全文获取类型
收费全文 | 334篇 |
免费 | 4篇 |
专业分类
化学 | 247篇 |
晶体学 | 3篇 |
力学 | 4篇 |
数学 | 20篇 |
物理学 | 64篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 2篇 |
2014年 | 15篇 |
2013年 | 25篇 |
2012年 | 22篇 |
2011年 | 48篇 |
2010年 | 11篇 |
2009年 | 11篇 |
2008年 | 15篇 |
2007年 | 18篇 |
2006年 | 26篇 |
2005年 | 14篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 14篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有338条查询结果,搜索用时 15 毫秒
101.
Clark JL Peinado J Stezowski JJ Vold RL Huang Y Hoatson GL 《The journal of physical chemistry. B》2006,110(51):26375-26387
The crystal structure of a triclinic 2:2 inclusion complex of beta-cyclodextrin with N-acetyl-L-phenylalanine methyl ester has been determined at several temperatures between 298 and 20 K to further study molecular recognition using solid-state supramolecular beta-cyclodextrin complexes. The study reveals kinetic energy dependent changes in guest molecule conformations, orientations, and positions in the binding pocket presented by the crystal lattice. Accompanying these changes are observable differences in guest-guest interactions and hydrogen-bonding interactions in the binding pocket that involve guest molecules, water of hydration molecules, and beta-cyclodextrin molecules. On the basis of the differences observed in the crystal structures, we present a solid-state example of a system that displays the properties of both classical and quantum chemical models. At higher temperatures, the structure conforms to a classical mechanical model with dynamic disorder. At lower temperatures, the observations conform to examples in which there is static disorder representative of models in which quantum states differing in conformation, position, and orientation of components in the crystal structure are occupied. Ab initio theoretical calculations on the different guest molecule conformations have been carried out. Superpositions of theoretical electrostatic surface potential diagrams on the observed molecular positions in the complexes provide confidence that the deconvolution of the guest molecule disorder is acceptable. Temperature-dependent solid-state magic angle spinning deuteron NMR measurements provide evidence for large-amplitude, diffusive motion on a microsecond time scale in the complex. 相似文献
102.
Leland Rosenberger Ronald Baird Erik McCullen Gregory Auner Gina Shreve 《Surface and interface analysis : SIA》2008,40(9):1254-1261
Ten samples of crystalline aluminum nitride (AlN) film were deposited on sapphire and silicon substrates by a plasma source molecular beam method. The samples were analyzed using X‐ray photoelectron spectroscopy (XPS) depth profiling and high‐resolution X‐ray diffraction. Oxygen levels were observed to decrease exponentially from the surface into the bulk film. Aluminum, nitrogen and oxygen peaks were fitted with subpeaks in a consistent manner and the subpeaks were assigned to chemical states. AlN subpeaks were observed at 73.5 eV for Al2p and 396.4 eV for N1s. An N1s subpeak at 395.0 eV was assigned to N? N defects. No direct N? O bonds are assigned; rather it is proposed that an N? Al? O bond sequence is the source of higher binding energy N1s subpeaks. The observations in this study support a model in which oxygen is bound only to aluminum in the form of Al? O octahedral complexes dispersed or clustered throughout the main AlN matrix or as Al? O bonds on the crystal grain boundaries. The data also suggest that the AlN lattice parameters are related to oxygen content, since the c‐axis is observed to increase with increasing oxygen content. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
103.
Irene Suarez del Villar 《Tetrahedron letters》2010,51(23):3095-2144
Cyclopropanation of α,β-unsaturated δ-lactams and δ-lactones mediated by nitrogen ylides is described. The process tolerates different alkyl halides and gives efficiently bicyclo[4.1.0]heptanes in a totally stereoselective manner. On the other hand, ε-lactams under our experimental conditions suffer a novel process involving a skeletal reorganization to give a bicyclic[3.3.0] system. 相似文献
104.
Estela M. Alvarez Patricia H. Risso Carlos A. Gatti Mariano Burgos Virginia Suarez Sala 《Colloid and polymer science》2007,285(5):507-514
The formation of colloidal particles by Ca2+ precipitation of whole caseinates in the presence of phosphate (Pi), citrate (Cit), or both of the anions in concentrations
found to be effective in previous works was followed comparing the colloidal particle size and the ionic and proteic composition
of the precipitates obtained. Ca2+ was incorporated to the precipitate and colloidal particles in a different way than Pi, differences which were related to
the presence of Pi and/or Cit in the media. A sequential salting-out process due to progressive Ca2+ binding to at least two kinds of sites was observed. The precipitation curves were fitted, and the affinity constants and
binding site numbers were calculated with a modification of the Farrell’s equation based on the concept of Wyman’s linked
functions. Precipitates obtained at low total Ca2+ concentrations in different conditions varied their casein composition. Colloidal particles appeared at the beginning of
the second salting-out step, in different amount, and in average size according to the presence or absence of Pi and/or Cit
in the media. Consideration of these differences showed that Cit favored the formation of bigger colloidal particles, acting
especially in the first steps of the casein aggregation and conditioning the mechanism of this process. 相似文献
105.
