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111.
Diazene dehydrogenation follows H2 addition to coordinated dinitrogen in an ansa-zirconocene complex
An activated side-on-bound ansa-zirconocene dinitrogen complex, [Me2Si(eta5-C5Me4)(eta5-C5H3-3-tBu)Zr]2(mu2,eta2,eta2-N2), has been prepared by sodium amalgam reduction of the corresponding dichloride precursor under an atmosphere of N2. Both solution spectroscopic and X-ray diffraction data establish diastereoselective formation of the syn homochiral dizirconium dimer. Addition of 1 atm of H2 resulted in rapid hydrogenation of the N2 ligand to yield one diastereomer of the hydrido zirconocene diazenido complex. Kinetic measurements have yielded the barrier for H2 addition and in combination with isotopic labeling studies are consistent with a 1,2-addition pathway. In the absence of H2, the hydrido zirconocene diazenido product undergoes swift diazene dehydrogenation to yield an unusual hydrido zirconocene dinitrogen complex. The N=N bond length of 1.253(5) A determined by X-ray crystallography indicates that the side-on-bound N2 ligand is best described as a two-electron reduced [N2]2- fragment. Comparing the barrier for deuterium exchange with [Me2Si(eta5-C5Me4)(eta5-C5H3-3-tBu)ZrH]2(mu2,eta2,eta2-N2H2) to diazene dehydrogenation is consistent with rapid 1,2-elimination of dihydrogen followed by rate-determining hydride migration to the zirconium. This mechanistic proposal is also corroborated by H2 inhibition and the observation of a normal, primary kinetic isotope effect for dehydrogenation. 相似文献
112.
Nishijima M Pace TC Nakamura A Mori T Wada T Bohne C Inoue Y 《The Journal of organic chemistry》2007,72(8):2707-2715
Photophysics and photochemistry of 2-anthracenecarboxylate (AC) bound to bovine serum albumin (BSA) were investigated in detail for the first time by electronic absorption, circular dichroism (CD), steady-state and time-resolved fluorescence, fluorescence quenching, and product analysis studies. Through the spectroscopic investigations, it was revealed that the four independent binding pockets of BSA, which are known to accommodate 1, 3, 2, and 3 AC molecules in the order of decreasing affinity, are distinctly different in hydrophobicity, chiral environment, and accessibility. Interestingly, AC bound to site 1 gave highly structured fluorescence with dual lifetimes of 4.8 and 2.1 ns in an intensity ratio of 3:2, which may be assigned to the existence of two positional or orientational isomers within the very hydrophobic site 1. In contrast, the lifetime of AC in site 2 was much longer (13.3 ns), and ACs in sites 3 and 4 have broader fluorescence spectra with lifetimes that were practically indistinguishable from that in bulk water (15.8 ns). Although each of sites 2-4 simultaneously binds multiple AC molecules, no CD exciton coupling or static fluorescence quenching was detected, indicating that ACs bound to each site are not in close proximity to each other. Quenching studies with nitromethane further confirmed the significant difference in accessibility among the binding sites; thus, ACs bound to sites 1 and 2 are highly protected from the attack of the quencher, affording 32 and 10 times smaller rate constants than that for free AC in water. Product studies in the presence and absence of nitromethane more clearly revealed the photochirogenic performance of each binding site. Although the addition of nitromethane did not greatly alter the product distribution, the enantiomeric excesses (ee's) of chiral cycloadducts 2 and 3 were critically manipulated by selectively retarding the photoreaction occurring at the more accessible binding sites. Thus, the highest ee of 38% was obtained for 2 in the presence of 18 mM nitromethane, while the highest ee of 58% was attained for 3 in the absence of nitromethane, both at [AC]/[BSA]=3.6. 相似文献
113.
Tamara Grummt P.‐D. Hansen P. Rettberg P. Waldmann J. Zipperle 《Chemie in Unserer Zeit》2004,38(4):248-255
The occurrence of genotoxic substances in aquatic systems is a serious problem because of their risk for both human and ecosystem health. The far‐reaching consequences of possible genotoxic impacts (in situ) for environmental policy, trying aims to discourage exposure to genotoxic substances, require a refined experimental concept is needed. The bioassays umu‐test, Ames‐test, comet assay, the DNA unwinding test, alkaline filter elution, the UDS‐test and the micronucleus test are sensitive enough to detect genotoxic effects in complex mixtures in a low dose range. They seem to be suitable for routine testing and should therefore be used in the first stage of a graduated testing battery for detection of genotoxicity in aquatic systems in combination with effect‐oriented chemical analysis. 相似文献
114.
We study the Cauchy problem for the Whitham modulation equations for increasing smooth initial data. The Whitham equations are a collection of one-dimensional quasi-linear hyperbolic systems. This collection of systems is enumerated by the genus g=0,1,2, ... of the corresponding hyperelliptic Riemann surface. Each of these systems can be integrated by the so-called hodograph transformation introduced by Tsarev. A key step in the integration process is the solution of the Tsarev linear overdetermined system. For each g>0, we construct the unique solution of the Tsarev system, which matches the genus g+1 and g–1 solutions on the transition boundaries. 相似文献
115.
116.
