全文获取类型
收费全文 | 1574篇 |
免费 | 42篇 |
国内免费 | 7篇 |
专业分类
化学 | 1121篇 |
晶体学 | 5篇 |
力学 | 21篇 |
数学 | 261篇 |
物理学 | 215篇 |
出版年
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 15篇 |
2016年 | 23篇 |
2015年 | 32篇 |
2014年 | 34篇 |
2013年 | 65篇 |
2012年 | 30篇 |
2011年 | 37篇 |
2010年 | 40篇 |
2009年 | 37篇 |
2008年 | 79篇 |
2007年 | 62篇 |
2006年 | 74篇 |
2005年 | 56篇 |
2004年 | 47篇 |
2003年 | 58篇 |
2002年 | 58篇 |
2001年 | 42篇 |
2000年 | 38篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 21篇 |
1996年 | 26篇 |
1995年 | 26篇 |
1994年 | 43篇 |
1993年 | 37篇 |
1992年 | 49篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 32篇 |
1986年 | 22篇 |
1985年 | 20篇 |
1984年 | 21篇 |
1983年 | 28篇 |
1982年 | 20篇 |
1981年 | 16篇 |
1980年 | 21篇 |
1979年 | 21篇 |
1978年 | 33篇 |
1977年 | 18篇 |
1976年 | 12篇 |
1975年 | 17篇 |
1974年 | 25篇 |
1973年 | 20篇 |
1972年 | 16篇 |
1971年 | 21篇 |
1970年 | 12篇 |
排序方式: 共有1623条查询结果,搜索用时 15 毫秒
81.
de Ropp JS Sham S Asokan A Newmyer S Ortiz de Montellano PR La Mar GN 《Journal of the American Chemical Society》2002,124(37):11029-11037
The functional higher oxidation states of heme peroxidases have been proposed to be stabilized by the significant imidazolate character of the proximal His. This is induced by a "push-pull" combination effect produced by the proximal Asp that abstracts ("pulls") the axial His ring N(delta)H, along with the distal protonated His that contributes ("pushes") a strong hydrogen bond to the distal ligand. The molecular and electronic structure of the distal His mutant of cyanide-inhibited horseradish peroxidase, H42A-HRPCN, has been investigated by NMR. This complex is a valid model for the active site hydrogen-bonding network of HRP compound II. The (1)H and (15)N NMR spectral parameters characterize the relative roles of the distal His42 and proximal Asp247 in imparting imidazolate character to the axial His. 1D/2D spectra reveal a heme pocket molecular structure that is highly conserved in the mutant, except for residues in the immediate proximity of the mutation. This conserved structure, together with the observed dipolar shifts of numerous active site residue protons, allowed a quantitative determination of the orientation and anisotropies of the paramagnetic susceptibility tensor, both of which are only minimally perturbed relative to wild-type HRPCN. The quantitated dipolar shifts allowed the factoring of the hyperfine shifts to reveal that the significant changes in hyperfine shifts for the axial His and ligated (15)N-cyanide result primarily from changes in contact shifts that reflect an approximately one-third reduction in the axial His imidazolate character upon abolishing the distal hydrogen-bond to the ligated cyanide. Significant changes in side chain orientation were found for the distal Arg38, whose terminus reorients to partially fill the void left by the substituted His42 side chain. It is concluded that 1D/2D NMR can quantitate both molecular and electronic structural changes in cyanide-inhibited heme peroxidase and that, while both residues contribute, the proximal Asp247 is more important than the distal His42 in imparting imidazole character to the axial His 170. 相似文献
82.
Daniel Carmona Joaquina Ferrer María Pilar Lamata Luis A. OroHans-Heinrich Limbach Gerd SchererJos Elguero María Luisa Jimeno 《Journal of organometallic chemistry》1994,470(1-2):271-274
The Arrhenius equation corresponding to the process P---Ag+P*---Ag*→---P---Ag*+P*---Ag has been determined for [(η6-p-cymene)Ru(μ-pz)3Ag(PPh3)] (1) by complete line-shape analysis of the 31P NMR spectra between −40°C and +30°C. It has the form K = 1011.8± e(−46±5 kJ mol−1/RT). The preexponential term, log A = 11.8 corresponds to a small activation entropy, whereas the activation energy, 46 kJ mol−1 is comparable to those determined for other phosphorus—metal compounds. 相似文献
83.
Ulrich Wannagat Hans Bürger Peter Geymayer Gerd Torper 《Monatshefte für Chemie / Chemical Monthly》1964,95(1):39-45
Zusammenfassung Gemischt substituierte Disilylamine (RRRSi)2NH lassen sich von NaNH2 zu reaktionsfähigen Natrium-bis-(silyl)-amiden metallieren, aus denen nach Umsetzung mit Organohalogensilanen und anschließender Alkoholyse der verbleibenden Si-Cl-Gruppen gemischt substituierte Trisilylamine (RRRSi)3N mit R=(me), R, R=O(me); R,R=(me), R=O(et); R=(vi), R, R=O(me); R=(me), R=(vi), R=O(ipr) dargestellt werden konnten.33. Mitt.:U. Wannagat, J. Pump undH. Bürger, Mh. Chem.94, 1013 (1963).Auszüge aus den DissertationenH. Bürger, TH Aachen 1962,P. Geymayer, TH Graz 1963, und der DiplomarbeitG. Torper, TH Graz 1962; vgl. auchU. Wannagat undH. Bürger, Angew. Chem.75, 95 (1963). 相似文献
84.
