全文获取类型
收费全文 | 192篇 |
免费 | 3篇 |
专业分类
化学 | 143篇 |
晶体学 | 1篇 |
力学 | 8篇 |
数学 | 16篇 |
物理学 | 27篇 |
出版年
2024年 | 1篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 8篇 |
2012年 | 7篇 |
2011年 | 6篇 |
2010年 | 4篇 |
2009年 | 14篇 |
2008年 | 8篇 |
2007年 | 13篇 |
2006年 | 16篇 |
2005年 | 9篇 |
2004年 | 18篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 3篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1988年 | 1篇 |
1985年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1929年 | 2篇 |
排序方式: 共有195条查询结果,搜索用时 15 毫秒
71.
Range K Riccardi D Cui Q Elstner M York DM 《Physical chemistry chemical physics : PCCP》2005,7(16):3070-3079
Benchmark calculations of proton affinities and gas-phase basicities of molecules most relevant to biological phosphoryl transfer reactions are presented and compared with available experimental results. The accuracy of proton affinity and gas-phase basicity results obtained from several multi-level model chemistries (CBS-QB3, G3B3, and G3MP2B3) and density-functional quantum models (PBE0, B1B95, and B3LYP) are assessed and compared. From these data, a set of empirical bond enthalpy, entropy, and free energy corrections are introduced that considerably improve the accuracy and predictive capability of the methods. These corrections are applied to the prediction of proton affinity and gas-phase basicity values of important biological phosphates and phosphoranes for which experimental data does not currently exist. Comparison is made with results from semiempirical quantum models that are commonly employed in hybrid quantum mechanical/molecular mechanical simulations. Data suggest that the design of improved semiempirical quantum models with increased accuracy for relative proton affinity values is necessary to obtain quantitative accuracy for phosphoryl transfer reactions in solution, enzymes, and ribozymes. 相似文献
72.
Biffis A Ricoveri R Campestrini S Kralik M Jeràbek K Corain B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(13):2962-2967
Most hydrogen peroxide is currently produced by the selective hydrogenation of 2-ethylanthraquinone (EAQ) to 2-ethylanthrahydroquinone (EAHQ), followed by treatment with dioxygen; this produces hydrogen peroxide and regenerates 2-ethylanthraquinone. We have developed novel catalysts for this process that are based on palladium supported on very lipophilic functional resins and that are able to promote a chemoselectivity for EAHQ slightly but definitely superior to that provided by an industrial catalyst under identical conditions. This finding demonstrates the potential of variations of the lipophilic/hydrophilic character of the support as a tool for the improvement of the chemoselectivity of resin-based metal catalysts. 相似文献
73.
Cox H Akibo-Betts G Wright RR Walker NR Curtis S Duncombe B Stace AJ 《Journal of the American Chemical Society》2003,125(1):233-242
An experimental gas-phase study of the intensities and fragmentation patterns of [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes shows the combinations [Mn.(H(2)O)(4)](2+) and [Mn.(ROH)(4)](2+) to be stable. Evidence in complexes involving the alcohols methanol, ethanol, 1-propanol, and 2-propanol favors preferential fragmentation to [Mn.(ROH)(4)](2+), whereas the fragmentation data for water is less clear. Supporting density functional calculations show that both [Mn.(H(2)O)(4)](2+) and [Mn.(MeOH)(4)](2+) adopt stable tetrahedral configurations, similar to those proposed for biochemical systems where solvent availability and coordination is restricted. Calculated incremental binding energies show a gradual decline on going from one to six solvent molecules, with a step occurring between four and five molecules. The addition of further solvent molecules to the stable [Mn.(MeOH)(4)](2+) unit shows a preference for [Mn.(MeOH)(4)(MeOH)(1,2)](2+) structures, where the extra molecules occupy hydrogen-bonded sites in the form of a secondary solvation shell. Very similar behavior is seen on the part of water. As part of an analysis of the experimental data, the calculations have explored the influence different spins states of Mn(2+) have on solvent geometry. It is concluded that the experimental observations are best reproduced when the central Mn(2+) ion is in the high-spin (6)S ground state. The results are also considered in terms of the biochemical activity of Mn(2+) where the ion is capable of isomorphous substitution with Zn(2+), which itself exhibits a preference for tetrahedral coordination. 相似文献
74.
