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21.
John T. W. Lai Frankie W. Lau Damon Robb Peter Westh Gerda Nielsen Christa Trandum Aase Hvidt Yoshikata Koga 《Journal of solution chemistry》1995,24(1):89-102
The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)–H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H2O. The values of the excess partial molar Gibbs energies for both DMSO and H2O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H2O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H2O molecules from exposure to the nonpolar alkyl groups of DMSO. 相似文献
22.
The synthesis of a biotinated amphiphile assembled from d-(+)-biotin, ethylene diamine as spacer, galactaric acid and 1-dodecylamine was achieved in six steps. The key step was the synthesis of a bisacetonide protected galactaric ester, the structure of which was determined by X-ray analysis. Aminolysis, spacer attachment, coupling with biotin and deprotection led to the amphiphilic galactaramide. 相似文献
23.
Sabine Schmid 《Journal of solid state chemistry》2003,174(1):221-228
The novel alkaline earth silicate borate cyanides Ba7[SiO4][BO3]3CN and Sr7[SiO4][BO3]3CN have been obtained by the reaction of the respective alkaline earth metals M=Sr, Ba, the carbonates MIICO3, BN, and SiO2 using a radiofrequency furnace at a maximum reaction temperature of 1350°C and 1450°C, respectively. The crystal structures of the isotypic compounds MII7[SiO4][BO3]3CN have been determined by single-crystal X-ray crystallography (P63mc (no. 186), Z=2, a=1129.9(1) pm, c=733.4(2) pm, R1=0.0336, wR2=0.0743 for MII=Ba and a=1081.3(1) pm, c=695.2(1) pm, R1=0.0457, wR2=0.0838 for MII=Sr). Both ionic compounds represent a new structure type, and they are the first examples of silicate borate cyanides. The cyanide ions are disordered and they are surrounded by Ba2+/Sr2+ octahedra, respectively. These octahedra share common faces building chains along [001]. The [BO3]3− ions are arranged around these chains. The [SiO4]4− units are surrounded by Ba2+/Sr2+ tetrahedra, respectively. The title compounds additionally have been investigated by 11B, 13C, 29Si, and 1H MAS-NMR as well as IR and Raman spectroscopy confirming the presence of [SiO4]4−, [BO3]3−, and CN− ions. 相似文献
24.
A simple and accurate spectrophotometric method is proposed for the determination of tannins in tea and beer samples based on the reduction of iron(III) to iron(II) by tannins at 80 degrees C for 20 min. The iron(II) was then reacted with 1,10-phenanthroline at pH 4.4 to form a coloured complex. Background correction could be effected by precipitating the tannins in the sample solution twice with gelatin and kaolin. Absorbance measurements were made at 540 nm and the calibration graph was linear from 0 to 5.5 micrograms ml-1 of tannic acid with a slope of 0.213 A p.p.m.-1. The precision for the determination of tannins in a tea sample containing 9.45% of tannins was 1.8%. Most of the ingredients commonly found in tea and beer samples do not interfere with the determination. Several tea and beer samples were analysed for their tannin content using the proposed method. 相似文献
25.
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
26.
Sabine Strohschein Matthias Pursch Heidrun Händel K. Albert 《Analytical and bioanalytical chemistry》1997,357(5):498-502
The separation of cis/trans isomers of β-carotene has been performed with a C30 stationary phase employing 1H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow
mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison
of simulated 1D 1H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of
three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously
reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies
in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of
the separated compounds is inhibited.
Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996 相似文献
27.
The ruthenium(II) complex cis-[Ru(6, 6'-Cl(2)bpy)(2)(OH(2))(2)](CF(3)SO(3))(2) (1) is a robust catalyst for C-H bond oxidations of hydrocarbons, including linear alkanes, using tert-butyl hydroperoxide (TBHP) as terminal oxidant. Alcohols can be oxidized by the "1 + TBHP" protocol to the corresponding aldehydes/ketones with high product yields at ambient temperature. Oxidation of 1 with Ce(IV) in aqueous solution affords cis-[Ru(VI)(6, 6'-Cl(2)bpy)(2)O(2)](2+), which is isolated as a green/yellow perchlorate salt (2). Complex 2 is a powerful stoichiometric oxidant for cycloalkane oxidations under mild conditions. Oxidation of cis-decalin is highly stereoretentive; cis-decalinol is obtained in high yield, and formation of trans-decalinol is not observed. Mechanistic studies showing a large primary kinetic isotope effect suggest a hydrogen-atom abstraction pathway. The relative reactivities of cycloalkanes toward oxidation by 2 have been examined through competitive experiments, and comparisons with Gif-type processes are presented. 相似文献
28.
K. S. Y. Lau W. J. Kelleghan R. H. Boschan N. Bilow 《Journal of polymer science. Part A, Polymer chemistry》1983,21(10):3009-3026
Three diethynylated bisbenzimidazole prepolymers were synthesized and their polymerization characteristics examined. N-phenyl-substituted diethynylated bisbenzimidazoles melted between 250 and 265°C and had gel times of several seconds to several minutes. In contrast, an N-phenoxyphenyl diethynylated bisbenzimidazole melted at a temperature sufficiently low (ca. 150°C) to provide a 5-min gel time at 170°C and a 4-min gel time at 210°C. This material showed promise as a potential laminating resin and a brief screening of the prepolymer for this application was carried out. The polymers of both N-phenyl- and N-phenoxyphenyl-substituted diethynylated bisbenzimidazoles showed apparent degradation temperatures of 500°C or greater when examined by thermogravimetric analysis. 相似文献
29.
Duma L Hediger S Brutscher B Böckmann A Emsley L 《Journal of the American Chemical Society》2003,125(39):11816-11817
We show that the resolution of homonuclear multidimensional solid-state NMR correlation experiments can be significantly improved using transition selection and spin-state-selective polarization transfer techniques. The selection and transfer of single states allow the removal of the J-coupling contribution from the line width in both the direct and indirect spectral dimensions. This is demonstrated with a new spin-state-selective CO-Calpha correlation experiment, applied to a microcrystalline 85-residue protein. With this new sequence, all four components of the CO-Calpha cross-peaks are separated into different spectra, obtained by linear combination of four recorded data sets. Line narrowing of up to 44% was obtained on the protein sample for the spin-state-selective CO-Calpha spectrum compared to a standard spin-diffusion experiment. The new technique also allows an easy distinction between "direct" and "relayed" transfer cross-peaks. 相似文献
30.
Lam WH Jia G Lin Z Lau CP Eisenstein O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(12):2775-2782
Theoretical calculations on the metathesis process, [Tp(PH3)MR(eta 2-H[bond]CH3)] --> [Tp(PH3)M(CH3)(eta 2-H[bond]R)] (M=Fe, Ru, and Os; R=H and CH3), have been systematically carried out to study their detailed reaction mechanisms. Other than the one-step mechanism via a four-center transition state and the two-step mechanism through an oxidative addition/reductive elimination pathway, a new one-step mechanism, with a transition state formed under oxidative addition, has been found. Based on the intrinsic reaction coordinate calculations, we found that the trajectories of the transferring hydrogen atom in the metathesis processes studied are similar to each other regardless of the nature of reaction mechanisms. 相似文献