T)) ab initio theoretical methods. We have calculated these potential energy surfaces as a function of single OH rotation, and simultaneous OH and methyl rotation. The results indicate that the low energy isomers coupled to the reduced barrier to methyl rotation can lead to several rotamers for this species to form. The values are compared to experimental quantities and implications for astrobiological research are also addressed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
432.
433.
M. Vivas J. Contreras F. López-Carrasquero A.T. Lorenzo M.L. Arnal V. Balsamo A.J. Müller E. Laredo H. Schmalz V. Abetz 《Macromolecular Symposia》2006,239(1):58-67
A first attempt was made to produce novel ABC triblock terpolymers with three potentially crystallisable blocks: polyethylene (PE), poly(ethylene oxide) (PEO), and poly(ε-caprolactone) (PCL). Polybutadiene-b-poly(ethylene oxide) diblock copolymers were synthesized by living anionic polymerization. Then, a non-catalyzed thermal polymerization of ε-caprolactone from the hydroxyl end group of the PB-b-PEO diblock precursors was performed. Finally, hydrogenation by Wilkinson catalyst produced PE-b-PEO-b-PCL triblock terpolymers. Side reactions were detected that lead to the formation of undesired PCL-b-PEO diblock copolymers, however, these impurities were successfully removed by purification. A range of triblock terpolymers with PCL and PEO minor components were prepared. Topological restrictions on the PEO middle block prevented this block from crystallizing while the complex crystallization behavior of the PE and PCL blocks was documented by DSC and WAXS measurements. 相似文献
434.
Pérez P Andrés J Safont VS Contreras R Tapia O 《The journal of physical chemistry. A》2005,109(18):4178-4184
Spin forbidden 1,2-cycloadditions of triplet methylene to alkenes have been theoretically studied as an example of the two-state reactivity paradigm in organic chemistry. The cycloadditions of triplet methylene to ethylene and the (E)- and (Z)-2-butene isomers show spin inversion after the transition state and therefore with no effect on the reaction rate. A local analysis shows that while triplet methylene addition to alkenes leading to the formation of a biradical intermediate is driven by spin polarization, the ring closure step to yield cyclopropane is a pericyclic process. We have found that at the regions in the potential energy surface where the spin crossover is likely to occur, the spin potential in the direction of increasing spin multiplicity, mu(+)(s), tends to equalize the one in the direction of decreasing spin multiplicity, mu(-)(s). This equalization facilitates the spin transfer process driven by changes in the spin density of the system. 相似文献
435.
Trino Suárez Barnardo Fontal Marisela Reyes Fernando Bellandi Ricardo R. Contreras José M. Ortega Gustavo León Pedro Cancines Belsymar Castillo 《Reaction Kinetics and Catalysis Letters》2004,82(2):325-331
RuCl2(TPPMS)3(DMSO) (complex (I)) is very soluble in the ionic liquid 1-nbutyl-3-methylimidazolium hexafluorophosphate (BMI.PF6). Complex (I) catalyzes 1-hexene hydrogenation (500 psi H2 and 100oC) in a two-phase system, with 80% conversion in 24 h, with little
substrate isomerization. Changes in catalytic behavior with temperature, pressure, time and substrate/catalyst relation are
presented. Complex (I) shows good stability and can be reused several times with little activity loss.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
436.
437.
A statistical procedure to obtain the intrinsic electronic contributions to the Hammett substituent constant sigma(p) is reported. The method is based on the comparison between the experimental sigma(p) values and the electronic electrophilicity index omega evaluated for a series of 42 functional groups commonly present in organic compounds. 相似文献
438.
New and useful aspects of chemical reactivity as described by reactivity indexes and used in connection with the maximum hardness and minimum polarizability principles (MHP and MPP, respectively) are discussed and illustrated for two classical reactions in organic chemistry. They include the Beckmann rearrangement and the condensation reactions of -amino acids. The MPP appears as a more general rule than the MHP. Another relevant result is related to the usefulness of both empirical reactivity rules to predict the most probable reaction mechanism among two different pathways displaying very close values in activation energy (competitive pathways). This is illustrated for the condensation reaction of a series of -amino acids: while the accepted stepwise route follows both the MHP and MPP rules, the alternative concerted channel does not, yet its associated activation energy is slightly lower than that corresponding to the nonconcerted reaction mechanism.From the Proceedings of the 28th Congresco de Quimicos Teóricos de Expresión Latina (QUITEL 2002) 相似文献
439.
Cadmium hexacyanometallates (II) crystallize in the same monoclinic cell (space group C5
2h–P21/n) reported for the analogs of manganous complexes. In that monoclinic framework, two outer cations (M = Mn2+ or Cd2+) of neighboring structural units M2[LII(CN)6], (L = FeII, RuII, OsII), are linked by two common water molecules. On heating above 100°C, these water bridges are removed. In dehydrated complexes, the outer cation (M) remains linked to only three CN ligands. The loss of aquo-ligands induces a remarkable -back donation from the inner cation (L) to CN ligands, which was studied using IR, Raman, and Mössbauer spectroscopies. 相似文献
440.
Gonzalo Contreras Rafael Oswaldo Ruggiero 《Bulletin of the Brazilian Mathematical Society》1997,28(1):43-71
We construct examples ofC
3 compact surfaces of non-positive curvature having non-Anosov geodesic flows and satisfying the following property: there existsL>0 such that the area of every ideal triangle in the universal covering of the surface is bounded above byL.Partially supported by CNPq of Brazilian Government 相似文献