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141.
Batchelor-McAuley C Gonçalves LM Xiong L Barros AA Compton RG 《Chemical communications (Cambridge, England)》2010,46(47):9037-9039
Graphite is a highly versatile electrode substrate material but the recorded voltammetric response is regularly complicated by varying degrees of adsorption of the analyte to the surface leading to voltammetry which is complex to analyse. We report how through the pre-adsorption of acetone the electro-activity of the substrate is unhindered but adsorption of an electro-active species is effectively blocked, hence the experimentalist is able to readily tailor the electrode so as to effectively switch the adsorption of the analyte 'on' or 'off'. 相似文献
142.
Barros TC Cuccovia IM Farah JP Masini JC Chaimovich H Politi MJ 《Organic & biomolecular chemistry》2006,4(1):71-82
The study of highly conjugated, carbonyl-containing molecules such as 1,4,5,8-naphthalene tetracarboxylic dianhydride, III, is of interest since reactivity differences and transmission of electronic effects through the conjugated framework can be evidenced. The kinetics of hydrolysis of III in aqueous solution were determined from 5 M acid to pH 10. In basic solution hydrolysis of III yields, sequentially, 1,4,5,8-naphthalene diacid monoanhydride, II, and 1,4,5,8-naphthalene tetracarboxylic acid, I. The second order rate constant for alkaline hydrolysis is 200 fold higher for the first ring opening. The water-catalyzed hydrolysis of III yields a pH-dependent mixture of ionic forms of I and II. The rate constant for water-catalyzed hydrolysis of III is 25 fold higher than that for II. In concentrated acid the rates for reaching equilibrium (I, II and III) increase and III is the major product. The pK(a)s of I (3.24, 5.13 and 6.25) and II (3.05, 5.90) were determined by potentiometric, fluorescence and UV spectroscopy titrations and by quantitative fit of the kinetic and equilibrium data. The apparent, pH-dependent, equilibrium constants, K(EqII), for anhydride formation between I and II were obtained from the UV spectra. The quantitative fit of kinetic and equilibrium data are consistent with the assumption that anhydride formation only proceeds with the fully protonated species for both I and II and permitted the estimation of the equilibrium constants for anhydride formation, K(EqII). The value of K(EqII) (I <==> II) between pH 1 and 6 was ca. 5. Geometry optimization calculations in the gas phase of the reactions of III in alkaline, neutral and acid conditions, at the DFT level of theory, gave electronic distributions that were qualitatively consistent with the experimental results. 相似文献
143.
The activation energies for the methyl exchange reactions between Cp2M-CH3 and H-CH3 have been calculated for M = Sc, Y and representative metals of the lanthanide family (La, Ce, Sm, Ho, Yb and Lu) with DFT(B3PW91) calculations with large-core pseudopotentials for M. The sigma-bond metathesis reactions are calculated to have lower activation energies for early lanthanides than for late lanthanides and any of group 3 metals. The relative activation barriers are analyzed using the NBO charge distributions in the reactant and in the transition states. It is shown that the methane needs to be polarized in the transition state as H((+delta))-CH3((-delta)) by the reactant, because this sigma-bond metathesis is best viewed as heterolytic cleavage of methane, leading to a proton transfer between two methyl groups in the field of an electropositive M metal. Early lanthanides, which are involved in strongly ionic metal-ligands bonds are thus associated with the lowest activation energies. The ionic radius and the steric effects influence the relative rates of reaction for the complexes of Sc, Y and Lu. In agreement with earlier works of Sherer et al., the experimental reactivity trends found by Tilley are reproduced best with Cp*2M-CH3 (Cp* = C5Me5) rather than Cp2M-CH3 (Cp = C5H5) because the steric bulk of C5Me5 deactivates most the complex where the metal has the smallest ionic radius (Sc). While the steric effects and the influence of the metal ionic radius cannot be neglected, these factors are not the only ones involved in determining the activation barriers of the sigma-bond metathesis reaction. 相似文献
144.
Synthesis and biodegradation studies of new copolymers based on sucrose derivatives and styrene 总被引:1,自引:0,他引:1
Novel unsaturated esters of sucrose were synthesised directly and in good yields from sucrose using a simple, mild, and selective Mitsunobu procedure. These regioisomerically pure vinyl sugars have been copolymerised with styrene by a free radical process, yielding unbranched linear polymer materials with pendant sucrose moieties. Their physical properties indicate that these polymers have potential technological relevance as amphiphilic and biodegradable materials. Biodegradation tests on the copolymer samples by a fungal (Aspergillus niger) culture method showed a fungal growth ?60%, indicating good biodegradability and that the copolymers were partially bio-assimilated during microbial attack. 相似文献
145.
