全文获取类型
收费全文 | 1865篇 |
免费 | 67篇 |
国内免费 | 6篇 |
专业分类
化学 | 1310篇 |
晶体学 | 15篇 |
力学 | 24篇 |
数学 | 94篇 |
物理学 | 495篇 |
出版年
2023年 | 13篇 |
2021年 | 21篇 |
2020年 | 16篇 |
2019年 | 33篇 |
2018年 | 30篇 |
2017年 | 21篇 |
2016年 | 40篇 |
2015年 | 32篇 |
2014年 | 55篇 |
2013年 | 96篇 |
2012年 | 86篇 |
2011年 | 77篇 |
2010年 | 69篇 |
2009年 | 78篇 |
2008年 | 95篇 |
2007年 | 79篇 |
2006年 | 108篇 |
2005年 | 80篇 |
2004年 | 91篇 |
2003年 | 52篇 |
2002年 | 65篇 |
2001年 | 59篇 |
2000年 | 37篇 |
1999年 | 20篇 |
1998年 | 23篇 |
1997年 | 28篇 |
1996年 | 32篇 |
1995年 | 14篇 |
1994年 | 24篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 23篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 23篇 |
1986年 | 34篇 |
1985年 | 29篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 25篇 |
1980年 | 13篇 |
1979年 | 27篇 |
1978年 | 24篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 22篇 |
1974年 | 12篇 |
1973年 | 11篇 |
排序方式: 共有1938条查询结果,搜索用时 69 毫秒
81.
Single crystal EPR studies of the reduced active site of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F 总被引:1,自引:0,他引:1
Foerster S Stein M Brecht M Ogata H Higuchi Y Lubitz W 《Journal of the American Chemical Society》2003,125(1):83-93
In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the "substrate hydrogen". In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J. Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H(-)) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(I) oxidation state is assumed for this species with a proton (H(+)) removed from the bridge between the nickel and the iron atom. 相似文献
82.
[reaction: see text] Phosphates from 3-substituted 4,4,4-trifluorobut-2-en-1-ols were found to be effective for construction of CF3-containing quaternary carbon centers by way of Cu(I)-catalyzed Grignard reactions in the presence of catalytic amounts of CuCN and trimethylsilyl chloride (TMSCl) in an anti S(N)2' manner. 相似文献
83.
Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a-c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave beta-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave alpha-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave alpha-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave beta-silicon-substituted Z-alkenes 3 selectively. 相似文献
84.
T. Yamazaki 《Zeitschrift für Physik A Hadrons and Nuclei》1992,341(2):223-225
A new way to promote antihydrogen formation via the recently discovered long-lived metastable states of antiprotonic helium atoms is discussed. Recombination processes such ase
–
¯pHe++ +e
+
e
– e
+
¯p + He0 are possible in this respect.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday 相似文献
85.
Y. Saito-Kokubu K. Yasuda M. Magara Y. Miyamoto S. Sakurai S. Usuda H. Yamazaki S. Yoshikawa 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(1):183-186
In order to know the distribution of plutonium derived from the Nagasaki atomic bomb, soil samples were measured to determine
the 240Pu/239Pu isotope ratio of and concentrations of 239+240Pu and 137Cs. The 239+240Pu concentrations in soils, except for Nishiyama area, were close to the average concentration of soil collected in Japan.
In soils collected at the Nishiyama area and at the eastern area of Nagasaki Prefecture and at part of northern area in Kumamoto
Prefecture, the 240Pu/239Pu ratios were lower than the global fallout values. This suggests that plutonium from the atomic bomb was deposited in the
eastern area from the hypocenter reaching up to 100 km eastwards. 相似文献
86.