Tamara Grava 《Physica D: Nonlinear Phenomena》2009,238(1):55-66
We study the Whitham equations for the Camassa-Holm equation. The equations are neither strictly hyperbolic nor genuinely nonlinear. We are interested in the initial value problem of the Whitham equations. When the initial values are given by a step function, the Whitham solution is self-similar. When the initial values are given by a smooth function, the Whitham solution exists within a cusp in the x-t plane. On the boundary of the cusp, the Whitham solution matches the Burgers solution, which exists outside the cusp. 相似文献
117.
Indrek Koppel Tamara Aid-Pavlidis Kaur Jaanson Mari Sepp Priit Pruunsild Kaia Palm T?nis Timmusk 《BMC neuroscience》2009,10(1):68
Background
Brain-derived neurotrophic factor (BDNF) is a small secreted protein that has important roles in the developing and adult nervous system. Altered expression or changes in the regulation of the BDNF gene have been implicated in a variety of human nervous system disorders. Although regulation of the rodent BDNF gene has been extensively investigated, in vivo studies regarding the human BDNF gene are largely limited to postmortem analysis. Bacterial artificial chromosome (BAC) transgenic mice harboring the human BDNF gene and its regulatory flanking sequences constitute a useful tool for studying human BDNF gene regulation and for identification of therapeutic compounds modulating BDNF expression. 相似文献118.
Matkovich KM Thorne LM Wolf MO Pace TC Bohne C Patrick BO 《Inorganic chemistry》2006,45(12):4610-4618
A series of Ru(II) complexes, tcc-RuX2(POC4Pyr-P,O)2 (X = Cl (3), Br (4), I (5)), containing the hemilabile phosphine pyrenyl ether ligand 4-{2-(diphenylphosphino)phenoxy}butylpyrene (POC4Pyr (1)) are reported. The synthesis and spectroscopic properties of both the ligand, POC4pyr (1), and ligand oxide, P(=O)OC4pyr 2, and the solid-state structure of 1 are reported. Complexes 3-5 react rapidly with CO to give complexes ttt-RuX2(CO)2(POC4pyr-P)2 (X = Cl (6), Br (7), I (8)). No pyrene excimer emission is detected from 3-5; however, different intensities of excimer emission are observed for 6-8. The intensity of excimer emission decreases through the series, with 6 showing the most intense response. The emission is solely due to intramolecular pyrene excimers at low concentrations (< or =10(-4) M). Comparison of the UV-vis and steady-state fluorescence spectra shows overlap between the low energy d-d absorption of 7 and 8 with excimer emission (480 nm), suggesting nonradiative energy transfer may be occurring. Once excess CO is removed, complexes 6-8 isomerize to cis-dicarbonyl complexes cct-RuX2(CO)2(POC4Pyr-P)2 (X = Cl (9), Br (10), I (11)). The intensity of excimer emission from 9-11 increases with respect to the excimer emission observed for 6-8, with 9 showing a significant increase in excimer intensity. 相似文献
119.
The silyl-substituted titanocene complex, (eta5-C5Me4SiMe2Ph)2Ti, coordinates dinitrogen upon cooling to -35 degrees C to yield an unprecedented example of a mono(dinitrogen) complex of a substituted bis(cyclopentadienyl) titanium compound, (eta5-C5Me4SiMe2Ph)2Ti(N2). Analogous monocarbonyl derivatives, (eta5-C5Me4R)2Ti(CO) (R = SiMe3, SiMe2Ph, CHMe2), have been prepared by mixing the dicarbonyl compounds with the corresponding sandwiches. Both (eta5-C5Me4SiMe2Ph)2Ti(N2) and (eta5-C5Me4SiMe2Ph)2Ti(CO) have been characterized by X-ray diffraction, and mixed N2-CO titanocene complexes have also been observed by in situ IR spectroscopy. 相似文献
120.
A novel approach for the separation of anions from aqueous mixtures was demonstrated, which involves their selective crystallization with metal-organic frameworks (MOFs) containing urea functional groups. Self-assembly of Zn2+ with the N,N'-bis(m-pyridyl)urea (BPU) linker results in the formation of one-dimensional MOFs including various anions for charge balance, which interact to different extents with the zinc nodes and the urea hydrogen-bonding groups, depending on their coordinating abilities. Thus, Cl-, Br-, I-, and SO4(2-), in the presence of BPU and Zn2+, form MOFs from water, in which the anions coordinate the zinc and are hydrogen-bonded to the urea groups, whereas NO3- and ClO4- anions either do not form MOFs or form water-soluble discrete coordination complexes under the same conditions. X-ray diffraction, FTIR, and elemental analysis of the coordination polymers precipitated from aqueous mixtures containing equivalent amounts of these anions indicated total exclusion of the oxoanions and selective crystallization of the halides in the form of solid solutions with the general composition ZnCl(x)Br(y)I(z).BPU (x + y + z = 2), with an anti-Hofmeister selectivity. The concomitant inclusion of the halides in the same structural frameworks facilitates the rationalization of the observed selectivity on the basis of the diminishing interactions with the zinc and urea acidic centers in the MOFs when going from Cl- to I-, which correlates with decreasing anionic charge density in the same order. The overall crystal packing efficiency of the coordination frameworks, which ultimately determines their solubility, also plays an important role in the anion crystallization selectivity under thermodynamic equilibration. 相似文献