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21.
Summary A new sensitive, spectrophotometric determination of gallium, based on the ternary gallium-Eriochrome Cyanine R-cetyltrimethylammonium system has been developed. At
max=588 nm the molar absorptivity is 1.20×105l·mole–1·cm–1. The maximum absorbance is attained in 5 min in the presence of a 10-fold molar ratio of Eriochrome Cyanine R and an approx. 50-fold ratio of cetyltrimethylammonium bromide (to Ga). Citrate, EDTA and to a lesser extent tartrate, interfere. Interference by metals can be eliminated by a preliminary extraction of gallium with di-isopropyl ether from 7 M hydrochloric acid in the presence of ascorbic acid as reductant. The results for gallium (at a concentration level of 1×10–4%) in aluminium metal and chloride indicate good precision and accuracy.
Empfindliche spektrophotometrische Bestimmung von Gallium mit Eriochromcyanin-R- und Cetyltrimethylammonium-Ionen
Zusammenfassung Ein neues, empfindliches spektrophotometrisches Verfahren zur Galliumbestimmung wurde ausgearbeitet. Es beruht auf der Verwendung des ternären Systems Gallium — EriochromcyaninR — Cetyltrimethylammonium. Bei max=588 nm beträgt die molare Extinktion 1,20·105 1·Mol–1·cm–1. Die maximale Absorption wird innerhalb von 5 min bei Gegenwart der 10fachen molaren Menge EriochromcyaninR und der etwa 50fachen Menge Cetyltrimethylammoniumbromid erreicht. Citrat, ÄDTA und in geringerem Maß Tartrat stören. Störende Wirkung durch Metalle kann durch vorhergehende Extraktion des Galliums mit Diisopropyläther aus 7 M Salzsäure in Gegenwart von Askorbinsäure als Reduktionsmittel vermieden werden. Die Ergebnisse (im Konzentrationsbereich von 10–4% Ga) in metallischem Aluminium und AICI3 sind genau.相似文献
22.
João Paulo R.F. André Hugh D. Burrows Carlos F.G.C. Geraldes Maria Da Graça M. Miguel M.Helena S.F. Teixeira L.F. Vilas Boas 《Polyhedron》1996,15(24):4331-4340
Complexation of europium(III) with glyceryl-1- and -2-phosphates has been studied by metal ion luminescence, 1H and 13C NMR spectroscopy and potentiometry. From the luminescence and NMR studies, the formation of a 1:1 inner-sphere complex, in which the glyceryl phosphate is directly bound to the metal, is confirmed. Similar apparent binding constants at pH 2 were obtained by the three methods. Values obtained by NMR at pH 2 are 53 M−1 and 12 M−1 for glyceryl-1- and -2-phosphate, respectively. By comparison with literature data on related systems it is suggested that the ligands bind through the phosphate group. To obtain structural information from the NMR data, complexation has also been studied with the lanthanide ions Dy(III), Er(III) and Gd(III) using both chemical shift and relaxation data. From this, metal-proton distance ratios have been calculated. Comparison of 1H and 13C NMR spectral data in the presence of paramagnetic lanthanides suggests conformational equilibria in the solutions. From the potentiometric studies, global formation constants have been determined, and speciation diagrams obtained over the pH range 1.5pH7.0 for ligand/metal ratios of 1 and 30. Implications of these results on lanthanide induced fusion of phospholipid membranes are discussed. 相似文献
23.
Maria Helena Araujo Maria D. Vargas Anthony G. Avent Fabrizia Grepioni 《Journal of organometallic chemistry》2004,689(22):3513-3519
The reactions of [HIr4(CO)9(Ph2PCCPh)(μ-PPh2)] (1) or [Ir4(CO)8(μ3-η2-HCCPh)(μ-PPh2)2] (2) with HCCPh gave two isomeric forms of [Ir4(CO)6(μ3-η2-HCCPh)(μ2-η4-C4H2Ph2)(μ-PPh2)2] (3 and 4) in good yields as the only products. These compounds were characterized with analytical and spectroscopic data including 1H, 13C and 31P NMR (1 and 2D) spectroscopy and their molecular structures were established by X-ray diffraction studies. Compounds 3 and 4 exhibit the same distorted butterfly metal polyhedral arrangement of metal atoms with two μ-PPh2 that occupy different positions in the structures of the two isomers. Both molecules contain a HCCPh ligand bonded in a μ3-η2-// mode to one of the wings of the butterfly and a metallacyclic ring, which resulted from head-to-tail coupling, in the case of [Ir4(CO)6(μ3-η2-HCCPh){μ2-η4-(H)CC(Ph)C(H)C(Ph)}(μ-PPh2)2] (3) and tail-to-tail coupling, in that of [Ir4(CO)6(μ3-η2-HCCPh){μ2-η4-(H)CC(Ph)C(Ph)C(H)}(μ-PPh2)2] (4), and which is linked to two metal atoms of the second wing of the butterfly. 相似文献
24.