Unsaturated transition metal carbonyls are important in processes such as organometallic synthesis, homogeneous catalysis, and photochemical decomposition of organometallics. In particular, a metal monocarbonyl offers a zeroth-order model for interpreting the chemisorption of a CO molecule on a metal surface in catalytic activation processes. Quite large numbers of theoretical papers have appeared which predict spectroscopic and structural properties of transition metal carbonyls. The nickel monocarbonyl NiCO has been one of the metal carbonyls most extensively studied by the theoretical calculations. At least 50 theoretical studies have been published on this simplest transition metal carbonyl up to the present time. However, experimental evidence of NiCO is much more sparse than theoretical predictions, and the actual structure of NiCO has never been determined by any experimental methods. This Communication reports the first preparation of free nickel monocarbonyl and observation of its rotational transitions. The NiCO molecule was generated by the sputtering reaction of a Ni cathode in the presence of CO. The accurate bond lengths of Ni-C and C-O were experimentally determined from isotopic data and were compared with the theoretical predictions for the first time. 相似文献
87.
Low-spin ferric porphyrin radical cations formed by the oxidation of chloro(meso-tetraalkylporphyrinato)iron(III) followed by the addition of bulky 2-methylimidazole show antiferromagnetic coupling, which is interpreted in terms of the interaction between porphyrin a2u and iron d(xy), orbitals caused by the S4 ruffling of the porphyrin core. 相似文献
88.
Crown ether dyes with pendent anionic side-arms were synthesized for extractionspectrophotometry of alkali and alkaline earth metal ions. Dramatic changes in metal selectivity were obtained simply by changing the nature of the anionic side-arm on the same crown ether skeleton. A structure/metal selectivity relationship is discussed in detail in terms of “chelate” and “intramolecular ion-pair” formation. Small metal cations (high charge density) are preferred in the extraction by a crown ether reagent with a charge-localized anionic side-arm through the formation of a “chelate”. Large metal cations (low charge density) are preferred in the extraction by reagents with a charge-delocalized anionic side-arm through the formation of an “intramolecular ion-pair”. Steric restrictions imposed by the side-arm on the metal ion approaching the crown ether are also important factor in controlling the selectivity of these reagents. 相似文献
89.
Y. Yamazaki H. Kataoka A. Miyazakl M. Watanabe T. Ootakl 《Photochemistry and photobiology》1996,64(2):387-392
The sexual development of the fungus Phycomyces is inhibited by light. The action spectra for this photoinhibitory effect were determined for 48 h continuous exposure between 350 and 700 nm wavelengths during the mating process. Effective wavelengths were shorter than 490 nm, but the most effective wavelengths depended on the stage of sexual development. In early stages of progametangium formation, the major peaks appeared near 360 nm with small shoulders at 410 nm, but in later stages, after gametangium formation, only single peaks were detected in the UVA range (350–390 nm). Low-fluence irradiation in the later stage, however, revealed inhibitory effectiveness at 370–410 nm, implying the existence of a dual photoresponse and multiple regulatiory systems in the mating process of Phycomyces. 相似文献
90.
The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) have been examined in solution by (1)H NMR, (13)C NMR, and EPR spectroscopy and by magnetic measurements. While [Fe(OMTPP)(DMAP)(2)](+) and [Fe(TBTXP)(DMAP)(2)](+) maintain the low-spin (S = (1)/(2)) state, [Fe(OMTPP)(THF)(2)](+) and [Fe(TBTXP)(THF)(2)](+) exhibit an essentially pure intermediate-spin (S = (3)/(2)) state over a wide range of temperatures. In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S = (3)/(2) to S = (1)/(2) as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is similar to that of [Fe(OETPP)Py(2)](+) reported in our previous paper (Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620) in the context that all these complexes exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) (L = Py, 4-CNPy) revealed, however, that these complexes adopt the less common (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature in contrast to [Fe(OETPP)Py(2)](+) which shows the common (d(xy))(2)(d(xz), d(yz))(3) electron configuration. These observations have been attributed to the flexible nature of the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores can ruffle the porphyrin ring and adopt the (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature. Therefore, this study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways. 相似文献