[reaction: see text] Haloacetal radical cyclizations of alpha- and beta-hydroxyhydrazones provide a direct access to aminosugarlike compounds. Stereocontrol of this process is influenced by stereogenic centers of both the hydroxyhydrazone and the acetal. The outcomes are consistent with chair and twist transition states with the anomeric alkoxy group in pseudoaxial orientations. 相似文献
106.
Extraction and high-performance liquid chromatographic (HPLC) procedures are described that permit the complete analysis of free and conjugated zeranol metabolites in plasma from pigs implanted with [3H]zeranol. Free metabolites (9.0%) are extracted and then analysed by radio-HPLC on a reversed phase C18 column. They are distributed between three compounds that have been identified by gas chromatography-mass spectrometry as taleranol, zeranol and zeralanone. Direct radio-HPLC of the pre-extracted, deproteinated and Sep-Pak C18-purified plasma on a reversed-phase C18 column using tetrabutylammonium as an ion-pairing agent showed four main peaks: one corresponds to a weakly retained unidentified compound(s) (20%) and the other three were identified as the taleranol, zeranol and zeralanone glucuro conjugates. However, the total recovery is only about 25% owing to strong affinity of this polar material for the plasma proteins. Enzymatic deconjugation of the pre-extracted plasma followed by radio-HPLC analysis of the freed metabolites led to a good recovery of the radioactivity (81.8%) and allowed the quantitation of the different metabolites. These preliminary results indicate that zeranol is metabolized in the pig following pathways similar to those in other tested species. 相似文献
107.
Recent studies have shown that semiempirical methods (e.g., PM3 and AM1) for zinc-containing compounds are unreliable for modeling structures containing zinc ions with ligand environments similar to those observed in zinc metalloenzymes. To correct these deficiencies a reparameterization of zinc at the PM3 level was undertaken. In this effort we included frequency corrected B3LYP/6-311G* zinc metalloenzyme ligand environments along with previously utilized experimental data. Average errors for the heats of formation have been reduced from 46.9 kcal/mol (PM3) to 14.2 kcal/mol for this new parameter set, termed ZnB for "Zinc, Biological." In addition, the new parameter sets predict geometries for the Bacillus fragilis active site model and other zinc metalloenzyme mimics that are qualitatively in agreement with high-level ab initio results, something existing parameter sets failed to do. 相似文献
108.
Florio GM Christie RA Jordan KD Zwier TS 《Journal of the American Chemical Society》2002,124(34):10236-10247
The hormone melatonin (N-acetyl-5-methoxytryptamine) is an indole derivative with a flexible peptide-like side chain attached at the C3 position. Using a combination of two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence excitation (LIF), resonant ion-dip infrared spectroscopy (RIDIRS), fluorescence-dip infrared spectroscopy (FDIRS), and UV-UV hole-burning spectroscopy, the conformational preferences of melatonin in a molecular beam have been determined. Three major trans-amide conformers and two minor cis-amide conformers have been identified in the R2PI spectrum and characterized with RIDIRS and FDIRS. Structural assignments are made using the infrared spectra in concert with density functional theory and localized MP2 calculations. Observation of cis-amide melatonin conformers in the molecular beam, despite the large energy gap (approximately 3 kcal/mol) between trans- and cis-amides, is striking because there are at least nine lower-energy trans-amide minima that are not detected. The implications of this observation for cooling and trapping conformational population in a supersonic expansion are discussed. 相似文献
109.
Challenges encountered in extending the sensitivity of US Environmental Protection Agency Method 314.0 for perchlorate in drinking water 总被引:1,自引:0,他引:1
Wagner HP Suarez FX Pepich BV Hautman DP Munch DJ 《Journal of chromatography. A》2004,1039(1-2):97-104
Concerns about the potential adverse health effects of perchlorate at concentrations below the minimum reporting level (MRL) of US Environmental Protection Agency (EPA) Method 314.0 (generally recognized as 4.0 microg/l) have led to an interest in increasing the sensitivity of the method. This work describes the use of 2 mm columns with a large-loop direct injection method, a column concentration technique and this concentration technique with a background reduction step, to increase the sensitivity for the analysis of trace levels of perchlorate in high ionic strength matrices. The concentrator columns studied were the Dionex TAC LP-1 and a new Dionex high capacity Cryptand concentrator column. The use of a surrogate to monitor trapping efficiency for the concentration technique and the use of confirmational columns to minimize the potential for false positives are also discussed. The large-loop direct injection method and the column concentration methods provided acceptable data when the samples were pre-treated with solid phase pretreatment cartridges. The background reduction technique did not provide acceptable data with either of the concentrator columns evaluated. 相似文献
110.