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131.
Julio Casado J. Ramón Leis Manuel Mosquera L. Carlos Paz M. Elena Peña 《Monatshefte für Chemie / Chemical Monthly》1984,115(2):155-163
Acetic acid/acetate ion buffer acts catalytically upon the nitrosation of amines under conditions in which the only nitrosating agents are N2O3 and NOBr, but inhibits nitrosation by H2NO
2
+
. The kinetic characteristics of these phenomena have been analysed quantitatively and compared with similar effects caused by the solventsTHF, DMSO and dioxane. The experimental results show that this behaviour is an effect of the medium.
Kinetische Untersuchungen zur Bildung von N-Nitroso-Verbindungen, 8. Mitt.: Nachweis eines Medium-Effekts von Essigsäure/Acetat-Puffer auf die Geschwindigkeitskonstante der Nitrosierung
Zusammenfassung Essigsäure/Acetat-Puffer wirkt bei der Nitrosierung von Aminen katalytisch, unter Bedingungen, wo die alleinigen nitrosierenden Agentien N2O3 und NOBr sind; andererseits wird die Nitrosierung durch H2NO 2 + unterbunden. Die kinetischen Charakteristika dieses Phänomens wurden quantitativ analysiert und mit ähnlichen Effekten der LösungsmittelTHF, DMSO und Dioxan verglichen. Die experimentellen Ergebnisse zeigen, daß dieses Verhalten auf einen Mediumeffekt zurückzuführen ist.相似文献
132.
133.
A successful route to prepare zeolites with controlled crystal size from the millimeter range down to nanometer scale by simulating the natural formation conditions is presented. Water is used as the solution and reaction medium at high temperatures and pressures (T=120-400°C, p=1 kbar); aluminosilicate glasses with identical chemical composition as the zeolite products are used as precursors. The synthesis of heulandite is reported as representative example. 相似文献
134.
Based on previously developed solution phase chemistry, an effective general approach to the synthesis of heparin-like oligosaccharides on a soluble polymer support is reported. 相似文献
135.
Griselda Corro M. Paz Elizalde Angeles Velasco 《Reaction Kinetics and Catalysis Letters》2002,76(1):117-122
NO oxidation was investigated over Pt-Sn/g-Al2O3. The addition of Sn to 1%Pt/g-Al2O3 enhances NO oxidation to NO2. This effect is related to the enhancement of chemisorbed oxygen on the metallic surface produced by Sn. 相似文献
136.
Javier Cataln Marta Snchez-Cabezudo Jos Luis G. De Paz Jos Elguero Robert W. Taft Frederick Anvia 《Journal of computational chemistry》1989,10(3):426-433
The annular tautomerism of 1,2,3-triazole and 3(5)-methylpyrazole is discussed by means of a combination of theoretical calculations and experimental (ICR) gas-phase basicities and acidities. In the gas phase 1,2,3-triazole exists as the 2H-tautomer, whereas both tautomers of 3(5)-methylpyrazole are of similar energy. The solvent effects on these prototropic equilibria are discussed taking into account solvent properties as polarity/polarizability, acidity, and basicity. In nonhydrogen bonding solvents, the difference in dipole moments between both tautomers plays a role that has usually been underestimated. 相似文献
137.
M. M. Mato S. M. Cebreiro J. L. Legido M. I. Paz Andrade 《Journal of Thermal Analysis and Calorimetry》2006,84(1):279-283
Experimental
excess molar volumes for the ternary system {x1MTBE+x21-propanol+(1–x1–x2)nonane}
and the three involved binary mixtures have been determined at 298.15 K and
atmospheric pressure. Excess molar volumes were determined from the densities
of the pure liquids and mixtures, using a DMA 4500 Anton Paar densimeter.
The ternary mixture shows maximum values
around the binary mixture MTBE+nonane and minimum values for the mixture MTBE+propanol.
