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991.
992.
The stereoselectivities of base-catalyzed enolizations of ketones have been studied by quantum mechanical methods. Transition structures of exo and endo deprotonation of camphor, norcamphor, and dehydronorcamphor have been located with two model bases. Stereoelectronic, torsional, and steric effects on activation energies were assessed. These calculations demonstrate the importance of torsional strain between partial bonds and vicinal bonds on the rates of deprotonation and on related reactions involving the formation of enolates or reactions of enols and enolates. 相似文献
993.
994.
Ono S Ando Y Murayama T Balakirev FF Betts JB Boebinger GS 《Physical review letters》2000,85(3):638-641
We measure the normal-state in-plane resistivity of Bi(2)Sr(2-x)La(x)CuO(6+delta) single crystals at low temperatures by suppressing superconductivity with 60 T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to an insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates. 相似文献
995.
996.
997.
Encapsulation of emitting CdTe QDs within silica beads to retain initial photoluminescence efficiency 总被引:1,自引:0,他引:1
Highly luminescent silica beads (30 nm-2 mum slashed circle) incorporating CdTe quantum dots (QDs) were prepared via a two-step preparation procedure, namely a modified St?ber synthesis and a subsequent reverse micelle route. In the modified St?ber synthesis, the silica molecules are deposited on the surface of the QDs. After this first step, these coated QDs were incorporated into silica beads via a reverse micelle route. Inductively coupled plasma analysis revealed a red-emitting silica bead of 30 nm in diameter thus prepared encapsulated roughly 14 CdTe QDs. These glass beads (30-40 nm slashed circle) retained the initial photoluminescence (PL) efficiencies of the colloidal QDs (27 and 65% for the green- and red-emitting beads, respectively). The protection of QDs by a silica layer at the first step, together with the short total reaction time, is the main reason for the retention of the PL efficiency. The size of the glass beads can be easily controlled over the wide range by adjusting the injection speed and the ratio of chemicals used for the reverse micelle preparation. Since the original efficiency was maintained in the beads and is the highest ever reported for QD-containing silica beads, the method presented here is of significant importance for applications of silica beads to biological probes. 相似文献
998.
The quantitative analysis of ketones using DNPH is usually conducted in the presence of an acid catalyst. However, this method may cause an analytical error because 2,4-dinitrophenylhydrazones have both E- and Z-stereoisomers. Purified ketone-2,4-dinitrophenylhydrazone comprised only the E-isomer. However, under the addition of acid, both E- and Z-isomers were seen. In the case of 2-butanone-, 2-pentanone- and 2-hexanone-2,4-dinitrophenylhydrazone, the equilibrium Z/E isomer ratios were 0.20, 0.21 and 0.22, respectively. In addition, when trace water was added to the hydrazone derivatives in acetonitrile solution, the concentration of ketone derivatives were seen to decrease and the concentration of free DNPH was seen to increase. The decomposition rate of 2-butanone-2,4-dinitrophenylhydrazone was dependent on the concentration of acid-catalysis and reached an equilibrium state - carbonyl, DNPH, hydrazone-derivative and H2O - within 10 h at 0.1 mol L−1 phosphoric acid solution. The equilibrium constants of ketone-2,4-dinitrophenylhydrazones, [carbonyl] [DNPH]/[hydrazone] [H2O], were relatively large and ranged from 0.74 × 10−4 to 5.9 × 10−4. Hydrazone derivatives formed from 2-ketones such as 2-pentanone, 2-hexanone and 4-methyl-2-pentanone showed lower equilibrium constants than corresponding 3-ketones. Consequently, only a minimum concentration of catalytic acid must be added. The best method for the determination of ketone-2,4-dinitrophenylhydrazones by HPLC or GC is to add phosphoric acid to both the standard reference solution and samples, forming a 0.001 mol L−1 acid solution, and analyze after 27 h. 相似文献
999.
The evolution of the electronic structure of molecular aggregates is investigated using anion photoelectron (PE) spectroscopy for anionic clusters of anthracene (Ac) and its alkyl derivatives: 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-methylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA). For their monomer anions (n=1), electron affinities are confined to the range from 0.47 to 0.59 eV and are well reproduced by density functional theory calculations, showing the isoelectronic character of these molecules. For cluster anions (n=2-100) of Ac and 2MA, two types of isomers I and II coexist over a wide size range: isomers I and II-1 (4< or =n<30) or isomers I and II-2 (n> or = approximately 40 for Ac and n> or = approximately 55 for 2MA). However, for the other alkyl-substituted Ac cluster anions (i.e., 1MA, 9MA, DMA, and 2TBA), only isomer I is exclusively formed, and neither isomer II-1 nor II-2 is observed. The vertical detachment energies (VDEs) of isomer I in all the anionic clusters depend almost linearly on n(-1/3). In contrast, the VDEs of isomers II-1 (n> or =14) and II-2 (n=40-100), appeared only in Ac and 2MA cluster anions, remain constant with n and are approximately 0.5 eV lower than those of isomer I. The PE spectra revealed the characteristics of each isomer: isomer I possesses a monomeric anion core that is gradually embedded into the interior of the cluster with increasing n. On the other hand, isomers II-1 and II-2 possess a multimeric (perhaps tetrameric) anion core, but they differ in the number of layers from which they are made up; monolayer (isomer II-1) and multilayers (isomer II-2) of a two-dimensionally ordered, finite herringbone-type structure, in which electron attachment produces only little geometrical rearrangement. Moreover, the agreement of the constant VDEs of isomer II-2 with the bulk data demonstrates the largely localized nature of the electronic polarization around the excess charge in a crystal-like environment, where about 50 molecules provide a charge stabilization energy comparable to the bulk. 相似文献
1000.
Sakata Y Kuramoto M Ando K Yamaguchi M Kawasaki I Kunitomo J Yokomizo T Ohishi Y 《Organic & biomolecular chemistry》2007,5(19):3083-3086
A series of 3-(4-chlorophenyl)-2-(2-aminothiazol-4-yl)benzo[b]furan derivatives 6-10 were prepared and their leukotriene B(4) inhibitory activity was evaluated. We found that several compounds showed strong inhibition of calcium mobilization in CHO cells overexpressing human BLT(1) and BLT(2) receptors. Among them, 3-(4-chlorophenyl)-2-[5-formyl-2-[(dimethylamino)methyleneamino]thiazol-4-yl]-5-methoxybenzo[b]furan 9b showed the most potent and selective inhibition for the human BLT(2) receptor, and its IC(50) value was smaller than that of the selected positive control compound, ZK-158252. 相似文献