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991.
Braunschweig H Gross M Hammond K Friedrich M Kraft M Oechsner A Radacki K Stellwag S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(29):8972-8979
We report a series of [2]borametallocenophanes of Ti, Zr, and Hf with various ligand systems. The ligands have been synthesized in high yields starting from 1,2-dibromo-1,2-bis(dimethylamino)diborane(4) upon reaction with Na[C5H5] and Li[C13H9], respectively. All compounds were fully characterized by multinuclear NMR spectroscopy and, for selected examples, by X-ray analysis. 相似文献
992.
Kuliszewska E Hanbauer M Hammerschmidt F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(28):8603-8614
The enantiomers of 1-phenylethylamine were phosphorylated with diethyl chlorophosphate/Et(3)N and then Boc-protected (Boc=tert-butoxycarbonyl) at the nitrogen atom. These phosphoramidates were metalated by using sBuLi/N,N,N',N'-tetramethylethylenediamine (TMEDA) to give alpha-aminobenzyllithiums that isomerised to alpha-aminophosphonates in yields of up to 80 % with retention of the configuration at the carbon atom. The intermediate tertiary organolithiums were found to be microscopically configurationally stable from -78 to 0 degrees C in Et(2)O. The protected alpha-aminophosphonates were deblocked by using boiling 6 M HCl or preferably Me(3)SiBr/(allyl)SiMe(3). When the Boc group was replaced by the diethoxyphosphinyl group, the alpha-aminobenzyllithium intermediate partially enantiomerised even at -78 degrees C and rearranged to yield an alpha-aminophosphonate with 50 % ee (ee=enantiomeric excess). Similarly, N-Boc-protected phosphoramidates derived from racemates and/or enantiomers of 1-(1-naphthyl)ethyl-, 1-indanyl- and 1,2,3,4-tetrahydro-1-naphthylamine or 1-azidoindan- and 1-azido-1,2,3,4-tetrahydronaphthalene were converted to aminophosphonates in good yields. Deblocking gave alpha-aminophosphonic acids of excellent enantiomeric excess (97-99 %), as determined by means of HPLC on a chiral ion-exchange stationary phase based on quinine carbamate. When racemic Boc-protected diethyl phosphoramidate derived from 1,2,3,4-tetrahydro-1-naphthylamine was metalated with LiTMP/TMEDA (TMP=2,2,6,6-tetramethylpiperidine), 1-hydroxyethylphosphonamidates resulted. The configuration of the main isomer was determined by means of a single-crystal X-ray structure analysis. 相似文献
993.
Lang S von Philipsborn AC Bernard A Bonhoeffer F Bastmeyer M 《Analytical and bioanalytical chemistry》2008,390(3):809-816
A microfluidic network (μFN) etched into a silicon wafer was used to deliver protein solutions containing different concentrations
of the axonal guidance molecule ephrinA5 onto a silicone stamp. In a subsequent microcontact printing (μCP) step, the protein
was transferred onto a polystyrene culture dish. In this way, stepwise substrate-bound concentration gradients of ephrinA5
were fabricated spanning a total distance of 320 μm. We tested the response of chick retinal ganglion cell (RGC) axons, which
are guided in vivo by ephrin gradients, to these in vitro gradients. Temporal, but not nasal axons stop at a distinct zone
in the gradient, which is covered with a certain surface density of substrate-bound ephrinA5. Within the temporal RGC population,
all axons respond uniformly to the gradients tested. The position of the stop zone depends on the slope of the gradient with
axons growing further into the gradient in shallow gradients than in steep gradients. However, axons stop at lower ephrinA5
concentrations in shallow gradients than in steep gradients, indicating that the growth cone can adjust its sensitivity during
the detection of a concentration gradient of ephrinA5.
Susanne Lang and Anne C. von Philipsborn contributed equally to this work: S.L. performed the experiments; A.P. evaluated
the data and wrote the paper. 相似文献
994.