Krzysztof A. Kunert 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(9):1469-1488
In this paper are presented the static and dynamic mechanical investigation of chemically cross-linked low density polyethylene (XLPE) prepared in our laboratory. This polymer has been tested mechanically at different frequencies, amplitudes, and temperatures as a function of cross-link density which is indicated to some extent by the amount of peroxide used in the cross-linking. The main findings can be described as follows: mechanical damping in XLPE at the α-relaxation point as a function of peroxide concentration is different for shear and compression modes of deformation. Moreover, the dynamic investigation at very small amplitudes indicated two relatively rigid structures. One structure at low concentration of peroxide is attributed to excessive crystallinity; the other one at the high peroxide concentration is probably due to the very regular and perfect polyethylene network. These two interesting structures are not detected by dynamical testing with large amplitudes. As far as Young's modulus as a function of peroxide concentration is concerned, we conclude that in this polymer this factor does not depend on the crystallinity but on changes of the so-called hard amorphous phase. These findings are consistent with our previous structural investigation. 相似文献
85.
86.
Dr. Alexey S. Kiryutin Dr. Grit Sauer Dr. Daniel Tietze Martin Brodrecht Stephan Knecht Prof. Dr. Alexandra V. Yurkovskaya Prof. Dr. Konstantin L. Ivanov Dr. Olga Avrutina Prof. Dr. Harald Kolmar Prof. Dr. Gerd Buntkowsky 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(16):4025-4030
Two-dimensional NMR spectroscopy is one of the most important spectroscopic tools for the investigation of biological macromolecules. However, due to the low sensitivity of NMR spectroscopy, it takes usually from several minutes to many hours to record such spectra. Here, the possibility of detecting a bioactive derivative of the sunflower trypsin inhibitor-1 (SFTI-1), a tetradecapeptide, by combining parahydrogen-induced polarization (PHIP) and ultrafast 2D NMR spectroscopy is shown. The PHIP activity of the inhibitor was achieved by labeling with O-propargyl-l -tyrosine. In 1D PHIP experiments a signal enhancement of a factor of approximately 1200 compared to standard NMR was found. This enhancement permits measurement of 2D NMR correlation spectra of low-concentrated SFTI-1 in less than 10 seconds, employing ultrafast single-scan 2D NMR detection. As experimental examples PHIP-assisted ultrafast single-scan TOCSY spectra of SFTI-1 are shown. 相似文献
87.
88.
Gerd Wellenreuther Venkataraman Parthasarathy Wolfram Meyer‐Klaucke 《Journal of synchrotron radiation》2010,17(1):25-35
In biological systems, X‐ray absorption spectroscopy (XAS) can determine structural details of metal binding sites with high resolution. Here a method enabling an automated analysis of the corresponding EXAFS data is presented, utilizing in addition to least‐squares refinement the prior knowledge about structural details and important fit parameters. A metal binding motif is characterized by the type of donor atoms and their bond lengths. These fit results are compared by bond valance sum analysis and target distances with established structures of metal binding sites. Other parameters such as the Debye–Waller factor and shift of the Fermi energy provide further insights into the quality of a fit. The introduction of mathematical criteria, their combination and calibration allows an automated analysis of XAS data as demonstrated for a number of examples. This presents a starting point for future applications to all kinds of systems studied by XAS and allows the algorithm to be transferred to data analysis in other fields. 相似文献
89.
Uwe Herzog Uwe Böhme Gerd Rheinwald 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):281-286
The reactions of Me2MCl2 (M = Si, Ge, Sn), Si2Me4Cl2, Si2Me2Cl3, Si2Me2Cl4 and CH2(SiCl2Me)2, and suitable mixtures thereof, with H2S / NEt3 and Li2E (E = Se, Te) have been investigated and lead to a variety of new group 14 chalcogenide systems. 相似文献
90.
Gerd Becker Winfried Becker Gudrun Uhl Werner Uhl Hans-Jürgen Wessely 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-3):7-10
Abstract Alkyl- or arylbis(trimethylsilyl)phosphines as well as tris(trimethylsilyl)phosphine and the corresponding arsines react with acyl chlorides to give [1-(trimethylsiloxy)alkylidene]phosphines 1 and -arsines 2; most of their 2,2-dimethylpropylidene derivatives are thermally stable at room temperature. With the same class of phosphines as starting compounds and carbon disulfide [bis(trimethylsilylsulfano)methylidene]phosphines 3 are formed, whereas [(dialkylamino)methylidene]-4 and [diarylmethylidene]phosphines 5 or the corresponding arsines 6 and 7 can be obtained from acyl amides or ketones.1 相似文献