Albrecht H Zbinden P Rizzi A Villetti G Riccardi B Puccini P Catinella S Imbimbo BP 《Combinatorial chemistry & high throughput screening》2004,7(8):745-756
A cell-based assay using homogeneous time-resolved fluorescence has been developed for high throughput screening of putative beta-amyloid (Abeta)production inhibitors. In this assay, total Abeta is detected by simply adding two commercially available antibody complexes. The first was a biotinylated monoclonal antibody (4G8), specifically recognizing an epitope comprising the residues 17-24 of the Abetapeptide, complexed with europium cryptate-streptavidin conjugate. The second was a polyclonal antibody (BioS-N), raised against the N-terminus of the Abeta peptide, complexed with an allophycocyanin-anti rabbit antibody conjugate. Binding of the two complexes to the Abeta peptide brought europium cryptate (fluorescence donor) and allophycocyanin (fluorescence acceptor) into close proximity, consequently a fluorescent resonance energy transfer signal was produced upon excitation at 337 nm. The resulting fluorescence signal (665 nm) was then detected using a Discovery or a ViewLux reader. Detection of Abeta by the proposed method is possible at concentrations of approximately 1 nM. The method was employed for the detection of Abeta secreted from a stable transfected human neuroglioma cell line (H4) overexpressing a mutated form of the human amyloid precursor protein (APP695NL) and developed for robotic automation. At optimized conditions, signal-to-background ratios exceeding 5 and Z' factors around 0.7 were achieved in a 384-well format. High throughput screening of 56,913 potential Abeta production inhibitors led to identification of new non-cytotoxic and cell permeable compounds with potencies in the submicromolar range. 相似文献
75.
C. Galas Umberto Sansone Maria Belli Sabrina Barbizzi Philippe Cyffroy Giovanni Paolo Fanzutti Volodya Kanivets Rita Ocone Raffaella Piani Milena Repetti Michele Riccardi Cinzia Terzoni Oleg V. Voitsekhovitch 《Accreditation and quality assurance》2002,7(5):202-208
Sampling and sample preparation/processing are known to carry large, but typically unknown uncertainty contribution to the
final analytical data and there is a lack of qualitative and quantitative data on the comparability of results achieved by
the different sampling methods.
To this end, an intercomparison programme was founded by the European Commission, in which different institutions participated
using their own ”in-situ” methods (”ANPA”, ”EDF” and ”MIDIYA”) for collection of water and suspended material in freshwater
bodies. The main criterion for this intercomparison was the agreement among the 137Cs activity concentrations in the dissolved phase (Bq l–1), those associated with the suspended particles (Bq g–1) and the concentrations of total suspended material (TSM) in the water body (mg l–1). The results show that the sampling systems provide TSM concentration results with low accuracy; on the contrary, they are
recommended for the determination of 137Cs activity concentration in the dissolved phase. Concerning the determination of radiocaesium activity concentration in the
suspended particles, the ”EDF” system provides the more reliable results.
Received: 10 December 2001 Accepted: 4 March 2002 相似文献
76.
Micol Santi Jakob Seitz Rossana Cicala Tomas Hardwick Dr. Nisar Ahmed Prof. Dr. Thomas Wirth 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(71):16230-16235
Amino acid derivatives undergo non-Kolbe electrolysis to afford enantiomerically enriched α-alkoxyamino derivatives through intermediate chiral carbenium ions. The products contain N,O-acetals which are important structural motifs found in bioactive natural products. The reaction is performed in a continuous flow electrochemical reactor coupled to a 2D-HPLC for immediate online analysis. This allowed a fast screening of temperature, electrode material, current, flow-rate and concentration in a DoE approach. The combination with online HPLC demonstrates that also stereoselective reactions can benefit from a hugely accelerated optimisation by combining flow electrochemistry with multidimensional analysis. 相似文献
77.
