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排序方式: 共有115条查询结果,搜索用时 31 毫秒
1.
D Früh J R Tolman G Bodenhausen C Zwahlen 《Journal of the American Chemical Society》2001,123(20):4810-4816
A novel NMR experiment allows one to characterize slow motion in macromolecules. The method exploits the fact that motions, such as rotation about dihedral angles, induce correlated fluctuations of the isotropic chemical shifts of the nuclei in the vicinity. The relaxation of two-spin coherences involving C(alpha) and Cbeta nuclei in proteins provides information about correlated fluctuations of the isotropic chemical shifts of C(alpha) and Cbeta. The difference between the relaxation rates of double- and zero-quantum coherences and is shown to be affected by cross-correlated chemical shift modulation. In ubiquitin, evidence for slow motion is found in loops or near the ends of beta-strands and alpha-helices. 相似文献
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Eugene Lerman Susan Tolman 《Transactions of the American Mathematical Society》1997,349(10):4201-4230
In the first part of the paper, we build a foundation for further work on Hamiltonian actions on symplectic orbifolds. Most importantly, we prove the orbifold versions of the abelian connectedness and convexity theorems. In the second half, we prove that compact symplectic orbifolds with completely integrable torus actions are classified by convex simple rational polytopes with a positive integer attached to each open facet and that all such orbifolds are algebraic toric varieties.
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General Relativity and Gravitation - 相似文献
5.
V. Tolman 《Journal of fluorine chemistry》1993,60(2-3):179-183
Synthetic methods for the preparation of 4-fluoroglutamic acid (1) are reviewed. Theutility of every method for routine laboratory work is evaluated with respect to theaccessibility of starting materials and reagents, the overall yields and also from theviewpoint of safety. Optimum reaction conditions are given for the selected pathway,suitable for large-scale synthesis. 相似文献
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M. Maccoss L. C. Meurer K. Hoogsteen J. P. Springer G. Koo L. B. Peterson R. L. Tolman E. Emini 《Journal of heterocyclic chemistry》1993,30(5):1213-1220
A number of novel C-nucleosides related to purine derivatives are described in which the purine moiety has been replaced by the isosteric heterocycle, 8-aminoimidazo[1,2-α]pyrazine. The nucleosides prepared include the ribo, 3′-deoxy, 2′,3′-dideoxy, and 2′,3′-unsaturated derivatives. These C-nucleosides represent derivatives containing acid stable glycosyl bonds and they can be considered as analogs of adenine- or 3-deazaade-nine-containing nucleosides. Preparation of the parent ribonucleoside was accomplished by reaction of the C-l functionalized sugar, (2ξ)-1-amino-3,6-anhydro-l-deoxy-4,5-O-isopropylidene-7-O-trityl-D-allo-heptitol with 2,3-dichloropyrazine, followed by ring closure to the 8-chloroimidazo[1,2-α]pyrazine nucleoside, conversion to the 8-amino derivative and deblocking. A single crystal X-ray structure of the parent 8-amino-3-(β-D-ribofuranosyl)imidazo[1,2-α]pyrazine is described and the conformation compared to that of formycin. The sugar-modified analogs were prepared by subsequent functional group manipulations on the sugar moiety. Biological evaluation against HIV in H9 T-lymphoid cell culture showed the nucleosides to be devoid of significant antiviral activity compared to DDA. The 3-deazaadenosine analog also demonstrated weak suppression of mouse splenic NK activity toward YAC cells (mouse lymphoma cell targets). The imidazo[1,2-α]pyrazine analog of 3-deazaadenosine showed antiinflammatory activity in vivo in the rat pleurisy carrageenan model in the same range with 3-deazaadenosine. 相似文献
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Brown EC York JT Antholine WE Ruiz E Alvarez S Tolman WB 《Journal of the American Chemical Society》2005,127(40):13752-13753
By treating Cu(I) complexes of neutral, bidentate N-donor ligands with S8, clusters with novel delocalized mixed-valence [Cu3(mu-S)2]3+ cores have been isolated. X-ray crystal structures and UV-vis and resonance Raman spectral features of these clusters reveal similarities to the tetracopper-sulfide "CuZ" site in nitrous oxide reductase. A delocalized S = 1 ground state for the mixed-valent CuIIICu2II cores is supported by the observation of high symmetry in the X-ray structures and 10-line hyperfine features arising from coupling to three equivalent Cu ions in EPR spectra obtained at room temperature (shown) and 10 K. The delocalization we observe contrasts with the localization reported previously for a [Cu3(mu-O)2]3+ analogue (Root, D. E.; Henson, M. J.; Machonkin, T.; Mukherjee, P.; Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 4982), which we rationalized through DFT calculations. 相似文献
10.
Kryatov SV Chavez FA Reynolds AM Rybak-Akimova EV Que L Tolman WB 《Inorganic chemistry》2004,43(6):2141-2150
Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes. 相似文献