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31.
Urea is widely employed to facilitate protein separations in ion exchange chromatography at various scales. In this work, five model proteins were used to examine the chromatographic effects of protein conformational changes induced by urea in ion exchange chromatography. Linear gradient experiments were carried out at various urea concentrations and the protein secondary and tertiary structures were evaluated by far UV CD and fluorescence measurements, respectively. The results indicated that chromatographic retention times were well correlated with structural changes and that they were more sensitive to tertiary structural change. Steric Mass Action (SMA) isotherm parameters were also examined and the results indicated that urea induced protein conformational changes could affect both the characteristic charge and equilibrium constants in these systems. Dynamic light scattering analysis of changes in protein size due to urea-induced unfolding indicated that the size of the protein was not correlated with SMA parameter changes. These results indicate that while urea-induced structural changes can have a marked effect on protein chromatographic behavior in IEX, this behavior can be quite complicated and protein specific. These differences in protein behavior may provide insight into how these partially unfolded proteins are interacting with the resin material. 相似文献
32.
2,2-Dimethyl- and 2-benzyl-2-methyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethenes were employed as ligand of mononuclear chlorogold(I) complexes, which catalyzed cycloisomerization of 1,6-enyne affording vinylcyclopentene exclusively in the absence of silver co-catalyst. The reaction mechanisms are discussed based on DFT calculations. In addition to the cycloisomerization, the phosphaalkene-chlorogold(I) complexes catalyzed cyclization of pent-4-ynoic acids providing γ-methylene-γ-lactones under basic conditions. 相似文献
33.
A simple and efficient total synthesis of ellipticine was developed via the Suzuki-Miyaura coupling of sterically sensitive 2-hydroxybenzeneboronic acid with a multifunctional aryl halide using Pd(OAc)2 as a catalyst and Cu(OAc)2·H2O as an additive in DMSO/H2O as a key step followed by double N-arylation and cyclization. 相似文献
34.
Reaction of substituted benzene rings with N‐bromophthalimide, under neutral conditions, gave the corresponding bromo derivatives with a preference for the formation of the para bromo isomer over the ortho isomer. The simple work‐up procedure minimizes loss of product and the yields are good. 相似文献
35.
Rudolf Pietschnig 《Journal of fluorine chemistry》2007,128(2):150-152
The 29Si-19F INEPT NMR method has been applied to significantly reduce the recording time of 29Si NMR spectra of sterically shielded fluorosilanes. This technique furthermore improves the signal to noise ratio to a degree which allows the simultaneous detection of minor byproducts which are below the detection limit using direct acquisition. 相似文献
36.
In reversed-phase chromatography (RPC), the restricted retention of "bulky" solutes can occur in one of two ways, giving rise to either "shape selectivity" or "steric interaction." Starting with data for 150 solutes and 167 monomeric type-B alkylsilica columns, the present study examines the steric interaction process further and compares it with shape selectivity. The dependence of column hydrophobicity and steric interaction on column properties (ligand length and concentration, pore diameter, end-capping) was determined and compared. The role of the solute in steric interaction was found to be primarily a function of solute molecular length, with longer solutes giving increased steric interaction. We find that there are several distinct differences in the way shape selectivity and steric interaction are affected by separation conditions and the nature of the sample. Of particular interest, steric interaction exhibits a maximum effect for monomeric C(18) columns, and becomes less important for either a C(1) or C(30) column; shape selectivity appears unimportant for monomeric C(1)-C(18) columns at ambient and higher temperatures, but becomes pronounced for C(30) - as well as polymeric columns with ligands ≥C(8). One hypothesis is that shape selectivity involves the presence or creation of cavities within the stationary phase that can accommodate a retained solute (a primarily enthalpic process), while steric interaction mainly makes greater use of spaces that pre-exist the retention of the solute (a primarily entropic process). The related dependence of hydrophobic interaction on column properties was also examined. 相似文献
37.
Summary The separation by thin-layer chromatography (TLC) is reviewed for 25 pairs of tetrasubstituted-ethane diastereomers on silica.
A total of 50 different mobile phases were used in order to define the basis of solvent selectivity for these separations.
Separation factors (α) for various diastereomeric pairs are determined mainly by the relative localization of the strong solvent
in these binary-solvent mobile phases (as measured by the mobile-phase parameter m). Relative retention of the diastereomers
expressed by positive vs. negative values of α appear understandable on the basis of steric hindrance to adsorption and solvent-solute
localization.
Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984 相似文献
38.
化学工业生产中,用氢气为还原剂,通过选择性加氢可以制备多种重要化学品。5-羟甲基糠醛是重要的生物质基平台化合物,而5-甲基糠醛是用途广泛的化学品。由5-羟甲基糠醛加氢得到5-甲基糠醛是一条非常理想的路径,但是选择性活化C-OH非常困难。本文设计并制备了Pt@PVP/Nb2O5(PVP: 聚乙烯吡咯烷酮)催化剂,该催化体系巧妙地结合了位阻效应、氢溢流和催化剂界面的电子效应,系统研究了该催化剂对5-羟甲基糠醛选择性加氢制备5-甲基糠醛催化性能,在最优条件下,5-甲基糠醛的选择性可达92%。利用密度泛函理论计算研究了5-羟甲基糠醛选择性加氢制备5-甲基糠醛反应路径。 相似文献
39.
A new nitrosation and oxidation process to synthesize gem-dinitro compounds was accomplished by using nitryl chloride as nitrosation reagent and ozone as oxidizing agent. The main features of the present protocol include the compatibility to substances with high steric hindrance, high yields and mild reaction conditions. A plausible mechanism involving the formation of an intermediate of gem-nitrosonitro compound by means of single electron transfer was also proposed. 相似文献
40.
The alkylation of ethyl 1H-pyrazole-3-carboxylate with a variety of alkylating agents in the presence of K2CO3 was found to largely favor the formation of ethyl 1-substituted pyrazole-3-carboxylates. The alkylation could be sterically redirected by the use of a triphenylsilyl group (ethyl 3-(triphenylsilyl)-1H-pyrazole-5-carboxylate) to provide synthetically useful yields of ethyl 1-substituted-3-(triphenylsilyl)-1H-pyrazole-5-carboxylates. The triphenylsilyl group could be removed with Bu4NF. Other triorganosilyl groups (TMS, TES, TBDMS) failed to provide significant redirection, while TIPS proved refractory to protodesilylation. 相似文献