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排序方式: 共有325条查询结果,搜索用时 156 毫秒
151.
A class of stationary, rigidly rotating perfect fluids coupled with nonlinear electromagnetic fields was investigated. An exact solution of the Einstein equations with sources for the Carter B(+) branch was found for the equation of state 3p+=const. We use a structure function for the Born-Infeld nonlinear electrodynamics which is invariant under duality rotations and a metric possessing a four-parameter group of motions. The solution is of Petrov type D and the eigenvectors of the electromagnetic field are aligned to the Debever-Penrose vectors. 相似文献
152.
153.
Davies SG Rodriguez-Solla H Tamayo JA Cowley AR Concellon C Garner AC Parkes AL Smith AD 《Organic & biomolecular chemistry》2005,3(8):1435-1447
The highly diastereoselective samarium diiodide and D(2)O-promoted conjugate reduction of homochiral (E)- and (Z)-benzylidene and isobutylidene diketopiperazines (E)-5,7 and (Z)-6,8 has been demonstrated. This methodology allows the asymmetric synthesis of methyl (2S,3R)-dideuteriophenylalanine 27 in > or = 95% de and >98% ee, and (2S,3R)- or (2S,3S)-dideuterioleucine-(S)-phenylalanine dipeptides 37 and 38 in moderate de, 66% and 74% respectively. A mechanism is proposed to account for this process. 相似文献
154.
Carolina Morales Moraes Priscila Abrami Daniele Riberio de Araujo Angélica F. A. Braga Michelle Georges Issa Humberto G. Ferraz Eneida de Paula Leonardo Fernandes Fraceto 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):313-316
An inclusion complex was prepared between the local anesthetic lidocaine (LDC) and hydroxypropyl-β-CD (HP-β-CD). The complex
was characterized by thermal analysis (differential scanning calorimetry, DSC), UV absorption and high-pressure liquid chromatography
(HPLC). DSC results were indicative of complexation, due to the loss of the characteristic endothermic peak of LDC (77 °C).
Phase-solubility diagrams allowed the determination of the association constant between LDC and HP-β-CD (35.7 ± 4.7 M−1). The rate of LDC release decreased after complexation and thermodynamic parameters from the HPLC studies (ΔG° = −2.65 kJ/mol)
revealed that a stable complex was formed. 相似文献
155.
156.
Hernández-Cobos J Saint-Martin H Mackie AD Vega LF Ortega-Blake I 《The Journal of chemical physics》2005,123(4):044506
Coexistence properties for water near the critical point using several ab initio models were calculated using grand canonical Monte Carlo simulations with multiple histogram reweighting techniques. These models, that have proved to yield a good reproduction of the water properties at ambient conditions, perform rather well, improving the performance of a previous ab initio model. It is also shown that bulk geometry and dipole values, predicted by the simulation, can be used and a good approximation obtained with a polarizable rigid water model but not when polarization is excluded. 相似文献
157.
Velásquez P Leinen D Pascual J Ramos-Barrado JR Grez P Gómez H Schrebler R Del Río R Córdova R 《The journal of physical chemistry. B》2005,109(11):4977-4988
Electrodic surfaces of natural chalcopyrite and natural pyrite minerals (El Teniente mine, Chile) have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy including microanalysis (SEM/EDX). For comparison, fractured and polished mineral surfaces were also studied by XPS. In both electrodes, the formation of Fe(III) species containing oxygen were detected and Cu(II) species containing oxygen were additionally detected for chalcopyrite at advanced oxidation states. The presence of Cu(II) species containing oxygen was not detected by XPS for the initial oxidation states of the chalcopyrite. For pyrite, the present results do not allow confirmation of the presence of polysulfurs such as have been previously proposed. In both minerals, the measurements of SEM and EDX show relevant alterations in the respective surfaces when different potential values were applied. The chalcopyrite surface shows the formation of protrusions with a high concentration of oxygen. The pyrite surface shows a layer of modified material with high oxygen content. The modifications detected by XPS, SEM, and EDX allowed the explanation of the complexity of the equivalent circuit used to simulate the experimental EIS data. At high oxidation states, both minerals showed a pseudoinductive loop in the equivalent circuit, which was due to the active electrodissolution of the minerals which takes place through a surface film previously formed. 相似文献
158.
Merlin Rosales Ysaías Alvarado Mirixa Boves Raúl Rubio Humberto Soscún Roberto A. Sánchez-Delgado 《Transition Metal Chemistry》1995,20(3):246-251
Summary A kinetic study of the regioselective homogeneous hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) was carried out using the cationic complex [RuH(CO)(NCMe)2(PPh3)2]BF4 (1) as the precatalyst. The experimentally determined rate law wasr = {k
2
K
1/(1+K
1[H2])}[Ru0][H2]2, which becomesr = {k
2
K
1[Ru0]–[H2]2 at low hydrogen concentrations (k
2
K
1 = 28.5M
–2 s–1 at 398 K). The corresponding activation parameters were found to be H
= 42 + 6 kJ mol–1, S
= – 115 ± 2JK–1mol–1 and G
= 92 ± 8 kJ mol–1. Complex(1) was found to react with Q in CHCl3 under reflux to yield [RuH(CO)(NCMe)(N-Q)(PPh3)2]BF4
(2) which was also isolated from the hydrogenation runs. These experimental findings, together with the results ofab initio self-consistent-field molecular orbital calculations on the free organic molecules involved, are consistent with a mechanism involving a rapid and reversible partial hydrogenation of(2) to yield the corresponding dihydroquinoline (DHQ) species [RuH(CO)(NCMe)(DHQ)(PPh3)2]BF4
(4), followed by a rate-determining second hydrogenation of DHQ to yield [RuH(CO)(NCMe)(THQ)(PPh3)2]BF4
(3). 相似文献
159.
Ann M Chippindale Stephen G Davies Keiji IwamotoRichard M Parkin Christian A.P SmethurstAndrew D Smith Humberto Rodriguez-Solla 《Tetrahedron》2003,59(18):3253-3265
Diastereoselective conjugate addition of lithium (S)-N-allyl-N-α-methylbenzylamide to a range of α,β-unsaturated esters followed by ring closing metathesis is used to afford efficiently a range of substituted cyclic β-amino esters in high d.e. Alternatively, conjugate addition to α,β-unsaturated Weinreb amides, functional group conversion and ring closing metathesis affords cyclic amines in high d.e. The further application of this methodology to the synthesis of a range of carbocyclic β-amino esters via conjugate addition, enolate alkylation and ring closing metathesis is also described. Application of this methodology affords, after deprotection, (S)-homoproline, (S)-homopipecolic acid, (S)-coniine and (1S,2S)-trans-pentacin. 相似文献
160.