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21.
Vonderheide AP Meija J Tepperman K Puga A Pinhas AR States JC Caruso JA 《Journal of chromatography. A》2004,1024(1-2):129-137
High-performance chelation ion chromatography (HPCIC) was employed to retain cationic Cr(III) on an anion-exchange column and hence allow the separation of the two most prevalent forms of chromium, Cr(II) and Cr(VI). A mobile phase of nitric acid was utilized at pH = 1.5; additionally, 2,6-pyridinedicarboxylic acid was used at a concentration of 6 mM. Additives with different structural characteristics were used in an effort to elucidate retention mechanisms. Inductively-coupled plasma mass spectrometry was used for chromium detection. A collision cell was utilized to reduce chloride-based polyatomic ions that may interfere with the detection of Cr(III), and a detection limit study yielded levels in the low part-per-billion range. The newly developed method was applied to the chromatographic analysis of samples of an incubation medium containing Cr(VI) incubated with cell nuclei. 相似文献
22.
Susan A. Bourne Katherine De Villiers Timothy J. Egan 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(2):o53-o57
The structures of three compounds with potential antimalarial activity are reported. In N,N‐diethyl‐N′‐(7‐iodoquinolin‐4‐yl)ethane‐1,2‐diamine, C15H20IN3, (I), the molecules are linked into ribbons by N—H⋯N and C—H⋯N hydrogen bonds. In N‐(7‐bromoquinolin‐4‐yl)‐N′,N′‐diethylethane‐1,2‐diamine dihydrate, C15H20BrN3·2H2O, (II), two aminoquinoline molecules and four water molecules form an R54(13) hydrogen‐bonded ring which links to its neighbours to form a T5(2) one‐dimensional infinite tape with pendant hydrogen bonds to the aminoquinolines. The phosphate salt 7‐chloro‐4‐[2‐(diethylammonio)ethylamino]quinolinium bis(dihydrogenphosphate) phosphoric acid, C15H22ClN32+·2H2PO4−·H3PO4, (III), was prepared in order to establish the protonation sites of these compounds. The phosphate ions form a two‐dimensional hydrogen‐bonded sheet, while the aminoquinoline cations are linked to the phosphates by N—H⋯O hydrogen bonds from each of their three N atoms. While the conformation of the quinoline region hardly varies between (I), (II) and (III), the amino side chain is much more flexible and adopts a significantly different conformation in each case. Aromatic π–π stacking interactions are the only supramolecular interactions seen in all three structures. 相似文献
23.
Ellipsometry, surface tensiometry, and contact-angle measurement have been used to study the transition between partial wetting and pseudo-partial wetting of surfactant solutions by alkanes. In the partial wetting regime, the air-water surface tension is the same with and without alkane. In the pseudo-partial wetting regime, the air-water surface tension is lowered by the presence of alkane, showing that oil is solubilised into the surfactant monolayer. A discontinuous change in the coefficient of ellipticity with increasing surfactant concentration provides unequivocal evidence for the first-order nature of the wetting transitions. Ellipsometry has been used to explore the generality of wetting transitions of alkanes (dodecane, hexadecane, and squalane) on surfactant solutions [dodecyltrimethylammonium bromide, tetredecyltrimethylammonium bromide, dibucaine hydrochloride, and Aerosol OT (AOT)]. Of the systems studied, only hexadecane on AOT solutions did not show a wetting transition. Excess alkane remains as a lens on the surface of the surfactant solutions at all concentrations, but the contact angle is a minimum at the wetting transition. A semiquantitative model for the variation of the contact angle with surfactant concentration is provided. 相似文献
24.
Koontz JL Phillips KM Wunderlich KM Exler J Holden JM Gebhardt SE Haytowitz DB 《Journal of AOAC International》2005,88(3):805-813
Analysis of total folate concentration measured by microbiological assay in a variety of foods submitted in a routine manner to experienced laboratories that regularly perform folate analysis on fee-for-service basis was evaluated. Homogenates of fresh strawberries, frozen spinach, orange juice, frozen meat and vegetable pizza, dry macaroni, and dried pinto beans were prepared and stored under conditions previously determined to maintain stability of folate content. An aliquot of each composite and of 3 certified reference materials were sent on each of 4 occasions to 4 laboratories. Results for macaroni and pizza, the only folic acid-fortified foods, had considerably lower between-laboratory variation (CV(B)) with CV(B) of 9-11% versus >45% for other foods. Mean total folate ranged from 14 to 279 microg/100 g for a mixed vegetable reference material, from 5 to 70 microg/100 g for strawberries, and from 28 to 81 microg/100 g for wholemeal flour. Only 1 laboratory reported using a tri-enzyme extraction, and all laboratories used folic acid fortified foods as internal control materials. Users of commercial total folate analysis should understand the uncertainty in values determined by microbiological assay, particularly for foods containing primarily naturally occurring folate, which may not be apparent when replicate samples are not submitted for analysis. 相似文献
25.
