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991.
Pomerantz AE Hammond MR Morrow AL Mullins OC Zare RN 《Journal of the American Chemical Society》2008,130(23):7216-7217
Defined by their solubility in toluene and insolubility in n-heptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and most polar fraction of crude oil. Although asphaltenes are critically important to the exploitation of conventional oil and are poised to rise in significance along with the exploitation of heavy oil, even as fundamental a quantity as their molecular weight distribution is unknown to within an order of magnitude. Laser desorption/ionization (LDI) mass spectra vary greatly with experimental parameters so are difficult to interpret: some groups favor high laser pulse energy measurements (yielding heavy molecular weights), arguing that high pulse energy is required to detect the heaviest components of this mixture; other groups favor low pulse energy measurements (yielding light molecular weights), arguing that low pulse energy is required to avoid aggregation in the plasma plume. Here we report asphaltene mass spectra recorded with two-step laser mass spectrometry (L2MS), in which desorption and ionization are decoupled and no plasma is produced. L2MS mass spectra of asphaltenes are insensitive to laser pulse energy and other parameters, demonstrating that the asphaltene molecular weight distribution can be measured without limitation from insufficient laser pulse energy or plasma-phase aggregation. These data resolve the controversy from LDI, showing that the asphaltene molecular weight distribution peaks near 600 Da and previous measurements reporting much heavier species suffered from aggregation effects. 相似文献
992.
Friedman JF Miller TM Friedman-Schaffer JK Viggiano AA Rekha GK Stevens AE 《The Journal of chemical physics》2008,128(10):104303
An experimental study has been made of thermal electron attachment to the transition-metal trifluorophosphine complexes Ni(PF(3))(4) and Pt(PF(3))(4) using a flowing-afterglow Langmuir-probe apparatus. Both complexes are efficient at electron attachment, although the rate constants are somewhat less than collisional. The rate constant for electron attachment to Ni(PF(3))(4) is 1.9 x 10(-7) cm(3) s(-1) at room temperature, about a factor of 2 less than collisional. The activation energy is 39+/-5 meV for the attachment reaction. The rate constant for electron attachment to Pt(PF(3))(4) is 5.4 x 10(-8) cm(3) s(-1) at room temperature, and the activation energy is 84+/-8 meV. For both complexes, a PF(3) ligand is lost on electron attachment, and only the M(PF(3))(3)(-) ion is observed in the negative-ion mass spectrum. Density functional calculations were carried out on Ni(PF(3))(4) and various fragments in order to describe the thermochemistry of the attachment reaction. 相似文献
993.
The CC to TT tandem mutation is induced by UV radiation exposure, though at relatively low frequencies when compared with the more commonly induced C to T mutation. Induction of the tandem mutation by UV is enhanced in mammalian cells with certain genetic deficiencies; however, conditions have not been described in which the frequency of this mutation is enhanced in DNA repair-proficient mammalian cells. For this study, an integrated construct that detects C to T and CC to TT mutations at a single codon in mouse Aprt was used to examine UVB mutagenesis under various conditions. Oxidative stress, in the form of intracellular hydrogen peroxide, increased the frequency of UVB-induced CC to TT mutations. Surprisingly, exposure of the cells to two antioxidants (N-acetylcysteine and trolox), either alone or in combination, also enhanced UVB induction of CC to TT tandem mutations. These results demonstrate, for the first time, that the frequency of UVB-induced CC to TT tandem mutations can be enhanced dramatically in DNA repair-proficient mammalian cells, and suggest that the enhancing effect does not require direct damage to DNA. 相似文献
994.
Bearinger JP Stone G Christian AT Dugan L Hiddessen AL Wu KJ Wu L Hamilton J Stockton C Hubbell JA 《Langmuir : the ACS journal of surfaces and colloids》2008,24(9):5179-5184
Photocatalytic lithography couples light with photoreactive coated mask materials to pattern surface chemistry. We excite porphyrins to create radical species that photocatalytically oxidize, and thereby pattern, chemistries in the local vicinity. The technique advantageously is suited for use with a wide variety of substrates. It is fast and robust, and the wavelength of light does not limit the resolution of patterned features. We have patterned proteins and cells to demonstrate the utility of photocatalytic lithography in life science applications. 相似文献
995.
