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81.
Hedi Hannachi Naoual Anoune Christian Arnaud Pierre Lant ri R mi Longeray Henry Chermette 《Journal of Molecular Structure》1998,434(1-3):183-191
The Baeyer-Villiger reaction of polymethoxybenzaldehydes with peroxysuccinic acid (PSA) is studied theoretically. The semi-empirical SCF-MO/PM3 method has been used to study the reaction and to calculate the energy profiles. In addition, heat of formation computations and bond order analyses of the transition species along the reaction coordinates have been performed and are interpreted qualitatively in terms of electron flow through the system during aryl migration. Our results support the hypothesis that the second step is a concerted reaction. Our data support a definite effect of the substituent on the aryl migration. The theoretical predictions are in good agreement with experimental results. 相似文献
82.
S. V. Shishkina T. G. Drushlyak L. A. Kutulya N. S. Pivnenko O. V. Shishkin 《Journal of Structural Chemistry》2004,45(5):889-895
X-ray analysis was carried out to study the molecular structure of β-hydroxyketone, which is one of the minor products of selective aldol condensation of (−)-menthone with 4-carbomethoxybenzaldehyde. The cyclohexanone ring of the compound has a practically undistorted chair conformation with an axial orientation of the 1-methyl group and the bulkiest 2-[1′-hydroxy-1′ -(4-carbomethoxyphenyl)]methyl substituent and the equatorial orientation of the isopropyl group in the 4 position. The stereochemical configuration was found to be (1R,2R,4 R, 1′ S), supporting that the compound belongs to the group of (+)-isomenthones; i.e., the starting (-)-menthone undergoes (to a certain extent) epimerization under the reaction conditions used. In crystal, there is intermolecular hydrogen bonding >C=O...HO- (the -O...H- bond length is 2.15 Å). The molecular conformation in hydroxyketone crystals differs from the one which prevails in solutions (according to previous NMR data) and which is characterized by an inverted cyclohexanone ring and intramolecular hydrogen bonding >C=O...HO.Original Russian Text Copyright © 2004 by S. V. Shishkina, T. G. Drushlyak, L. A. Kutulya, N. S. Pivnenko, and O. V. Shishkin__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 933–940, September–October, 2004. 相似文献
83.
V. Yu. Buz’ko I. V. Sukhno V. T. Panyushkin D. N. Ramazanova 《Journal of Structural Chemistry》2005,46(4):596-602
The structure and relative stability of 1,4-dioxane-water, (Diox)n·(H2O )m (n = 1, 2, m = 1–6), molecular complexes have been calculated by semiempirical MNDO/PM3 method. A considerable variety of (Diox)n·(H2O )m isomeric structures was stated. The mean energy of ODiox…HW-OW hydrogen bond in (Diox)n·(H2O)m complexes formed by 1,4-dioxane molecules in the chair conformation amounts to ?2.293 ± 0.210 kcal/mol with the average bond length 2.797 0.015 Å. 相似文献
84.
A sensitive and confirmatory analytical method has been developed for the determination of 12 ionic per- and polyfluorinated alkyl substances (PFAS) in fine airborne particulate matter (PM2.5) at trace levels. The proposed method includes extraction of PM2.5-bound PFAS by microwave-assisted extraction (MAE) followed by centrifugation and injection into the liquid chromatograph coupled to a triple quadrupole tandem mass spectrometry system (LC-MS/MS). The main parameters affecting the performance of MAE were optimised using statistical design of experiments (DoE). Recoveries ranged from 83 to 120% and the method quantification limit (MQL) was 1.4 pg m(-3), when air volumes of 720 m(3) were sampled. This method was successfully applied to 41 samples collected from five stations of the monitoring network of the Valencian Regional Government (Spain) during April-July 2010. Eight out of 12 PFCs investigated were quantified in at least one sample (PFBA, PFPeA, PFHxS, 6:2 FTS, PFOA, PFNA, PFOS and PFDA). The measured concentrations ranged from 1.4 to 34.3 pg m(-3). 相似文献
85.
