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281.
Baba K Ono H Itoh E Itoh S Noda K Usui T Ishihara K Inamo M Takagi HD Asano T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(20):5328-5333
Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4'-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R = butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4'-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers-Grote-Hynes model or by the Agmon-Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 10(4)-10(7) s(-1). 相似文献
282.
Eri Yoshida 《Colloid and polymer science》2010,288(3):341-345
In order to clarify the initiator factor dominating the molecular weight distribution of the resulting polymer, the nitroxide-mediated
photo-living radical polymerization of methyl methacrylate was performed using eight different kinds of azoinitiators: i.e.,
2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile),
racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile), meso-(2RS,2′SR)-azobis(4-methoxy-2,4-dimethylvaleronitrile),
dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(N-butyl-2-methylpropionamide). The bulk polymerization was carried out at room temperature for 3 h using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl
(MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as the photo-acid generator. All
the initiators provided a molecular weight distribution below 1.7 for the MTEMPO/initiator ratio of 2, although at the ratio
of unity, about half of the initiators produced the molecular weight distribution around 2.3–3.4. The UV analysis revealed
that the initiators having a higher ε value tended to more strictly control the molecular weight and provide a higher initiator
efficiency. The half-lives of the initiators had little effect on the molecular weight control and initiator efficiency. 相似文献
283.
Eri Yoshida 《Colloid and polymer science》2010,288(9):1027-1030
The stability of the growing polymer chain ends for the nitroxide-mediated photo-living radical polymerization of methyl methacrylate
(MMA) was explored through block copolymerization with isopropyl methacrylate (
i
PMA). The block copolymerization of
i
PMA was performed with the PMMA prepolymer prepared by the photopolymerization of MMA using the racemic-(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) (r-AMDV) as the initiator, 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator, and (4-tert-butylphenyl)-diphenylsulfonium triflate (
t
BuS) as the photo-acid generator. When the polymerization of MMA was carried out for 6.5 h, the resulting block copolymer
showed a bimodal GPC due to the deactivation of part of the growing chain ends of the prepolymer. On the other hand, when
the MMA polymerization was shortened to 5 h, the unimodal block copolymer was obtained without deactivation of the prepolymer. 相似文献
284.