全文获取类型
收费全文 | 1625篇 |
免费 | 36篇 |
国内免费 | 10篇 |
专业分类
化学 | 1233篇 |
晶体学 | 33篇 |
力学 | 31篇 |
数学 | 109篇 |
物理学 | 265篇 |
出版年
2023年 | 6篇 |
2021年 | 6篇 |
2020年 | 14篇 |
2019年 | 28篇 |
2018年 | 19篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 33篇 |
2014年 | 38篇 |
2013年 | 96篇 |
2012年 | 92篇 |
2011年 | 108篇 |
2010年 | 56篇 |
2009年 | 71篇 |
2008年 | 90篇 |
2007年 | 88篇 |
2006年 | 91篇 |
2005年 | 90篇 |
2004年 | 99篇 |
2003年 | 90篇 |
2002年 | 72篇 |
2001年 | 27篇 |
2000年 | 21篇 |
1999年 | 17篇 |
1998年 | 23篇 |
1997年 | 25篇 |
1996年 | 28篇 |
1995年 | 17篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 9篇 |
1985年 | 24篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 9篇 |
排序方式: 共有1671条查询结果,搜索用时 31 毫秒
81.
This study shows the ESR spectra of oxoiron(IV) porphyrin pi-cation radicals of 1-8 in dichloromethane-methanol (5:1) mixture. We reported in a previous paper that oxoiron(IV) porphyrin pi-cation radicals of 1-4 are in an a(1u) radical state while those of 5-8 are in an a(2u) radical. The ESR spectra (g( perpendicular)(eff) approximately 3.1 and g( parallel)(eff) approximately 2.0) for the a(1u) radical complexes, 1-4, appear quite different from those reported previously for the oxoiron(IV) porphyrin pi-cation radical of 5 (g(y) = 4.5, g(x) = 3.6, and g(z) = 1.99). The unique ESR spectra of the a(1u) radical complexes rather resemble those of compound I from Micrococcus lysodeikticus catalase (CAT) and ascorbate peroxidase (ASP). This is the first examples to mimic the ESR spectra of compound I in the enzymes. From spectral analysis based on a spin Hamiltonian containing an exchange interaction, the ESR spectra of 1-4 can be explained as a moderate ferromagnetic state (J/D approximately 0.3) between ferryl S = 1 and the porphyrin pi-cation radical S' = (1)/(2). The magnitudes of zero-field splitting (D) for ferryl iron and isotropic J value, estimated from the temperature-dependence of the half-saturation power of the ESR signals, are approximately 28 and approximately +8 cm(-1), respectively. A change in the electronegativity of the beta-pyrrole substituent hardly changes the ESR spectral feature while that of the meso-substituent slightly does owing to the change in the E/D value. On the basis of the present ESR results, we propose the a(1u) radical state for compound I of CAT and ASP. 相似文献
82.
The title compound, (NEt(4))[[Mn(salen)](2)Fe(CN)(6)] (1), was synthesized via a 1:1 reaction of [Mn(salen)(H(2)O)]ClO(4) with (NEt(4))(3)[Fe(CN)(6)] in a methanol/ethanol medium (NEt(4)(+) = tetraethylammonium cation, salen(2)(-) = N,N'-ethylenebis(salicylidene)iminate). The two-dimensional layered structure of 1 was revealed by X-ray crystallographic analysis: 1 crystallizes in monoclinic space group P2(1)/c with cell dimensions of a = 12.3660(8) A, b = 15.311(1) A, c = 12.918(1) A, beta = 110.971(4) degrees, Z = 2 and is isostructural to the previously synthesized compound, (NEt(4))[[Mn(5-Clsalen)](2)Fe(CN)(6)] (5-Clsalen(2-) = N,N'-ethylenebis(5-chlorosalicylidene)iminate; Miyasaka, H.; Matsumoto, N.; Re, N.; Gallo, E.; Floriani, C. Inorg. Chem. 1997, 36, 670). The Mn ion is surrounded by an equatorial salen quadridentate ligand and two axial nitrogen atoms from the [Fe(CN)(6)](3-) unit, the four Fe[bond]CN groups of which coordinate to the Mn ions of [Mn(salen)](+) units, forming a two-dimensional network having [[bond]Mn[bond]NC[bond]Fe[bond]CN[bond]](4) cyclic repeating units. The network is spread over the bc-plane of the unit cell, and the layers are stacked along the a-axis. The countercation NEt(4)(+) is located between the layers. Compound 1 is a ferrimagnet with T(c) = 7.7 K and exhibits hysteresis with a remnant magnetization of 13.44 cm(3).mol(-1) (M/N mu(B) = 2.4) at zero field and a coercivity of 1000 Oe when the powder sample was measured at 1.9 K. Magnetic measurements of a direction-arranged single crystal were also carried out. The orientation of the crystallographic axes of a selected single crystal was determined by X-ray analysis, and magnetization was measured when an external field was applied in the a*, b, and c directions. The magnetization in the a* direction increased more easily than those in the b and c directions below the critical temperature. No hysteresis was observed only for the measurement in the a* direction, indicating the presence of strong structural anisotropy with potential anisotropy on Mn(III) ions. 相似文献
83.
84.
85.
86.
87.
Akira Mori Hitoshi Takeshita Ryoji Mori Shinji Takematsu Manabu Takemoto Seiji Ujiie 《Liquid crystals》2013,40(2):171-178
Two kinds of monocyclic troponoid mesogens, 2,5-dialkanoyloxytropones (4) and 5-alkanoyloxy2-alkoxytropones (5), were prepared. The former showed monotropic smectic A phases and the virtual isotropic liquid-smectic A transitions of the latter were determined by extrapolation of results in a binary phase diagram. Comparing the mesogenic properties between the tropones 4 and the 2-alkanoyloxy-5-alkoxytropones (1), the alkanoyloxy group at C-5 enhances both the melting points and the transition temperatures of the smectic A phases. From the comparison between 5 and 1, the alkanoyloxy group at C-2 lowers the melting points. 相似文献
88.
89.
90.
Abstract Bacteriochlorophyll d analogues that possessed a series of bulky substituents of OMe, OCOMe, and OCO t Bu at the C71-position were synthesized (Zn-complexes of methyl 71-substituted-3-(1-hydroxy-methyl)pyropheophorbide a). Aggregation behavior of these pigments was examined in comparison with a natural-type compound (C71-H) to elucidate the effects of size of the C71-moiety on self-assembly of the chlorins. These C71-substituted Zn-chlorins formed self-aggregates in 1% THF/hexane. The aggregates gave almost the same visible absorption bands, especially a ca. 2000-cm?1 red-shifted Qy peak; the spectra were essentially similar to those of the natural-type Zn-chlorin (C71-H). All the aggregates of the C71-substituted compounds showed similar exciton-type CD couplets in the red-shifted Qy region, which were different from the feature of the couplet of the C71-H Zn-chlorin aggregate. These spectral data suggested that the C71-moiety did not interrupt the intermolecular linkages between chlorin molecules but altered the supramolecular structure which influences the long-range chirality. 相似文献