全文获取类型
收费全文 | 251篇 |
免费 | 6篇 |
专业分类
化学 | 198篇 |
力学 | 5篇 |
数学 | 26篇 |
物理学 | 28篇 |
出版年
2023年 | 2篇 |
2022年 | 1篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 7篇 |
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 14篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 10篇 |
2009年 | 10篇 |
2008年 | 15篇 |
2007年 | 13篇 |
2006年 | 20篇 |
2005年 | 14篇 |
2004年 | 13篇 |
2003年 | 4篇 |
2002年 | 10篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1981年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有257条查询结果,搜索用时 296 毫秒
91.
Hernán Pessoa‐Mahana R. Acevedo Ramiro Araya‐Maturana Claudio Saitz C. David Pessoa‐Mahana 《合成通讯》2013,43(20):3559-3567
New benzothiophene arylpiperazine derivatives 8 (a–f) were synthesized as potential serotoninergic agents with 5‐HT1A receptor affinity. Preparation of the derivatives was performed by treating N‐[2‐(chloromethyl)phenyl]‐4,7‐dimethoxybenzo[b]thiophene‐2‐carboxamide (7) with a series of substituted 4‐arylpiperazines. 相似文献
92.
R. I. Mattos C. E. Tambelli E. Raphael I. D. A. Silva C. J. Magon J. P. Donoso A. Pawlicka 《Cellulose (London, England)》2014,21(4):2247-2259
Polysaccharide- and gelatin-based bioblends and polyblends were synthesized and characterized by complex impedance spectroscopy, proton nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). Higher ionic conductivities of 7.9 × 10?5 S/cm at room temperature and 2.5 × 10?3 S/cm at 80 °C were obtained for the agar-chitosan polyblends. For all samples, the activation energies, calculated from the Arrhenius plot of ionic conductivity and from the onset of NMR line narrowing, are in the range 0.30–0.86 and 0.38–0.57 eV, respectively. The glass transition temperatures (T g NMR ) varied from 200 to 215 K, depending on the sample composition. The temperature dependence of the 1H spin–lattice relaxation revealed two distinct proton dynamics. The EPR spectra are characteristic of Cu2 ions in tetragonally distorted octahedral sites. Quantitative analysis of the EPR spin Hamiltonian g || and A || parameters revealed copper ions complexed by nitrogens and oxygens in the samples containing chitosan or gelatin and only by oxygens in agar-based ones. The in-plane π bonding is less covalent for the gelatin and chitosan blends. Results suggest that natural bioblends and polyblends are interesting systems to be used in materials science engineering. 相似文献
93.
This paper studies a dynamic production system where multiple products must visit stations where inventories are constrained
by maximum and minimum sojourn times with neither negative flow nor backlog being allowed. A resource availability constraint
limits the aggregate throughput of the stations. The objective is to minimize the sum of flow and inventory cost. The problem
is broken down into several single-product serial systems that serve as subroutines of a Lagrangian relaxation routine. This
model is implemented in a spreadsheet so that it can be used by the officials of a Chilean institution for planning the operations
and defining the optimal allocation of resources. 相似文献
94.
The integral method to numerically calculate the time evolution of kinetic systems is discussed and improved for one-dimensional problems. The new approach is applied to the solution of the spherically symmetric one-component plasma kinetic evolution. The results are compared with those obtained by means of the finite-difference solution to the equivalent Fokker-Planck kinetic equation. 相似文献
95.
Ramiro Araya-Maturana Jorge Heredia-Moya Hernán Pessoa-Mahana Boris Weiss-López Juan Carlos Muñoz 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(4):o237-o239
The title compound, C12H7NO3, consists of a chromone moiety substituted in position 3 with an acrylonitrile group in a Z configuration. The two planar groups are twisted with respect to one another. The only significant hydrogen bond in the structure is an intramolecular O—H·O bond. π–π contacts connecting aromatic groups and C—H·O intermolecular weak interactions lead to a supramolecular layer arrangement. 相似文献
96.
97.
