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排序方式: 共有125条查询结果,搜索用时 15 毫秒
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D'Souza F Chitta R Gadde S Zandler ME McCarty AL Sandanayaka AS Araki Y Ito O 《The journal of physical chemistry. A》2006,110(13):4338-4347
Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads. 相似文献
96.
Kristina L. Stensaas Brent V. McCarty Natacha M. Touchette James B. Brock 《Tetrahedron》2006,62(46):10683-10687
The singlet oxygen (1Δg) photooxidations of 2-methyl-3-phenylthio-2-butene (1a), 1-[(4-nitrophenyl)thio]-2,3-dimethyl-2-butene (2c), 2-methyl-3-phenylsulfinyl-2-butene (3), 2-methyl-3-phenylsulfonyl-2-butene (6), and 1-[(4-nitrophenyl)sulfonyl]-2,3-dimethyl-2-butene (7c) were conducted in the following deuterated solvents: acetonitrile, benzene, chloroform, methanol, or methanol/water mixture. In each case the ene allylic hydroperoxide products and/or the [2+2] cycloaddition products were quantified and inspected for possible hydrogen bonding induced differences in product selectivity and regiochemistry. After comparison to literature values for related substrates, the results indicate that only photooxidations of vinyl sulfides are susceptible to hydrogen bonding solvent effects. 相似文献
97.
Hill JP Hewitt IJ Anson CE Powell AK McCarty AL Karr PA Zandler ME D'Souza F 《The Journal of organic chemistry》2004,69(18):5861-5869
The structures and electrochemistry of N-benzylated meso-tetrakis (3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogens have been investigated. Structural determinations reveal the isomeric identity of the products obtained from the N-alkylation of the parent meso-tetra (oxo-cyclohexadienylidene) porphyrinogen. The compounds are subject to increased macrocyclic deformations upon increasing N-substitution culminating in the tetra-N-benzyl derivative, which has a buckling superimposed on the already highly puckered macrocycle. The electrochemical analyses emphasize the electron deficiency of the N-benzylated meso-tetra(oxo-cyclohexadienylidene) porphyrinogens and indicate that they can be considered as quinones conjugated via the unsaturated tetrapyrrolic macrocycle. The N-benzylated compounds studied form stable and well-defined pi-cation radical and pi-anion radical species because of their highly conjugated nature. Ab initio molecular orbital calculations at the B3LYP/3-21G() level confirmed the high degree of conjugation between tetrapyrrole and meso substituents and also gave good agreement between calculated and experimentally determined HOMO-LUMO band gap energies. 相似文献
98.
The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N(c) limit and found to be comparable to that of octet baryons. Large N(c) selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained. 相似文献
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符合Ore条件的简单图称为Ore图,Ore条件指图的两个不相邻的顶点的度数之和不小于图的顶点数。本文搞清了Ore图的泛连通性,同时得到了Ore图的点(边)泛圈性的结果,推广了Bondy的一个结论。 相似文献