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991.
May Joy Contado Jeanette Adams Nancy J. Jensen Michael L. Gross 《Journal of the American Society for Mass Spectrometry》1991,2(2):180-183
The collision-induced allylic cleavage reactions of deuterium-labeled [M ? H + 2Li)+ and [M ? H]- ions of monounsaturated fatty acids were investigated. Three concerted mechanistic possibilities were considered for this process: a l,4-elimination of a vinylic H, a retro-ene reaction, and a l,4-conjugate elimination. A fourth mechanistic possibility, a two-step radical version of the retro-ene and l,4-conjugate elimination reactions, was also considered. The radical reactions are in accord with the isotopic labeling results and offer certain mechanistic consistencies for cleavage of both C-C allyl bonds; they are expected, however, to have large activation energies. The lower-energy concerted alternatives, the retro-ene reaction for cleavage of the proximal and the l,4-conjugate elimination for cleavage of the distal C-C allyl bond, are also consistent with experimental results. The alternative of two different concerted mechanisms for cleavage of the two allyl bonds, however, is at odds with the charge-remote concept. 相似文献
992.
The explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is of particular interest due to its extreme insensitivity to impact, shock and heat, while providing a good detonation velocity. To determine its fate under environmental conditions, TATB powder was irradiated with simulated sunlight and, in water, under UV light at 254 nm. The hydrolysis of particles submerged in neutral and alkaline solutions was also examined. We found that, by changing experimental conditions (e.g., light source, and mass and physical state of TATB), the intermediates and final products were slightly different. Mono-benzofurazan was the major transformation product in both irradiation systems. Two minor transformation products, the aci-nitro form of TATB and 3,5-diamino-2,4,6-trinitrophenol, were detected under solar light, while 1,3,5-triamino-2-nitroso-4,6-dinitrobenzene, 1,3,5-triamino-2,4-dinitrobenzene and mono-benzofuroxan were produced under UV light. The product identified as 3,5-diamino-2,4,6-trinitrophenol was identical to the one formed in the dark under alkaline conditions (pH 13) and in water incubated at either 50 °C or aged at ambient conditions. Interestingly, when only a few milligrams of TATB were irradiated with simulated sunlight, the aci-isomer and mono-benzofurazan derivative were detected; however, the hydrolysis product 3,5-diamino-2,4,6-trinitrophenol formed only much later in the absence of light. This suggests that the water released from TATB to form mono-benzofurazan was trapped in the interstitial space between the TATB layers and slowly hydrolyzed the relatively stable aci-nitro intermediate to 3,5-diamino-2,4,6-trinitrophenol. This environmentally relevant discovery provides data on the fate of TATB in surface environments exposed to sunlight, which can transform the insoluble substrate into more soluble and corrosive derivatives, such as 3,5-diamino-2,4,6-trinitrophenol, and that some hydrolytic transformation can continue even without light. 相似文献
993.
Maximum phonation duration (MPD) is a common assessment procedure in speech-language pathology. However, the specific contributions of the respiratory and phonatory components of the speech-production mechanism to this task are not typically assessed. Six women and 6 men with normal speech and voice were monitored for lung volume during a standard MPD task, and for laryngeal airway resistance (Rlaw) during a modified MPD (slow syllable-repetition) task. On average, subjects used 90% of their vital capacity (VC) for their best MPD trial. There was no systematic relation between MPD and VC for these subjects. Rlaw was strongly correlated with MPD for the men (r(s) = 0.886 for /a/; r(s) = 0.829 for /i/), but not for the women. Rlaw increased linearly as lung volume decreased (slope > 0.15) for a subset of trials (32%). This was a common pattern for four of the subjects. The clinical utility of MPD to assess breathing for speech is questioned because of the lack of association between MPD and VC, and some atypical laryngeal-valving strategies. 相似文献
994.
Nancy Ching Long Pan Xiao‐Ying Huang Jing Li 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(9):1124-1125
In the title compound, C5H4N2O4·H2O, the 3,5‐pyrazoledicarboxylic acid (H3pdc) molecules are joined into one‐dimensional chains by O—H?O and N—H?O hydrogen bonds, with distances of 2.671 (2) and 2.776 (2) Å, respectively. The one‐dimensional chains form a three‐dimensional structure via O—H?OW and OW—HW?N hydrogen bonds, with distances of 2.597 (3) and 2.780 (3) Å, respectively. In addition to the potential for forming open‐channel frameworks, access to the six coordination atoms of H3pdc can be directly controlled by varying the pH of the reaction environment, allowing further control over the design and synthesis of novel coordination polymers using various metal centers. 相似文献
995.
D. Abbott A. Ahmidouch H. Anklin J. Arvieux J. Ball S. Beedoe E.J. Beise L. Bimbot W. Boeglin H. Breuer 《The European Physical Journal A - Hadrons and Nuclei》2000,7(3):421-427
A rigorous extraction of the deuteron charge form factors from tensor polarization data in elastic electron-deuteron scattering,
at given values of the 4-momentum transfer, is presented. Then the world data for elastic electron-deuteron scattering is
used to parameterize, in three different ways, the three electromagnetic form factors of the deuteron in the 4-momentum transfer
range 0-7 fm−1. This procedure is made possible with the advent of recent polarization measurements. The parameterizations allow a phenomenological
characterization of the deuteron electromagnetic structure. They can be used to remove ambiguities in the form factors extraction
from future polarization data.