Fantazzini P Bortolotti V Garavaglia C Gombia M Riccardi S Schembri P Virgili R Soresi Bordini C 《Magnetic resonance imaging》2005,23(2):359-361
The current systems are unable to control and predict the cured meat composition nondestructively and in a reasonable time for production needs. In this work, T1 and T2 maps were obtained, with a monoexponential model, on internal sections of Longissimus dorsi muscle at increasing salting times. The maps allow one to visualize the salting process nondestructively and noninvasively. The method goes beyond the simple qualitative visualization, because, for each section of the sample and in any region of the section, it is possible to obtain quantitative information on the progress of salting and to predict salt-to-moisture ratios. In addition, detailed relaxation measurements were performed on samples cored after imaging in order to define better the relaxation properties of the dry-cured meat. 相似文献
78.
C. Marchioni E. Riccardi S. Spinelli F. DellUnto P. Grimaldi A. Bedini C. Giliberti L. Giuliani R. Palomba A. Congiu Castellano 《Ultrasonics》2009,49(6-7):569-576
The structural effect induced by therapeutic ultrasound on proteins in aqueous solution has been investigated with FTIR spectroscopy, UV–VIS spectroscopy, circular dichroism and light scattering. Six proteins (cytochrome, lysozyme, myoglobin, bovine serum albumin, trypsinogen, and α-chymotrypsinogen A) with different molecular weight and secondary structure have been studied. The experiment has been performed using an ultrasound source at resonant frequency of 1 MHz and sonication times of 10, 20, 30, 40, 50, and 60 min. A different behaviour of proteins under sonication depends on the dominant secondary structure type (α-helix or β-sheets) and on the grade of the ordered structure. The results suggest that the free radicals, produced by water sonolysis, have an important role in the changes of structural order. 相似文献
79.
An archaeometric investigation on plaster samples taken from the cellars of the Visconti Castle in Pavia has brought out the
existence of several layers and colour films of various periods. The oldest plaster spread over the masonry is formed with
three main layers. Above them, one film of bluish-black colour can be found on the arcs and on the bearing walls. On this
black layer, a white film of calcitic composition, containing also apatite fragments, was spread. These ancient materials
were subsequently covered in more recent times with plasters having different compositions and textures.
In the present paper the properties of the three main plaster layers and those of the black film are reported in detail. The
thermal behaviour of these materials was studied by means of differential thermal analysis and thermogravimetric analysis.
The study was completed with scanning electron microscopy,microprobe analysis and X-ray diffraction on powders.
The results obtained provide interesting clues for advancing reasonable hypotheses both on the methods adopted in ancient
building yards, and on the techniques for the production of the pigments employed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
80.
Raphael Beumer Pau BayónPiergiuliano Bugada Sylvie Ducki Nicola MongelliFederico Riccardi Sirtori Joachim Telser Cesare Gennari 《Tetrahedron》2003,59(44):8803-8820
The synthesis of a number of novel simplified eleutheside analogs with potent tubulin-assembling and microtubule-stabilizing properties is described, using ring closing metathesis as the key-step for obtaining the 6-10 fused bicyclic ring system. The RCM precursors were synthesized starting from aldehyde 3 [prepared in 6 steps on a multigram scale from R-(−)-carvone in 30% overall yield] via multiple stereoselective Brown allylations. Second generation RCM catalyst 13 gave the desired ring closed 10-membered carbocycles as single Z stereoisomers in good yields. The RCM stereochemical course (100% Z) likely reflects thermodynamic control. The crucial role of the protecting groups of the homoallylic and allylic substituents for the efficiency of the RCM reactions is discussed. These simplified analogs of the natural product (lacking inter alia the C-4/C-7 ether bridge) retain potent microtubule-stabilizing activity. However, the cytotoxicity tests did not parallel the potent tubulin-assembling and microtubule-stabilizing properties: limited cytotoxicity was observed against three common tumor cell lines (human ovarian carcinoma and human colon carcinoma cell lines, IC50 in the μM range given in Table 2), three orders of magnitude less than paclitaxel (IC50 in the nM range). 相似文献