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461.
Pancharatna PD Méndez-Rojas MA Merino G Vela A Hoffmann R 《Journal of the American Chemical Society》2004,126(46):15309-15315
A recently proposed system with a central planar tetracoordinate carbon linking two three-membered rings, C(5)(2-), lends itself to extension in one, two, and three dimensions. Our construction of potential realizations begins with an analysis of the electronic structure of C(5)(2-). Dimers such as C(10)Li(3-), C(10)Li(4), and a trimer C(15)Li(6) are then examined, and their geometries are optimized to find clues for ways the C(5)(2-) unit may polymerize in the presence of countercations. Coordination through the terminal carbons is favored in the oligomers and polymers; several electronically and structurally reasonable systems of the stoichiometry C(5)M(x) (M = Li, x = 2; M = Be, Pt, Zn, x = 1) emerge from band structure calculations and energetic considerations. 相似文献
462.
Montserrat Baucells Núria Ferrer Pilar Gómez Gloria Lacort Montserrat Roura 《Mikrochimica acta》1993,112(1-4):87-98
Fourier transform infrared spectroscopy has been used for the quantitative determination of caffeine in several pharmaceutical products: acetyl salicylic acid, paracetamol and propyphenazone. The method is simple, rapid and selective, and allows the determination of caffeine without sample pretreatment and without separation from the matrix. Two techniques for measuring the infrared spectra of caffeine are described: transmission through pellets and diffuse reflectance from powder (DRIFT). In both methods, samples were diluted (1%) with KBr. Caffeine in pharmaceutical matrices was recovered within 5% error in pellets and 10% by DRIFT. Relative standard deviations were generally 1.5% for repeated measurements on a single pellet and 5% for measurements on different pellets. DRIFT in the vacuum mode gave rather large RSDs. The limit of detection of the pellet technique was about 0.5% caffeine in the original sample. 相似文献
463.
The condensation reaction between α-keto-β-aroyl (or acyl) -γ-butyrolactones, 4a-4e and o-phenylenediamine or 2, 3-diaminonaphthalene leads under retrograde aldol condensation involving loss of formaldehyde to formation of 3-substituted-3, 4-dihydro-2 (1H) quinoxalinones or benzo [g] quinoxalinones, 7a-7g , respectively as a new convenient synthesis of this type of heterocyclic systems. The reaction of type 4 compound with 4, 5-diaminopyromidine, 8 , was found to proceed differently. 2-[(4-Amino-5-pyrimidinyl)amine]-4-oxo-3-(hydroxymethyl)-4-phenyl-2-butenoic acid 9 was the only product formed when the reaction between 4a and 8 was run in ethanol. The same reaction in glacial acetic acid proceeds with loss of formaldehyde, to afford 7-phenacylidene-7,8-dihydro-6 (1H)-pteridione 10 . The reaction between type 4 compounds and ethylenediamine or 1, 4-phenylenediamine leads to the formation of the bis-condensation products 13–15 , respectively. 相似文献
464.
Iria Fraga S. Montserrat J. M. Hutchinson 《Journal of Thermal Analysis and Calorimetry》2007,87(1):119-124
TOPEM is a new temperature modulated DSC technique, introduced by Mettler-Toledo
in late 2005, in which stochastic temperature modulations are superimposed
on the underlying rate of a conventional DSC scan. These modulations consist
of temperature pulses, of fixed magnitude and alternating sign, with random
durations within limits specified by the user. The resulting heat flow signal
is analysed by a parameter estimation method which yields a so-called ‘quasi-static’
specific heat capacity and a ‘dynamic’ specific heat capacity
over a range of frequencies. In a single scan it is thus possible to distinguish
frequency-dependent phenomena from frequency-independent phenomena. Its application
to the glass transition is examined here. 相似文献
465.
Contaminated industrial effluents often contain a variety of organic pollutants that can disturb the development of the endocrine system and the organs that respond to endocrine signals in organisms. Total characterisation of the effluents to detect all these potential endocrine disruptors by standard GC-MS methods is difficult since this technique often misses the more polar or non-volatile fraction of these organic compounds. The wide spectrum of endocrine-toxic substances and their low concentrations require powerful analytical approaches like LC-MS for their identification. Furthermore, the methods for the detection of endocrine effects developed up to now do not show all the possible oestrogenic effects and results of the different tests are not comparable to one another. Therefore, further ecotoxicological research should be carried out so that new strategies can be proposed. In this article, an overview of the analytical methods for determining organic contaminants in industrial effluents and tests for the identification of endocrine disruptors and their effects is presented. 相似文献
466.
In this work, the simultaneous quantification of three alkaline ions (potassium, sodium and ammonium) from a single impedance spectrum is presented. For this purpose, a generic ionophore - dibenzo-18-crown-6 - was used as a recognition element, entrapped into a polymeric matrix of polypyrrole generated by electropolymerization. Electrochemical impedance spectroscopy (EIS) and artificial neural networks (ANNs) were employed to obtain and process the data, respectively. In fact, EIS detected the ions exchanged between the medium and the sensing layer whereas ANNs, after an appropriated training process, could turn the impedance spectrum into concentrations values. A sequential injection analysis (SIA) system was employed for operation and to automatically generate the information required for the training of the ANN. Best results were obtained by using a backpropagation neural network made up by two hidden layers: the first one contained three neurons with the radbas transfer function and the second one ten neurons with the tansig transfer function. Three commercial fertilizers were tested employing the proposed methodology on account of the high complexity of their matrix. The experimental results were compared with reference methods. 相似文献
467.