Received: 28 January 2000 / Revised version: 25 February 2000 相似文献
996.
Different combinations of acetals with trimethylsilyl iodide have been explored as new initiating systems for the vinyl ether polymerization. The resulting polymers are characterized by controlled molecular weights and narrow molecular weight distributions, confirming the living polymerization mechanism. Acetals can also be used as transfer agents in the polymerization of vinyl ethers. When using 1,1-diethoxyethane (DEE) as transfer agent and isobutyl vinyl ether (IBVE) as monomer, a transfer constant of 0.2 was obtained (at −40°C in toluene). This method, transposed to functional acetals, provides a new way to prepare polyvinyl ethers with one or two functional end groups. The cationic polymerization of isobutyl vinyl ether initiated with the combination triflic acid/thietane, where thietane acts as electron donating moderator, leads to star-shaped polyvinylether-polythietane block-copolymers (at −40°C in dichloromethane). The block-copolymer structure is obtained because the vinyl ether polymerization is stopped when the α-alkoxy thietanium ion (active species) is attacked by a thietane molecule, which is at the same time an initiation reaction for the thietane polymerization. The star-shaped structure of the block-polymer is the result of the intermolecular termination in the cationic polymerization of thietane. When using a bifunctional initiator system, a polymer network is obtained consisting of linear polyIBVE-segments interconnected by branched polythietane segments. These findings support the sulfonium ion structure of the active species in the cationic polymerization of vinyl ethers initiated by the acid-sulfide system. 相似文献
997.
Nancy DArminio Deborah Giordano Bernardina Scafuri Carmen Biancaniello Mauro Petrillo Angelo Facchiano Anna Marabotti 《Molecules (Basel, Switzerland)》2022,27(15)
The SARS-CoV-2 variant Omicron is characterized, among others, by more than 30 amino acid changes occurring on the spike glycoprotein with respect to the original SARS-CoV-2 spike protein. We report a comprehensive analysis of the effects of the Omicron spike amino acid changes in the interaction with human antibodies, obtained by modeling them into selected publicly available resolved 3D structures of spike–antibody complexes and investigating the effects of these mutations at structural level. We predict that the interactions of Omicron spike with human antibodies can be either negatively or positively affected by amino acid changes, with a predicted total loss of interactions only in a few complexes. Moreover, our analysis applied also to the spike-ACE2 interaction predicts that these amino acid changes may increase Omicron transmissibility. Our approach can be used to better understand SARS-CoV-2 transmissibility, detectability, and epidemiology and represents a model to be adopted also in the case of other variants. 相似文献
998.
Multiple Blocking Sets and Arcs in Finite Planes 总被引:3,自引:0,他引:3
This paper contains two main results relating to the size ofa multiple blocking set in PG(2, q). The first gives a verygeneral lower bound, the second a much better lower bound forprime planes. The latter is used to consider maximum sizes of(k, n)-arcs in PG(2, 11) and PG(2, 13), some of which are determined.In addition, a summary is given of the value of mn(2, q) forq 13. 相似文献
999.
Yuxuan Ding Simon S. Pedersen Alex Lin Ruoyu Qian Zachary T. Ball 《Chemical science》2022,13(47):14101
Sulfoximines are emerging moieties for medicinal and biological chemistry, due in part to their efficacy in selective inhibition of amide-forming enzymes such as γ-glutamylcysteine synthetase. While small-molecule sulfoximines such as methionine sulfoximine (MSO) and its derivatives are well studied, structures with methionine sulfoximine residues within complex polypeptides have been generally inaccessible. This paper describes a straightforward means of late-stage one-step oxidation of methionine residues within polypeptides to afford NH-sulfoximines. We also present chemoselective subsequent elaboration, most notably by copper(ii)-mediated N–H cross-coupling at methionine sulfoximine residues with arylboronic acid reagents. This development serves as a strategy to incorporate diverse sulfoximine structures within natural polypeptides, and also identifies the methionine sulfoximine residue as a new site for bioorthogonal, chemoselective bioconjugation.Sulfoximines are emerging moieties for medicinal and biological chemistry. This work describes the late-stage incorporation of methionine sulfoximine residues into polypeptides and chemoselective subsequent elaboration of NH-sulfoximines. 相似文献
1000.
生物酶必须在细胞内拥挤环境下行使功能。最新的研究表明,生物酶或许利用这种复杂环境而更加高效的工作。驱动蛋白是一类可以沿着微管定向输运物质的马达蛋白,当施加某种扰动时可以加快驱动蛋白的运动。实验与模拟结果同时表明,许多蛋白在嘈杂环境中的工作效率比在安静环境中的更高。 相似文献