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991.
Jack Huet 《Tetrahedron》1981,37(4):731-741
13C NMR chemical shifts of ethylenic carbon atom Cβ of α alkoxy-indenes and α-alkoxy-styrenes are affected by the conformation of the OR group with respect to the double bond and hence the efficiency of p- Π conjugation but, contrary to previous results, δCα is not affected. The phenyl group exerts a diamagnetic Π- effect on Cα and Cβ which is a function of its dihedral angle with the double bond. Although δCβ is especially sensitive to the Π-electron density, the chemical shifts of the ethylenic carbons are best understood by considering the total charge density. The σ and Π contributions to the total charge density are discussed using the preferred conformation of the molecules. This approach strongly suggests that the correlation between the gradual shift of δCα to low frequency and the gradual shift of δCβ to high frequency (relative to TMS) as R changes from RCH3 to RtC4H9 in certain series of enol-ethers is not due to pΠ conjugation variations. This method of interpreting 13C NMR shifts, as opposed to previous methods, is compatible with explanations of other physico-chemical properties of alkyl vinyl ethers. Although it is sometimes possible to correlate the gradual shifts of δCβ with (σ + Π) electron density variations in a homogeneous series, it seems impossible to predict the relation. In all cases, there is no evident correlation between gradual shifts of δCβ(orδCα) and the reactivity of enol ethers because one cannot consider the role of the out-of-plane conformation of the alkoxy group (which increases from RCH3 to RtC4H9); however, the out of plane conformation does not have an identical effect on δCβ and the reactivity. 相似文献
992.
A finite groupG isQ-admissible if there exists a division algebra finite dimensional and central overQ which is a crossed product forG. AQ-admissible group is necessarily Sylow-metacyclic (all its Sylow subgroups are metacyclic). By means of an investigation into the structure of Sylow-metacyclic groups, the inverse problem (is every Sylow-metacyclic groupQ-admissible?) is essentially reduced to groups of order 2 a 3 b and to a list of known “almost simple” groups. 相似文献
993.
V—V energy transfer from a large molecule excited to vibrational energies of chemical interest has been demonstrated by detection of ≈ 1.5% yield of CO2(001) due to energy transfer from azulene (Evib ≈ 30600 cm?1. Also, the average enery lost per collision by azulene was measured as a function of Evib, and the rate constant for CO2(001) deactivation by azulene was determined. 相似文献
994.
Dr. Flavio della Sala Dr. Jack L.‐Y. Chen Simona Ranallo Dr. Denis Badocco Prof. Dr. Paolo Pastore Prof. Dr. Francesco Ricci Prof. Dr. Leonard J. Prins 《Angewandte Chemie (International ed. in English)》2016,55(36):10737-10740
A catalytic system based on monolayer‐functionalized gold nanoparticles (Au NPs) that can be electrochemically modulated and reversibly activated is reported. The catalytic activity relies on the presence of metal ions (Cd2+ and Cu2+), which can be complexed by the nanoparticle‐bound monolayer. This activates the system towards the catalytic cleavage of 2‐hydroxypropyl‐p‐nitrophenyl phosphate (HPNPP), which can be monitored by UV/Vis spectroscopy. It is shown that Cu2+ metal ions can be delivered to the system by applying an oxidative potential to an electrode on which Cu0 was deposited. By exploiting the different affinity of Cd2+ and Cu2+ ions for the monolayer, it was also possible to upregulate the catalytic activity after releasing Cu2+ from an electrode into a solution containing Cd2+. Finally, it is shown that the activity of this supramolecular nanosystem can be reversibly switched on or off by oxidizing/reducing Cu/Cu2+ ions under controlled conditions. 相似文献
995.
MoS2/WS2‐Graphene Composites through Thermal Decomposition of Tetrathiomolybdate/Tetrathiotungstate for Proton/Oxygen Electroreduction 下载免费PDF全文
Jan Luxa Jack Fawdon Zdeněk Sofer Vlastimil Mazánek Martin Pumera 《Chemphyschem》2016,17(18):2890-2896
MoS2 and WS2 have been prepared on a conductive graphene support by thermal reduction of tetrathiotungstate/tetrathiomolybdate and graphite oxide. Whereas the catalytic properties towards hydrogen evolution are strongly influenced by the Magnéli phases formed as a byproduct during the synthesis, the catalytic activity towards oxygen reduction of these composite materials is not affected by this phenomenon and these materials exhibit high catalytic activity towards this industrially important reaction. 相似文献
996.
997.
Glasson CR Meehan GV Clegg JK Lindoy LF Turner P Duriska MB Willis R 《Chemical communications (Cambridge, England)》2008,(10):1190-1192
A rigid linear bis-bidentate quaterpyridine undergoes metal directed self-assembly with iron(ii) salts yielding M(4)L(6) host-guest complexes; selective anion binding for PF(6)(-) over BF(4)(-) is observed. 相似文献
998.
Waters T Boulton J Clark T Gallen MJ Williams CM O'Hair RA 《Organic & biomolecular chemistry》2008,6(14):2530-2533
The gas phase anion proton affinities of the periodinane anions IBX- and IBA- were examined using mass spectrometry-based experiments, and estimated as 1300 +/- 25 and 1390 +/- 10 kJ mol(-1), respectively. The experimental results were supported by theoretical calculations, which yielded proton affinities of 1336 and 1392 kJ mol(-1) for IBX- and IBA- respectively, at the B3LYP/aug-cc-PVDZ level of theory. 相似文献
999.
The nitroxide spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-methyl-methanethiosulfonate (MTSSL), commonly used in site-directed spin labeling of proteins, is studied with molecular dynamics (MD) simulations. After developing force field parameters for the nitroxide moiety and the spin label linker, we simulate MTSSL attached to a polyalanine alpha-helix in explicit solvent to elucidate the factors affecting its conformational dynamics. Electron spin resonance spectra at 9 and 250 GHz are simulated in the time domain using the MD trajectories and including global rotational diffusion appropriate for the tumbling of T4 Lysozyme in solution. Analysis of the MD simulations reveals the presence of significant hydrophobic interactions of the spin label with the alanine side chains. 相似文献
1000.
Jack Huizenga 《Discrete Mathematics》2008,308(11):2134-2148
The chromatic capacityχcap(G) of a graph G is the largest k for which there exists a k-coloring of the edges of G such that, for every coloring of the vertices of G with the same colors, some edge is colored the same as both its vertices. We prove that there is an unbounded function f:N→N such that χcap(G)?f(χ(G)) for almost every graph G, where χ denotes the chromatic number. We show that for any positive integers n and k with k?n/2 there exists a graph G with χ(G)=n and χcap(G)=n-k, extending a result of Greene. We obtain bounds on that are tight as r→∞, where is the complete n-partite graph with r vertices in each part. Finally, for any positive integers p and q we construct a graph G with χcap(G)+1=χ(G)=p that contains no odd cycles of length less than q. 相似文献