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21.
The G3(MP2) method has been employed to study the 1,4-addition reactions between singlet oxygen and five acenes, including benzene, naphthalene, anthrecene, tetracene, and pentacene. In all, nine pathways between O(2) and the five acenes have been investigated. Our calculated results indicate that all nine pathways are concerted and exothermic and that the most reactive sites on the acenes are the center ring's meso-carbons. In addition, reactivity increases along the series benzene < naphthalene < anthrecene < tetracene < pentacene. This trend is identical to that of aromaticity for the five acenes. A correlation between reactivity and aromaticity is briefly rationalized with natural bond orbital (NBO) analysis and frontier molecular orbital (FMO) analysis. Furthermore, some experimental kinetics data from the literature supporting the calculated results are cited. 相似文献
22.
Ju-Chun Huang Chien Hong Cheng Shyang Roeng Sheen Zhi Ling Lee Chien Ming Lei Maw Kuen Wu 《中国化学会会志》1996,43(2):139-143
The synthesis of bulk Y2Ba4Cu7O15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y2Ba4Cu7O15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y2Ba4Cu7O15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described. 相似文献
23.
Nanometer-sized metallic palladium particles can be synthesized by hydrogen reduction of Pd2+ ions dissolved in the water core of a water-in-CO2 microemulsion. The Pd nanoparticles, stabilized by the micromeulsion and uniformly dispersed in the supercritical fluid phase, are effective catalysts for hydrogenation of olefins. Examples of rapid and efficient hydrogenation of water-soluble and CO2-soluble olefins catalyzed by the Pd nanoparticles in supercritical CO2 are given. 相似文献
24.
Redox reactions of Co(edta)? with Ru(NH3)5L2+ (L = 3- and 4-aminopyridine (AmPy)) were found to follow an outer-sphere electron transfer mechanism. The specific rate constants are (3.26 ± 0.03) × 102 and (3.07 ± 0.04) × 103 M?1S?1, for L = 3- and 4-AmPy, respectively, at μ, = 0.10 M LiClO4, pH = 8.0 (tris) and T = 25 °C. The rate constants of oxidations for a series of Ru(NH3)5L2+ complexes are higher than those of the corresponding Fe(CN)5L3- complexes by factors of 4 to 15 even after corrections for differences in reduction potentials and in charges of the complexes. Nonadiabaticity in the reactions of Fe(CN)5L3 complexes may account for the difference in the relative reactivities. 相似文献
25.
Teng-Kuei Yang Dong-Sheng Lee Tsung-Fan Teng Ting-Ting Jong Mei-Yueh Chien 《中国化学会会志》1991,38(4):401-403
Introducing two chiral auxiliaries, sulfoxide and menthol, into an acrylate would have either matched or mismatched pair systems. The presence of chiral menthol in α-sulfinylacrylates can change the diastereoselectivity from 24% d.e. to a single diastereomer by manipulation of the double asymmetric strategy. 相似文献
26.
To facilitate the determination of the fatty acid composition of tissues and the investigation of fatty acid metabolism, we developed a method for the rapid separation by high-performance liquid chromatography and quantitation (by ultraviolet light absorption) of p-bromophenyl esters of fatty acids which vary in chain length from 10 to 22 carbon atoms. The utility of the method was demonstrated by evaluating the fatty acid composition of human uterine decidua vera tissue and human endometrial stromal cells that are maintained in monolayer culture. 相似文献
27.
You-Xian Chen Marvin D. Rausch James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1995,33(13):2093-2108
Bis(1-indenyl)-di[1′S, 2′R, 5′S)-methoxy]silane ( 1 ) was converted into a mixture of corresponding ansa-diastereomeric zirconocenes. Further purification afforded a single dia-stereomer, di[(1′S, 2′R, 5′S)-methoxy] silylene-bis[η5-1(R, R)-(+)-indenyl] dichlorozirconium ( 2 ), which is optically active and hydrocarbon soluble. Extremely rapid ethylene, propylene, and ethylene-hexene polymerizations were observed both in toluene and n-heptane solutions; for instance, at 50°C, activity for ethylene polymerization reaches ~ 1.5×1010 (g of PE/((mol of Zr) · [C2H4] · h). The “bare” zirconocenium ion generated from 2/TIBA/Ph3CB(C6F5)4 exhibits unusual polymerization behaviors; the polymerization activity increases monotonically with temperature of polymerization (Tp) up to a conventional polymerization condition (50–70°C), and the 13C NMR study shows that the isotactic poly-propylene obtained has fairly high [mmmm] methyl pentad distributions at high Tp (?25°C with [mmmm] ~ 0.93–0.75) and a perfect stereoregularity at low Tp (?0°C with [mmmm] > 0.99). The catalyst precursors 2 and Et(Ind)2ZrCl2 ( 3 ) supported on silica by different approaches produced poly(olefins) of different molecular weights and stereoregularities, and a methylaluminokane and Ph3CB(C6F5)4 free silica-supported zirconocene system was found to be activated by triisobutylaluminum. © 1995 John Wiley & Sons, Inc. 相似文献
28.
Zhengtian Yu James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1995,33(1):125-135
Stereocontrol energy (ΔE0) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η5-indenyl) - ( 1 ) and rac-ethylenebis (1-η5-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The 13C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE0. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc. 相似文献
29.
James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1979,17(8):2555-2565
The following criteria are proposed to judge whether a coordination polymerization may be diffusion controlled or not: (1) If the number-average molecular weight and polydispersity of the polymer calculated from kinetic rate constants as a function of time agree with the experimental values, the polymerization is not diffusion controlled. (2) The polymerization may be diffusion controlled if the Thiele modulus, the ratio of the characteristic diffusion time to the characteristic reaction time, is much greater than unity; if it is much smaller than unity, the polymerization is reaction controlled. (3) If an initial linear dependence of rate of polymerization on catalyst concentration changes over to a square-root dependence, the polymerization may be diffusion limited. (4) The polymerization is likely to be diffusion limited if the instantaneous rate of polymerization is proportional to the rate of particle growth when the proportionality coefficient is the surface area of the particle. Criterion (1) is a necessary and sufficient condition as stated, as its converse is not true. All the other criteria are merely necessary but not sufficient conditions. The established Ziegler–Natta catalysts have activities too low to cause diffusion limitation; the Phillips catalyst system is likely to be diffusion limited. The polydispersity of polyolefins produced with Ziegler–Natta catalysts are not the consequence of diffusion control but are the characteristics of the catalysts in their kinetics of initiation, propagation, chain transfer, and termination. 相似文献
30.