The construction of good extensible rank-1 lattices 总被引:1,自引:0,他引:1
Josef Dick Friedrich Pillichshammer Benjamin J. Waterhouse. 《Mathematics of Computation》2008,77(264):2345-2373
It has been shown by Hickernell and Niederreiter that there exist generating vectors for integration lattices which yield small integration errors for for all integers . This paper provides algorithms for the construction of generating vectors which are finitely extensible for for all integers . The proofs which show that our algorithms yield good extensible rank-1 lattices are based on a sieve principle. Particularly fast algorithms are obtained by using the fast component-by-component construction of Nuyens and Cools. Analogous results are presented for generating vectors with small weighted star discrepancy.
995.
996.
Friedrich MG Kirste VU Zhu J Gennis RB Knoll W Naumann RL 《The journal of physical chemistry. B》2008,112(10):3193-3201
A systematic study of the influence of the packing density of proteins on their activity is performed with cytochrome c oxidase (CcO) from R. sphaeroides as an example. The protein was incorporated into a protein-tethered bilayer lipid membrane and CcO was genetically engineered with a histidine-tag, attached to Subunit II, and then tethered by an interaction with functionalized thiol compounds bound to a gold electrode. The packing density was varied by diluting the functionalized thiol with a nonfunctionalized thiol that does not bind to the enzyme. After attaching the CcO to the gold surface, a lipid bilayer was formed to incorporate the tethered proteins. The reconstituted protein-lipid bilayer was characterized by surface enhanced infrared reflection absorption spectroscopy (SEIRAS), electrochemical impedance spectroscopy, surface plasmon resonance, and atomic force microscopy. The activity of the proteins within the reconstituted bilayer was probed by direct electrochemical electron injection and was shown to be very sensitive to the packing density of protein molecules. At low surface density of CcO, the bilayer did not effectively form, and protein aggregates were observed, whereas at very high surface density, very little lipid is able to intrude between the closely packed proteins. In both of these cases, redox activity, measured by the efficiency to accept electrons, is low. Redox activity of the enzyme is preserved in the biomimetic structure but only at a moderate surface coverage in which a continuous lipid bilayer is present and the proteins are not forced to aggregate. Electrostatic and other interaction forces between protein molecules are held responsible for these effects. 相似文献
997.
Xiaobo Sun Thomas Hansen Jordi Poater Trevor A. Hamlin Friedrich Matthias Bickelhaupt 《Journal of computational chemistry》2023,44(4):495-505
We have quantum chemically studied the iron-mediated C X bond activation (X = H, Cl, CH3) by d8-FeL4 complexes using relativistic density functional theory at ZORA-OPBE/TZ2P. We find that by either modulating the electronic effects of a generic iron-catalyst by a set of ligands, that is, CO, BF, PH3, BN(CH3)2, or by manipulating structural effects through the introduction of bidentate ligands, that is, PH2(CH2)nPH2 with n = 6–1, one can significantly decrease the reaction barrier for the C X bond activation. The combination of both tuning handles causes a decrease of the C H activation barrier from 10.4 to 4.6 kcal mol−1. Our activation strain and Kohn-Sham molecular orbital analyses reveal that the electronic tuning works via optimizing the catalyst–substrate interaction by introducing a strong second backdonation interaction (i.e., “ligand-assisted” interaction), while the mechanism for structural tuning is mainly caused by the reduction of the required activation strain because of the pre-distortion of the catalyst. In all, we present design principles for iron-based catalysts that mimic the favorable behavior of their well-known palladium analogs in the bond-activation step of cross-coupling reactions. 相似文献
998.
Cover Picture: Graphene screen‐printed radio‐frequency identification devices on flexible substrates (Phys. Status Solidi RRL 11/2016) 下载免费PDF全文
999.
1000.
Kai Pahnke Josef Brandt Dr. Ganna Gryn'ova Dr. Ching Y. Lin Dr. Ozcan Altintas Dr. Friedrich G. Schmidt Dr. Albena Lederer Prof. Michelle L. Coote Prof. Christopher Barner‐Kowollik 《Angewandte Chemie (International ed. in English)》2016,55(4):1514-1518
We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high‐temperature (HT) NMR spectroscopy of self‐assembled supramolecular diblock systems as well as temperature‐dependent size‐exclusion chromatography (TD SEC) of covalently bonded Diels–Alder step‐growth polymers. 相似文献