G. A. Olah G. K. Surya Prakash P. Donald Katherine B. Loker Koop Lammertsma 《Research on Chemical Intermediates》1989,12(2):141-159
In this short review we have shown the importance of protosolvation of onium ions (containing non-bonded pairs of electrons)
in superacid catalyzed reactions. Such activation can result in unusual reactions such as aromatic alkylation with Meerwein’s
salts, aliphatic nitration with nitronium ion, alkylation of saturated hydrocarbons, greatly enhanced activity of acyl cations,
etc. Possibly such phenomena may be operative in hydroxylation reactions using protonated hydrogen peroxide in strong acid
solutions. Even the reactivity of halonium ions could be enhanced by protosolvation. Consequently, electrophilic protosolvation
may play a significant role in strogg acid catalyzed reactions. 相似文献
26.
Lemaître F Lucas D Groison K Richard P Mugnier Y Harvey PD 《Journal of the American Chemical Society》2003,125(18):5511-5522
The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd(3)(dppm)(3)(CO)(2+) cluster (Pd(3)(2+)) are investigated in detail. A series of alkyl halides (R-X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd(3)(2+) to form the corresponding Pd(3)(X)(+) adduct and "R(+)". This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd(3)(+). The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C-X bond strength and the steric factors. The highly reactive "R(+)" species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd(3)(2+) where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl --> RCO(+) + Cl(-); R = t-Bu, Ph) prior to Cl(-) scavenging by the Pd(3)(2+) species. For the other acid chlorides (R = n-C(6)H(13), Me(2)CH, Et, Me, Pr), a second associative process (Pd(3)(2+) + RCOCl --> Pd(3)(2+.....)Cl(CO)(R)) is seen. Addition of Cu(NCMe)(4)(+) or Ag(+) leads to the abstraction of Cl(-) from Pd(3)(Cl)(+) to form Pd(3)(2+) and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion ("RCO(+)") operates cleanly and rapidly. The trapping of "RCO(+)" with PF(6)(-) or BF(4)(-) leads to the corresponding acid fluorides and, with an alcohol (R'OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd(3)(Cl)(+)...M(+)" (M(+) = Cu(+), Ag(+)), which was successfully performed for Pd(3)(ClAg)(2+), as characterized by (31)P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF(6)(-) hydrolysis has been observed, where the product PF(2)O(2)(-) anion is observed in the complex Pd(3)(PF(2)O(2))(+), where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd(3)(2+) center. This work shows that Pd(3)(2+) is a stronger Lewis acid in CH(2)Cl(2) and THF than AlCl(3), Ag(+), Cu(+), and Tl(+). 相似文献
27.
Masaru Kawakami Katherine Byrne Bhavin S Khatri Tom C B McLeish D Alastair Smith 《Chemphyschem》2006,7(8):1710-1716
The viscoelastic properties of single poly(ethylene glycol) (PEG) molecules were measured by analysis of thermally and magnetically driven oscillations of an atomic force microscope (AFM) cantilever/molecule system. The molecular and monomer stiffness and friction of the PEG polymer were derived using a simple harmonic oscillator (SHO) model. Excellent agreement between the values of these two parameters obtained by the two approaches indicates the validity of the SHO model under the experimental regimes and the excellent reproducibility of the techniques. A sharp minimum in the monomeric friction is seen at around 180 pN applied force which we propose is due to a force induced change in the shape of the energy landscape describing the conformational transition of PEG from a helical to a planar state, which in turn affects the timescale of the transition and therefore modifies the measured internal friction. A knowledge of the viscoelastic response of PEG monomers is particularly important since PEG is widely used as a linker molecule for tethering groups of interest to the AFM tip in force spectroscopy experiments, and we show here that care must be exercised because of the force-dependent viscoelastic properties of these linkers. 相似文献
28.
C. Allen Chang Tamar E. Woodhouse Qihui Wu Ceshing Sheu Katherine McManus 《中国化学会会志》1988,35(4):253-260
Substitution-inert metal complexes, [Co(edda)(H2O)2]+, (Co(edda)(en)]+, [Co(edda)(dmen)]+, [Co(en)2-(gly)]2+, [Co(en)2(acac)]2+, and [Co(trien)(gly)]2+ in their nitrate salt solutions are used as eluents in nonsuppressed cation chromatography (where edda = ethylenediamine-N,N′-diacetic acid, en = ethylenediamine, dmen = N,N′-dime-thylethylenediamine, gly = glycine, acac = acetylacetone, and trien = triethylenetetraamine). It is found that all the mono- and di-valent charged complexes can be used to separate alkali and alkaline earth metal cations, respectively. The separations for monovalent cations are sometimes comparable to those using ultrapure HNO3 solutions. However, the divalent Ca2+ and Sr2+ ions cannot be resolved using the metal complex eluents. On the other hand, a selected, direct non-suppressed IC separation of zinc(II) and cadmium(II) ions is demonstrated for the first time using a substitution-inert metal complex eluent and a conductivity detector. Comparisons of these eluents with those reported previously, i. e. HNO3 and ethylenediammonium salt solution are made and explanations are proposed to account for the different selectivities observed where possible. The future development of this technique is also briefly discussed. 相似文献
29.
30.
Chen Hui Sharma Sunil K. Sharma Priyanka R. Chi Kai Fung Eric Aubrecht Katherine Keroletswe Ngonye Chigome Samuel Hsiao Benjamin S. 《Cellulose (London, England)》2021,28(13):8611-8628
Cellulose - Mercury contamination in drinking water is a worldwide problem due to its severely harming effects on the human body. A nanostructured natural bioadsorbent, carboxycellulose nanofiber... 相似文献