Fontes M Verdaguer X Solà L Pericàs MA Riera A 《The Journal of organic chemistry》2004,69(7):2532-2543
Here we report the use of 2-piperidino-1,2,2-triphenylethanol (5) as an outstanding catalyst for the ligand-catalyzed arylation of aldehydes. The use of 5 and a 2/1 mixture of Et(2)Zn/Ph(2)Zn provided the corresponding chiral diarylcarbinols with enantiomeric excess of up to 99% ee. The effect of temperature on the reaction enantioselectivity was studied and the inversion temperature (T(inv)) was determined to be 10 degrees C for reaction with p-tolylaldehyde. Most remarkably, lowering the amount of catalyst (5) to 0.5 mol % still afforded excellent levels of enantiocontrol (93.7% ee). Kinetics of the catalyzed and uncatalyzed arylation of aldehydes was studied by means of in situ FT-IR. The background uncatalyzed addition rates to p-tolylaldehyde when using pure Ph(2)Zn and Et(2)Zn/Ph(2)Zn (2/1) suggest that in the latter case a mixed zinc species forms (EtPhZn) minimizing the undesired nonselective addition. Formation of EtPhZn was modeled at the DFT calculation level. A four-center TS (TS-V) corresponding to the Et/Ph scrambling was localized along with two dimers (D-IV and D-VI). The model supports the hypothesis that Et/Ph exchange is a kinetically facile process. Gas evolution experiments during the formation of the active catalyst showed that the formation of an active site with a ONZn-Et (10) moiety is kinetically favored over ONZn-Ph (11). Finally, the phenyl transfer to benzaldehyde was modeled at the PM3(tm) level through anti and syn 5/4/4 tricyclic TS structures for both 10 and 11. The model could correctly predict the sense and selectivity of the overall process and predicted that 11 should be more selective than 10. 相似文献
468.
V. L. Cebolla M. Matt E. M. Gálvez L. Membrado M. P. Domingo J. Vela N. Beregovtsova V. Sharypov B. N. Kuznetsov N. Marin J. V. Weber 《Chromatographia》2002,55(1-2):87-93
Summary Two alternative methods, based on Thin-Layer Chromatography (TLC) with Fluorescence Scanning Densitometry have been developed
for characterization of heavy liquids from copyrolysis of different kinds of biomass and plastics in autoclaves under inert
atmosphere.
A conventional TLC system, which includes a vertical developing tank, and a High Performance TLC (HPTLC) system, with a horizontal
developing chamber and the use of HPTLC plates, have been used. The analytical method involves in both cases the measurement
of two chromatograms per sample: the first, on a silica gel berberine-impregnated plate, for detection of saturates using
the phenomenon of berberine-induced fluorescence; and the second, on a silica gel plate, for detection of aromatic-polars
and polars, by native fluorescence.
Although the HPTLC system is more sensitive and faster, both techniques represent an improvement with regard to current methods
for analyzing these kinds of products. However their application depends on the particular solubility of each sample and on
its slope of the fluorescent response-sample load regression. 相似文献
469.
Summary Twelve dinuclear complexes, each with a bridging cyano group, of formulaecis-M[(aa)2FCr-NC-Cr(CN)5] andcis- M[(aa)2FCr-NC-CrNO(CN)4] (aa=ethylenediamine or 1,3-diaminopropane; M=K+, Na+ or NH
4
+
) have been prepared by solid state heating of the corresponding doubly complexed saltstrans-M[CrF(H2O)(aa)2][Cr(CN)6] andtrans-M [CrF(H2O)(aa)2][CrNO(CN)5]. All have been characterized by chemical analysis, electronic and i.r. spectra and conductivity measurements.Magnetic susceptibilities down to liquid helium temperature have been determined forcis-K[(en)2FCr-NC-Cr(CN)5],cis-K[(tn)2FCr-NC-Cr(CN)5] andcis-K[(en)2FCr-NC-CrNO-(CN)4]. Slight antiferromagnetic interactions have been found in the first two compounds (J=–1.4 and –1.2 cm–1, respectively). Thecis-K[(en)2FCr-NC-CrNO(CN)4] compound behaves as a Curie paramagnet and a structural model is proposed to explain this fact. 相似文献
470.
S. S. Krishnan R. E. Jervis L. D. Vela 《Journal of Radioanalytical and Nuclear Chemistry》1992,161(1):181-187
We have examined the leachability of the toxic elements cadmium, arsenic, mercury, and selenium from solid wastes. The solid wastes studied are municipal incinerator ash, coal fly ash, hospital incinerator ash, raw sewage sludge, sewage incinerator bottom ash, and sewage incinerator lagoon ash (which is a combination of bottom and fly ashes). Cadmium displayed the greatest leachability in all waste types, with 76% leached from the municipal refuse incinerator ash. Although the sources of elements in the wastes are diverse, the leachability and hence the bioavailability in the incinerator ash appears mainly determined by the volatility of the element. 相似文献