The basidiomycete Hypholoma sublateritium produces the triterpenoid antitumor clavaric acid, an inhibitor of the human Ras-farnesyl transferase. The H. sublateritium squalene epoxidase gene (erg1) has been cloned and shown to encode a flavoprotein monooxygenase that requires FAD, NADPH, and P450 cofactors. Silencing of the erg1 gene in H. sublateritium using constructions expressed from the gdh promoter of Agaricus bisporus showed that the squalene epoxidase is involved in clavaric acid formation and in ergosterol biosynthesis; silenced expression of erg1 resulted in an ergosterol-dependent phenotype for full growth. Overexpression of erg1 gene resulted in up to 32% to 97% increment of clavaric acid production confirming its involvement in the biosynthesis of this antitumor product. Oxidosqualene (or dioxidosqualene) appears to be the branching point for primary metabolism (sterols) and secondary metabolites in basidiomycetes. 相似文献
98.
Carey DM Muñoz-Castro A Bustos CJ Manríquez JM Arratia-Pérez R 《The journal of physical chemistry. A》2007,111(28):6563-6567
The electronic structures of Lindqvist type functionalized polyoxometalates (POM) ([Mo6O18R]n- R=O, NO, NAr, NNAr, NNAr2; n=2, 3) have been investigated using density functional methods. We discuss the role of the replacement of terminal oxo ligands by pi-donor/acceptor multiple-bonded nitrogenous ligands on the basis of geometrical parameters, charge analyses, reactivity indexes, and vibrational spectra. The calculated reactivity indexes (chemical potential, electronegativity, hardness, and electrophilicity) indicate that the most reactive functionalized POMs are those substituted by pi-acceptor ligands. These pi-acceptor ligands induce a decrease in the hardness and an increase in the chemical potential and electrophilicity, thus increasing the reactivity. Our calculations are in reasonable agreement with reported experimental data. 相似文献
99.
Ximena Zarate Eduardo Schott Ramiro Arratia‐Pérez 《International journal of quantum chemistry》2011,111(15):4186-4196
Density functional theory and time dependent density functional theory calculations at the level of LDA/BP86/TZ2P were performed systematically on several Ti(IV) complexes of porphyrazines and one phthalocyanine. We performed an analysis of the frontier molecular orbitals of the ground state electronic structures and also discuss in particular the good concordance of our results with the experimental data, which affords to predict the geometrical and optical properties of new complexes ( 3 , 4 , and 7 ). We also emphasize the characterization of the UV–vis absorption spectra and propose transitions that contribute to the Q and B bands. Some useful calculated properties in complexes 2 , 3 , and 7 , like: high light absorption in the visible region of the spectra, transitions involved in these bands with a determined direction, charge separation, bigger highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO‐LUMO) gaps than complexes 4 and 5 , and the energy of their LUMO orbitals (that are higher than the lowest energy level of the conduction band of the TiO2) indicate that system complexes 2 , 3 , and 7 could act as light‐harvesting sensitizers for dye‐sensitized solar cells (DSCs). These proposals were made using a model of the previously experimentally known phthalocyanine, which was used as sensitizer in DSCs devices, comparing its electronic properties with the herein proposed sensitizers. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
100.
Ramírez-Tagle R Alvarado-Soto L Arratia-Perez R Bast R Alvarez-Thon L 《The Journal of chemical physics》2011,135(10):104506
In this study we report about the aromaticity of the prototypical [(H(t)Ac)(3)(μ(2)-H)(6)], [(H(t)Th)(3)(μ(2)-H)(6)](+), and [(H(t)Pa)(3)(μ(2)-H)(6)] clusters via two magnetic criteria: nucleus-independent chemical shifts (NICS) and the magnetically induced current density. All-electron density functional theory calculations were carried out using the two-component zeroth-order regular approach and the four-component Dirac-Coulomb Hamiltonian, including scalar and spin-orbit relativistic effects. Four-component current density maps and the integration of induced ring-current susceptibilities clearly show that the clusters [(H(t)Ac)(3)(μ(2)-H)(6)] and [(H(t)Th)(3)(μ(2)-H)(6)](+) are non-aromatic whereas [(H(t)Pa)(3)(μ(2)-H)(6)] is anti-aromatic. However, for the thorium cluster we find a discrepancy between the current density plots and the classification through the NICS index. Our results also demonstrate the increasing influence of f orbitals, on bonding and magnetic properties, with increasing atomic number in these clusters. We think that the enhanced electron mobility in [(H(t)Pa)(3)(μ(2)-H)(6)] is due the significant 5f character of its valence shell. Also the participation of f orbitals in bonding is the reason why the protactinium cluster has the shortest bond lengths of the three clusters. This study provides another example showing that the magnetically induced current density approach can give more reliable results than the NICS index. 相似文献