Wingen LM Moskun AC Johnson SN Thomas JL Roeselová M Tobias DJ Kleinman MT Finlayson-Pitts BJ 《Physical chemistry chemical physics : PCCP》2008,10(37):5668-5677
Heterogeneous reactions of sea salt aerosol with various oxides of nitrogen lead to replacement of chloride ion by nitrate ion. Studies of the photochemistry of a model system were carried out using deliquesced mixtures of NaCl and NaNO3 on a Teflon substrate. Varying molar ratios of NaCl to NaNO3 (1 : 9 Cl- : NO3-, 1 : 1 Cl- : NO3-, 3 : 1 Cl- : NO3-, 9 : 1 Cl- : NO3-) and NaNO3 at the same total concentration were irradiated in air at 299 +/- 3 K and at a relative humidity of 75 +/- 8% using broadband UVB light (270-380 nm). Gaseous NO2 production was measured as a function of time using a chemiluminescence NO(y) detector. Surprisingly, an enhanced yield of NO2 was observed as the chloride to nitrate ratio increased. Molecular dynamics (MD) simulations show that as the Cl- : NO3- ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl- and subsurface Na+. This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO2+O*-, increasing the effective quantum yield and hence the production of gaseous NO2. The implications of enhanced NO2 and likely OH production as sea salt aerosols become processed in the atmosphere are discussed. 相似文献
996.
Two gem-difluoromethylenated nucleoside moieties of liposidomycins, 3 and 4, were designed and synthesized. Compound 3 was assembled from lactol 5 and gem-difluoromethylenated nucleoside 6. In the synthesis of target molecule 4, the coupling of the trichloroacetimidate derivative of gem-difluoromethylenated furanose 7 with nucleoside 8 in the presence of TMSOTf gave the unexpected compound 16 when CH3CN was used as solvent. This results from acetonitrile acting as a nucleophile and participating in the glycosylation reaction. This unusual process may be correlated with the presence of the electron-withdrawing gem-difluoro substituents at the C-2 position of furanose. Compound 3 demonstrated 29% inhibition of MraY at 11.4 mM. 相似文献
997.
Mechanisms for the recently described reactions of isonitriles with carboxylic acids (Li, X.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130, 5446) are explored with the B3LYP density functional method. The mechanism involves the formation of a carboxylate mixed formimidic anhydride intermediate via a concerted mechanism. This intermediate is then transformed to an N-formylamide by a concerted pseudopericyclic [1,3]-acyl shift. Mechanisms involving zwitterions or diradicals are discounted. 相似文献
998.
Son A Dhirapong A Dosev DK Kennedy IM Weiss RH Hristova KR 《Analytical and bioanalytical chemistry》2008,390(7):1829-1835
Rapid and accurate detection of genetic mutations based on nanotechnology would provide substantial advances in detection
of polycystic kidney disease (PKD), a disease whose current methods of detection are cumbersome due to the large size and
duplication of the mutated gene. In this study, a nanotechnology-based DNA assay was developed for detection of SNPs (single
nucleotide polymorphisms) in a feline autosomal dominant PKD (ADPKD) model which can readily be adapted to diagnosis of human
ADPKD type 1. Europium and terbium phosphors were doped into gadolinium crystal hosts with a magnetic core, providing stable
luminescence and the possibility of magnetic manipulations in a solution-based assay. A hybridization-in-solution DNA assay
was optimized for feline PKD gene SNP detection using genomic DNA extracted from feline kidney tissue and blood. This assay
showed a substantial differentiation between PKD and control specimens. The nanotechnology-based DNA assay is attractive from
the viewpoint of rapid availability, simple methodology, and cost reduction for clinical use to detect mutations involved
in human ADPKD and other genetic diseases.
Figure Schematic diagram of PKD (Polycystic Kidney Disease) SNPs detection assay using feline genomic DNA in magnetic/luminescent
nanoparticle-based DNA hybridization 相似文献
999.
Let H(N) denote the Hurwitz class number. It is known that if p is a prime, then
1000.
We have investigated secondary ion yield enhancement using Bin2+ (n=1, 3, 5) primary ions impacting phenylalanine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), cholesterol, Irganox 1010, and polymer films adsorbed on silicon and aluminum. Secondary ion yields are increased using Bi2+and Bi3(2+) primary ions for the molecular layers and polymers that can undergo allyl cation rearrangements. For Irganox 1010, the deprotonated molecular ion yields (m/z 1175; [M-H]-) are one to two times larger for Bi2+ and Bi(3)2+ primary ions than for Bi+ and Bi3+ at the same primary ion velocities. In the positive ion mode, the largest fragment ion yield (m/z 899) is 1.5 times larger for Bi2+ ions than for Bi+. For Bi3(2+) the largest fragment ion yield is only 70% of the ion yield using Bi3+, but the secondary ion yields of the fragment ions at m/z 57 and 219 are enhanced. For polymers that can undergo allyl cation rearrangement reactions the secondary ion yield enhancements of the monomer ions range from 1.3 to 4.3. For Bi(5)2+ primary ions, secondary ion yields were the same or slightly larger than for Bi5+ in the negative ion mass spectra for Irganox 1010, but lower in the positive ion mode. No secondary ion yield enhancements were measured on polymer samples for Bi5(2+). For all polymer films studied, secondary ion intensities from the oligomer regions are substantially decreased using Bin2+ (n=1, 3, 5). We discuss differences in the ionization mechanisms for doubly and singly-charged Bi primary ion bombardment. 相似文献