A [3+2] cycloaddition of diethyl 1,2-oxiranephosphonate to aryl isocyanates catalyzed by lanthanide cations is described. The reaction is highly regioselective and 5-substituted 2-oxazolidinone phosphonates are obtained with a regioselectivity greater than 95:5 with respect to the 4-substituted regioisomer, and in up to 84% yield. When 20 mol % of Pybox-Yb3+ is used as a catalyst, enantiomerically enriched products are obtained in up to 75% ee, depending on the reaction conditions, and the nature of the isocyanate. Low temperatures benefit asymmetric induction, but have an adverse effect on the regioselectivity for para-substituted aryl isocyanates. 相似文献
146.
R. J. N. Bettencourt da Silva M. Filomena G. F. C. Camões João Seabra e Barros 《Accreditation and quality assurance》1998,3(4):155-160
Every analytical result should be expressed with some indication of its quality. The uncertainty as defined by Eurachem ("parameter
associated with the result of a measurement that characterises the dispersion of the values that could reasonably be attributed
to the, . . ., quantity subjected to measurement") is a good tool to accomplish this goal in quantitative analysis. Eurachem
has produced a guide to the estimation of the uncertainty attached to an analytical result. Indeed, the estimation of the
total uncertainty by using uncertainty propagation laws is components-dependent. The estimation of some of those components
is based on subjective criteria. The identification of the uncertainty sources and of their importance, for the same method,
can vary from analyst to analyst. It is important to develop tools which will support each choice and approximation. In this
work, the comparison of an estimated uncertainty with an experimentally assessed one, through a variance test, is performed.
This approach is applied to the determination by atomic absorption of manganese in digested samples of lettuce leaves. The
total uncertainty estimation is calculated assuming 100% digestion efficiency with negligible uncertainty. This assumption
was tested.
Received: 3 November 1997 · Accepted: 2 January 1998 相似文献
147.
Ana I. R. N. A. Barros André F. R. Dias Artur M. S. Silva 《Monatshefte für Chemie / Chemical Monthly》2007,8(7):585-594
A one-pot synthesis of novel 2-arylquinolines and 2-aryl-4-hydroxyquinolines was developed from the intramolecular reductive
coupling reactions of 2-nitrochalcones and 3-hydroxy-1-phenyl-3-(2-nitrophenyl)-2-propen-1-ones. Depending on the reduction
method and on the presence of electron donating substituents on the A ring of 2-nitrochalcones one can modulate the formation
of 2-arylquinolines, their N-oxides, and of 2-aminochalcones. The reduction of 3-hydroxy-1-(2-hydroxyphenyl)-3-(2-nitrophenyl)-2-propen-1-ones with stannous
chloride in hydrochloric acid gave 2′-aminoflavones and with ammonium formate and Pd/C yielded 2-(2-hydroxyaryl)-4-hydroxyquinolines. 相似文献
148.
C. V. Barros Leitte E. A. Schweikert 《Journal of Radioanalytical and Nuclear Chemistry》1985,90(2):325-332
The backscattering performance of 2 MeV He+ and N+ beams was studied using Ta2O5 targets as test targets. To allow ready comparison, the scattering geometry, projectile energy, and detection system were kept identical for both beams. Tantalum oxide films with thicknesses of 200 Å to 4000 Å were examined. For thickness determinations, beam straggling was found to be the major limiting factor. For thickness measurements below 1000 Å the N+ beam is best suited for larger thicknesses; the He+ beam is superior. For stoichiometric determinations both beams provide equally accurated and precise data. 相似文献
149.
Maria D. M. C. Ribeiro da Silva N. R. M. Araújo A. L. R. Silva L. C. M. da Silva N. P. S. M. Barros J. M. Gonçalves M. A. V. Ribeiro da Silva 《Journal of Thermal Analysis and Calorimetry》2007,87(1):291-296
The standard (p
0=0.1
MPa) molar enthalpies of formation, at T=298.15
K, in the gaseous phase, for three tetradentate Schiff bases involving a N2O2
set, N,N’-bis(salicylaldehydo)cyclohexanediimine
(H2salch), N,N’-bis(acetylacetone)cyclohexanediimine (H2acacch)
and N,N’-bis(benzoylacetone)cyclohexanediimine
(H2bzacch), were determined from their enthalpies of
combustion and sublimation, obtained by static bomb calorimetry in oxygen
and by the Knudsen effusion technique, respectively. The results are compared
with identical parameters for related compounds previously studied, resulting
from the condensation of salicylaldehyde or β-diketone with aliphatic
diamines. 相似文献
150.
Djalma A. de Barros Filho Pompeu P. Abreu Filho U. Werner Michel A. Aegerter 《Journal of Sol-Gel Science and Technology》1997,8(1-3):735-742
Structural, optical, electro and photoelectrochemical properties of amorphous and crystalline sol-gel Nb2O5 coatings have been determined. The coatings are n-type semiconductor with indirect allowed transition and present an overall low quantum efficiency ( < 4%) for UV light to electric conversion. The photoconducting behavior of the coatings is discussed within the framework of the Gärtner and Södergren models. Improvement can be foreseen if Nb2O5 coatings can be made of 10–20 nm size nanoparticles. 相似文献