Teobald Kupka 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》1997,53(14):2649-2658
IR studies were preformed to determine possible transition metal ion binding sites of penicillin. the observed changes in spectral position and shape of characteristic IR bands of cloxacillin in the presence of transition metal ions (both in solutions and in the solid state) indicate formation of M–L complexes with engagement of –COO− and/or –CONH– functional groups. The small shift of νC=O towards higher frequencies rules out direct M–L interaction via β-lactam carbonyl. PM3 calculations on simple model compounds (substituted formamide, cyclic ketones, lactams and substituted monocyclic β-lactams) have been performed. All structures were fully optimized and the calculated bond lengths, angles, heats of formation and C=O stretching frequencies were discussed to determine the β-lactam binding sites and to explain its susceptibility towards nucleophilic attack (hydrolysis in vitro) and biological activity. The relative changes of calculated values were critically compared with available experimental data and same correlation between structural parameters and in vivo activity was shown. 相似文献
86.
Static second-order nonlinear optical effects of amino acid zinc(Ⅱ) porphyrins 1, 2,3 and 4 were calculated by the TDHF/PM3 method based on the molecular structures optim.zed at the semiempirical PM3 quantum chemistry level, showing due to the cancellation of symmetric center, these amino acid zinc(H) porphyrins exhibit second order nonlinear optical response. The analysis of β components indicated that these amino acid zinc(Ⅱ) porphyrins are of multipolarizabilities, and they may be ascribed as the “mixture” of octupolar and dipoar molecules with ||βJ=3||/||βJ=1||≈5. It is found that there are no significant differences between the static β values of non-chiral and chiral amino acid zinc(Ⅱ) porphyrins. However, the βxyz component, which is quite important to quadratic macroscopic X^(2) susceptibility of chiral material, is increased significantly with the increase of side chain group of amino acids. 相似文献
87.
88.
表面活性剂增敏2.5次微分极谱测定香豆素 总被引:1,自引:0,他引:1
王福民 《理化检验(化学分册)》2003,39(3):138-141
拟定了表面活性剂增敏、2 .5次微分极谱测定香豆素的方法。在含 6.0× 10 -6 mol·L-13 氯 2 羟丙基 三甲基氯化铵的 0 .1mol·L-1KH2 PO4 Na2 HPO4 (pH 6.8)缓冲液中 ,香豆素还原波 [峰电位Ep=- 1.60V(vs .SCE) ]的 2 .5次微分极谱峰峰电流e″p 与其浓度在 4 .0× 10 -7~ 6.0× 10 -5mol·L-1范围内呈良好线性关系 (r =0 .9998,n =10 ) ,检出限为 1.2× 10 -7mol·L-1。 13次平行测量 8.0× 10 -6 mol·L-1香豆素的峰电流 ,RSD为 1.5 %。该方法可用于中药白芷中香豆素的直接测定 相似文献
89.
Krecar D Vassileva V Danninger H Hutter H 《Analytical and bioanalytical chemistry》2004,379(4):610-618
Powder metallurgy is a highly developed method of manufacturing reliable ferrous parts. The main processing steps in a powder metallurgical line are pressing and sintering. Sintering can be strongly enhanced by the formation of a liquid phase during the sintering process when using phosphorus as sintering activator. In this work the distribution (effect) of phosphorus was investigated by means of secondary ion mass spectrometry (SIMS) supported by Auger electron spectroscopy (AES) and electron probe micro analysis (EPMA). To verify the influence of the process conditions (phosphorus content, sintering atmosphere, time) on the mechanical properties, additional measurements of the microstructure (pore shape) and of impact energy were performed. Analysis of fracture surfaces was performed by means of scanning electron microscopy (SEM). The concentration of phosphorus differs in the samples from 0 to 1% (w/w). Samples with higher phosphorus concentrations (1% (w/w) and above) are also measurable by EPMA, whereas the distributions of P at technically relevant concentrations and the distribution of possible impurities are only detectable (visible) by means of SIMS. The influence of the sintering time on the phosphorus distribution will be demonstrated. In addition the grain boundary segregation of P was measured by AES at the surface of in-situ broken samples. It will be shown that the distribution of phosphorus depends also on the concentration of carbon in the samples. 相似文献
90.
M.H. Thoma S. Leupold U. Mosel 《The European Physical Journal A - Hadrons and Nuclei》2000,7(2):219-223
Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the
dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π+-π− annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the
quark-gluon plasma.
Received: 8 September 1999 / Revised version: 22 October 1999 相似文献