全文获取类型
收费全文 | 2774篇 |
免费 | 97篇 |
国内免费 | 8篇 |
专业分类
化学 | 1899篇 |
晶体学 | 25篇 |
力学 | 52篇 |
数学 | 114篇 |
物理学 | 452篇 |
无线电 | 337篇 |
出版年
2022年 | 19篇 |
2021年 | 24篇 |
2020年 | 35篇 |
2019年 | 36篇 |
2018年 | 33篇 |
2017年 | 19篇 |
2016年 | 49篇 |
2015年 | 48篇 |
2014年 | 53篇 |
2013年 | 103篇 |
2012年 | 119篇 |
2011年 | 122篇 |
2010年 | 82篇 |
2009年 | 89篇 |
2008年 | 143篇 |
2007年 | 105篇 |
2006年 | 145篇 |
2005年 | 145篇 |
2004年 | 118篇 |
2003年 | 112篇 |
2002年 | 83篇 |
2001年 | 64篇 |
2000年 | 68篇 |
1999年 | 48篇 |
1998年 | 37篇 |
1997年 | 46篇 |
1996年 | 49篇 |
1995年 | 32篇 |
1994年 | 43篇 |
1993年 | 35篇 |
1992年 | 52篇 |
1991年 | 46篇 |
1990年 | 45篇 |
1989年 | 40篇 |
1988年 | 36篇 |
1987年 | 47篇 |
1986年 | 38篇 |
1985年 | 42篇 |
1984年 | 38篇 |
1983年 | 29篇 |
1982年 | 39篇 |
1981年 | 43篇 |
1980年 | 39篇 |
1979年 | 40篇 |
1978年 | 27篇 |
1977年 | 39篇 |
1976年 | 26篇 |
1975年 | 19篇 |
1974年 | 17篇 |
1973年 | 17篇 |
排序方式: 共有2879条查询结果,搜索用时 15 毫秒
71.
Ueda J Takeshita K Matsumoto S Yazaki K Kawaguchi M Ozawa T 《Photochemistry and photobiology》2003,77(2):165-170
The reaction of singlet oxygen (1O2) generated by ultraviolet-A (UVA)-visible light (lambda > 330 nm) irradiation of air-saturated solutions of hematoporphyrin with phenolic compounds in the presence of a spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), gave an electron spin resonance (ESR) spectrum characteristic of the DMPO-hydroxyl radical spin adduct (DMPO-*OH). In contrast, the ESR signal of 5,5-dimethyl-2-pyrrolidone-N-oxyl, an oxidative product of DMPO, was observed in the absence of phenolic compounds. The ESR signal of DMPO-*OH decreased in the presence of either a *OH scavenger or a quencher of *O2 and under anaerobic conditions, whereas it increased depending on the concentration of DMPO. These results indicate both 1O2- and DMPO-mediated formation of free *OH during the reaction. When DMPO was replaced with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO), no DEPMPO adduct of oxygen radical species was obtained. This suggests that 1O2, as an oxidizing agent, reacts little with DEPMPO, in which a strong electron-withdrawing phosphoryl group increases the oxidation potential of DEPMPO compared with DMPO. A linear correlation between the amounts of DMPO-*OH generated and the oxidation potentials of phenolic compounds was observed, suggesting that the electron-donating properties of phenolic compounds contribute to the appearance of *OH. These observations indicate that 1O2 reacts first with DMPO, and the resulting DMPO-1O2 intermediate is immediately decomposed/reduced to give *OH. Phenolic compounds would participate in this reaction as electron donors but would not contribute to the direct conversion of 1O2 to *OH. Furthermore, DEPMPO did not cause the spin-trapping agent-mediated generation of *OH like DMPO did. 相似文献
72.
Yasuhiro Tachibana Ryusuke Muramoto Hajime Matsumoto Susumu Kuwabata 《Research on Chemical Intermediates》2006,32(5):575-583
We investigated the photoelectrochemical characteristics and photo-stability of Cu2O layered on a copper plate using a hydrophobic ionic liquid. Our findings revealed that Cu2O is stable under white light irradiation, provided water is removed from the electrolyte. Methyl viologen derivative, a well-established
electron acceptor, was introduced to the ionic liquid electrolyte, allowing the photo-induced electron transfer reaction at
the Cu2O/electrolyte interface to be characterized. The methyl viologen derivative exhibited two distinct redox reactions at −0.56
V and −0.98 V vs. Ag/AgCl, clearly indicating that no dimer formation or co-proportionation reaction occurred. The excessive photocurrents
being continuously generated resulted from a viable photo-induced electron transfer reaction from the Cu2O to the acceptor. However, in contrast, the reduction of the Cu2O by water in the aqueous solution causes this electron transfer to be inhibited. We further demonstrate that these findings
are vital to understanding the role of the Cu2O and its photoelectrochemical applications. 相似文献
73.
Takeo Saegusa Hirosuke Imai Shu-Ichi Matsumoto 《Journal of polymer science. Part A, Polymer chemistry》1968,6(3):459-465
Kinetic studies were carried out on the polymerization of tetrahydrofuran with catalyst systems of aluminum alkyl–epichlorohydrin. As aluminium alkyl species AlEt3, AlEt3–H2O (1:0.1 to 1:1.0), and “oxyaluminum ethyl” were employed. The polymerizations with these catalysts are characterized by a mechanism of stepwise addition without chain transfer or termination, which is expressed by the kinetic relation Rp = Kp[P*] ([M]–[M]e), where [M] and [M]e are the instantaneous and equilibrium concentrations of monomer and [P*] is the concentration of propagating species calculated from the amount and molecular weight of the product polymer. The determination of the rate constant kp for these catalysts has shown that the polymerization rate varied considerably with the change of aluminum alkyl species, i.e., with the water-to-aluminum ratio, but the propagation rate constant itself varied very little. The variation of polymerization rate was, therefore, attributed primarily to the differences in concentration of the propagating species, i.e. the efficiency of the catalyst in forming propagating species. The catalyst efficiency was closely related to the acid strength of the aluminum alkyl species, which was estimated from the magnitude of shift of the xanthone carbonyl band in the infrared spectrum of its coordination complex with aluminum alkyl. The maximal catalyst efficiency was attained at about [H2O]/[AlEt3] = 0.75. 相似文献
74.
Naoji Matsumoto Hiroshi Daimon Ju Kumanotani 《Journal of polymer science. Part A, Polymer chemistry》1980,18(6):1665-1678
Cycloalkanone (C5–C8, C12, and C15) or cycloalkane (C5 and C6) ring-containing monomeric diesters and the polyesters derived from them were prepared, and their configurations were studied by 13C-NMR spectroscopy. Absolute configurations were determined for ring sizes 5, 6, and 7. Configurational change during reduction of a ketonic group of monomeric diesters with ring sizes 5 and 6 was discussed in terms of the steric effect of ring substituents on the ketonic group. In the polycondensation reaction epimerization of the ring units was observed in 5-, 6-, and 7-membered cycloalkanones and not in others, which is explained by steric hindrance by the ring substituents against attack of tetrabutyl titanate catalyst on the ketonic group. 相似文献
75.
76.
77.
78.
79.
80.