The ternary contribution to the excess molar volume is negative, with the
exception of a range located around the rich compositions of 1-propanol.
Several empirical equations predicting ternary mixture properties from
experimental binary mixtures have been applied. 相似文献
138.
Mary C.?SalazarEmail author José L.?Paz Antonio J.?Hernández Ramón?López-Planes Marcia L.?Llanos 《Theoretical chemistry accounts》2003,110(1):42-46
We have performed ab initio fourth-order Møller–Plesset perturbation theory calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals N2–He dimer. They indicate a ``T-shaped' stablest ground N2(X1g+)–He(1S) electronic state with a well depth, De, of 21.63 cm–1 at a minimum distance, Re, of 3.44 Å and zero-point vibration correction, Do, of 7.07 cm–1. They also indicate a ``T-shaped' stablest excited conformer with Re=3.25 Å, De=36.85 cm–1 and Do=17.06 cm–1 for the N2(B3g)–He(1S) triplet electronic level. In order to investigate the use of less-demanding correlation methods, test density functional theory calculations using the mPW1PW exchange–correlation functional are also presented for comparison. 相似文献
139.
Muñoz EM López de la Paz M Jiménez-Barbero J Ellis G Pérez M Vicent C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(8):1908-1914
Carbohydrates with suitably positioned intramolecularly hydrogen-bonded hydroxyl and amide groups have the potential to act as efficient bidentate phosphate binders by taking advantage of sigma- and/or ,sigma,pi-H-bonding cooperativity in nonpolar solvents. Donor-donor 1,2-trans-diaxial amido alcohol (1) and diol (3), in which one of the donor centres is cooperative, are very efficient carbohydrate-phosphate binding motifs. We have proven and quantified the key role of hydrogen-bonding centres indirectly involved in complexation, which serve to generate an intramolecular H-bond (six-membered cis H-bond) in 1 and 3. This motif enhances the donor nature of the H-bonding centres that are directly involved in complexation. A comparison of the thermodynamic parameters of the complexes formed between phosphate and a cooperative (1-Phos) or anti-cooperative (2-Phos) bidentate H-bonded motif of a carbohydrate has allowed us to quantify the energetic advantage of H-bonding cooperativity in CDCl3 and CDCl3/CCl4 (1:1.3) (Delta Delta G degrees=-2.2 and -2.0 kcal mol(-1), respectively). The solvent dependences of the entropy and enthalpy contributions to binding provide a valuable example of the delicate balance between entropy and enthalpy that can arise for a single process, providing effective cooperative binding in terms of Delta G degrees. 相似文献
140.
M. Paz Fernndez-Liencres Emilio Calle Samuel Gonzlez-Mancebo Julio Casado Bartolom Quintero 《国际化学动力学杂志》1997,29(2):119-125
The kinetics of the reactions between sodium nitrite and phenol or m-, o-, or p-cresol in potassium hydrogen phthalate buffers of pH 2.5–5.7 were determined by integration of the monitored absorbance of the C-nitroso reaction products. At pH > 3, the dominant reaction was C-nitrosation through a mechanism that appears to consist of a diffusion-controlled attack on the nitrosatable substrate by NO+/NO2H2+ ions followed by a slow proton transfer step; the latter step is supported by the observation of basic catalysis by the buffer which does not form alternative nitrosating agents as nitrosyl compounds. The catalytic coefficients of both anionic forms of the buffer have been determined. The observed order of substrate reactivities (o-cresol ≈ m-cresol > phenol ≫ p-cresol) is explained by the hyperconjugative effect of the methyl group in o- and m-cresol, and by its blocking the para position in p-cresol. Analysis of a plot of ΔH# against ΔS# shows that the reaction with p-cresol differs from those with o- and m-cresol as regards the formation and decomposition of the transition state. The genotoxicity of nitrosatable phenols is compared with their reactivity with NO+/NO2H2+. © 1997 John Wiley & Sons, Inc. 相似文献