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201.
202.
The pyrolysis of ethylene–butene-2 mixtures has been studied in a static system over the temperature range of 689°-754°k and for initial pressures of each olefin of 20–200 torr. The two main addition products were cyclopentene and 3-methylpentene-1. Kinetic evidence indicated that cyclopentene was formed from radical processes while 3-methylpentene-1 was formed by the molecular “ene¨?” addition of ethylene to butene-2 through a six-center transition state. The following rate constants were obtained: The pyrolysis of 3-methylpentene-1 has been studied over the same temperature range and for initial pressures of 20–100 torr. Kinetic evidence showed that the products ethylene and butenes were formed in both radical and molecular processes. Estimates of the rate constant k?1t and k?1c were, however, in reasonable agreement with the measurements of k1t and k1c. The mechanism of the ene reaction is discussed, and it is concluded that the transition state does not involve the formation of a biradical. 相似文献
203.
204.
A series of aliphatic diselenides and selenides containing coordinating substituents was tested for glutathione peroxidase (GPx)-like catalytic activity in a model system in which the reduction of tert-butyl hydroperoxide with benzyl thiol to afford dibenzyl disulfide and tert-butyl alcohol was performed under standard conditions and monitored by HPLC. Although the diselenides showed generally poor catalytic activity, allyl selenides proved more effective. In particular, allyl 3-hydroxypropyl selenide (25) rapidly generated 1,2-oxaselenolane Se-oxide (31) in situ by a series of oxidation and [2,3]sigmatropic rearrangement steps. The remarkably active cyclic seleninate 31 proved to be the true catalyst, reacting with the thiol via a postulated mechanism in which the thioseleninate 32 is first produced, followed by further thiolysis to selenenic acid 33 and oxidation-dehydration to regenerate 31. In contrast to catalysis with GPx, formation of the corresponding selenenyl sulfide 34 comprises a competing deactivation pathway in the catalytic cycle of 31, as a separate experiment revealed that authentic 34 was a much less effective catalyst than 31. 1,2-Oxaselenane Se-oxide (37), the six-membered homologue of 31, was formed similarly from allyl 4-hydroxybutyl selenide (26), but proved a less effective catalyst than 31. Compounds 31 and 37 are the first examples of unsubstituted monocyclic seleninate esters. 相似文献
205.
Morita-Baylis-Hillman reaction and cyclization of 1-(p-toluenesulfonyl)-1,3-butadiene with aldimines
[reaction: see text] Aldimines 2 underwent Morita-Baylis-Hillman reaction with 1-(p-toluenesulfonyl)-1,3-butadiene (3) in the presence of 3-hydroxyquinuclidine (HQD) to afford adducts 4. The E-isomers of the products cyclized to the corresponding functionalized piperidines 8 under base-catalyzed conditions. Simultaneous equilibration of (E)-4 and (Z)-4 was effected by photoisomerization to improve the efficiency of the cyclization. 相似文献
206.
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208.
The decomposition of ethylene by pulsed, unfocussed CO2-laser radiation has been studied at pressures from 500 to 3000 Torr, using the P(14) line of the 10.6m band (v=949.48cm–1) at incident fluences from about 0.1 to 1.0J/cm2. Major products in order of decreasing importance were 1,3-butadiene, acetylene, ethane, propane, 1-butene and methane. These are known products of the thermal free-radical chain decomposition, and it is concluded that the laser-induced decomposition under our conditions is a transient bulk thermal reaction occurring in a thin disc of heated gas close to the entrance window of the reaction vessel at temperatures ranging from about 1000 to 1500K. As in the thermal decomposition, cyclobutane was observed to be a minor product, which in a sequence of laser pulses approached a final constant concentration. The possibility that this corresponded to an equilibrium concentration at some effective reaction temperature was explored. Computer simulation was used to model the accumulation of cyclobutane in the system, both in a single pulse and in a sequence of pulses, and predictions of this model were compared with experiment. It was concluded that cyclobutane could be used in this way as an approximate internal thermometer, within certain limits. Mechanisms of formation of the free-radical chain products are discussed. It is concluded that the chains are initiated by the bimolecular disproportionation reaction, 2C2H4 C2H3+C2H5, and that secondary initiation by dissociation of the product, 1-butene, becomes increasingly important as the reaction proceeds, leading to autocatalysis. It is further concluded that the radical chain decomposition in this system is a transient process occurring in a brief time interval following the short laser pulse (FWHM=110ns), and is far from steady-state conditions. 相似文献
209.
Rafaela G.M. da Costa Francisco R.L. Farias Davi Back Jones Limberger 《Tetrahedron letters》2018,59(8):771-775
A useful protocol for arylation of the olefin double bond of chalcones to afford tri- and tetra-substituted chalcone derivatives is reported. The protocol begins with the Heck reaction between chalcones and aryl iodides providing β-arylchalcones. This reaction tolerates various functional groups on both rings, as well as deactivated aryl iodides. The products are obtained in moderate to excellent yields and the (E)-β-arylchalcones (E:Z?>?96:4) can be isolated via precipitation. Competitive Heck reactions pointed to a significant effect of ring one substituents on the reaction rate, while substituents on ring two have a much smaller effect. To access α,β-diarylchalcones, a sequential bromination-Suzuki cross coupling strategy was applied to the β-arylated compounds which afforded double arylated chalcone derivatives in 60–99% yield over two steps. 相似文献
210.
The enamide moeity of the title azasteroid reacted with benzeneseleninic anhydride to afford products derived chiefly from a Pummerer-type reaction at C-6. The intermediate selenoxide was also prepared by oxidation of the corresponding selenide. 相似文献