Gustavsson S Rosén A Grennberg H Bolton K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2223-2227
Semiempirical and density functional electronic structure theory methods were used to study SWNT-X--R bond strengths, where the single-walled carbon nanotube (SWNT) had an armchair or zigzag structure, the link heteroatom X was O, N(H), or S and the hydrocarbon chain R was CH(2)CH(3), CH(OH)CH(3), CHCH(2), or CH(CF(3))CH(3). In all systems the hydrocarbon was bonded to the end of the nanotube. The SWNT-X--R bond (that is, the bond joining the link atom to the hydrocarbon) is more than 0.4 eV stronger for armchair than for zigzag nanotubes with the same diameters, irrespective of whether O, N, or S are used as link atoms or whether OH, C==C, or CF(3) groups are present in the hydrocarbon chain. This raises the possibility for selective manipulation of armchair/zigzag nanotubes using a variety of link atoms and hydrocarbon structures. The SWNT-O--CH(CF(3))CH(3) bond is weaker than the SWNT-O--CH(2)CH(3) bond (for both armchair and zigzag nanotubes), while inclusion of a double bond in the ethyl chain increases the bond strengths. Also, SWNT-S--CH(2)CH(3) and SWNT-N(H)--CH(2)CH(3) bonds are stronger than SWNT-O--CH(2)CH(3) bonds. 相似文献
25.
Helena?H?kanssonEmail author Per?Ahlgren Ulf?Germg?rd 《Cellulose (London, England)》2005,12(3):327-335
The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the
point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one
conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results
indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore
only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed
kraft pulp. A model explaining the LODP-results is proposed. 相似文献
26.
The effect of process variables on uronic acids (UAs) and hexenuronic acids (HexAs) in the annual crop Arundo donax L. during ethanol–alkali pulping has been examined. A substantial loss of UA moieties (up to 90%) was observed by the end of pulping (target kappa number 18) performed with 25% NaOH and 40% EtOH (by volume) within the temperature range of 130–150 °C. At the same time, the progressive formation of HexA in pulp was detected from the early phases of delignification. The proportion of HexA in the residual UA of the final pulp was found to be 84%, indicating almost complete conversion of 4-O-methylglucuronic acid side groups (MeGlcA) of heteroxylan into HexA. The kinetics of UA degradation and HexA formation has been described in terms of three consecutive first-order reaction stages. The overall rate of UA degradation was one order higher than the rate of UA conversion into HexA. The values of apparent activation energy were estimated as 68.6 and 94.7 kJ mol–1, respectively. The reaction medium alkalinity was shown to be the controlling factor for UA and HexA stability during ethanol–alkali pulping. An increase in alkali charge from 5% to 35% (as NaOH) led to UA loss of 40%, but promoted HexA formation from 11.8 to 20.1 mol g–1. The addition of organic solvent to the alkaline pulping solution had a similar effect, and about 10% of UA was lost and the content of HexA increased from 6.9 to 10.9 mol g–1 with an increase in ethanol proportion in the liquor from 20% to 60%. 相似文献
27.
28.
Carlos A.M Fraga Lis Helena P Teixeira Carla Maria de S Menezes Maria da Conceição K.V Ramos Eliezer J Barreiro 《Tetrahedron》2004,60(12):2745-2755
Reduction of 2-allyl-2-carboalkoxycyclohexanones (3d-f), 2-propyl-2-carboethoxycyclohexanone (3g) and 2-benzyl-2-carboethoxycyclohexanone (3h) with boron hydrides in the presence and absence of several chelating agents were studied. Molecular modeling studies using semiempirical PM3 method were performed in order to find a suitable explanation of the diastereoselection of ketone carbonyl faces during the reductive process, which yielded trans-2-allyl-2-carboethoxycyclohexanol (6e) and cis-2-allyl-2-carboethoxycyclohexanol (7e) in good diastereomeric excess by using inexpensive sodium and tetrabutylammonium borohydrides. 相似文献
29.
Phytochromes with noncovalently bound chromophores: the ability of apophytochromes to direct tetrapyrrole photoisomerization 总被引:1,自引:0,他引:1
Jorissen HJ Quest B Lindner I Tandeau de Marsac N Gärtner W 《Photochemistry and photobiology》2002,75(5):554-559
Chromophore-apoprotein interactions were studied with recombinant apoproteins, oat phytochrome (phyA) and CphB of the cyanobacterium Calothrix PCC7601, which were both incubated with the bilin compounds biliverdin (BV) IXalpha, phycocyanobilin (PCB) and the 3'-methoxy derivative of PCB. Previously it was shown that CphB and its homolog in Calothrix, CphA, show strong sequence similarities with each other and with the phytochromes of higher and lower plants, despite the fact that CphB carries a leucine instead of a cysteine at the chromophore attachment position and thus holds the chromophore only noncovalently. CphA binds tetrapyrrole chromophores in a covalent, phytochrome-like manner. For both eyanobacterial phytochromes, red and far-red light-induced photochemistry has been reported. Thus, the role of the binding site of CphB in directing the photochemistry of noncovalently bound tetrapyrroles was analyzed in comparison with the apoprotein from phyA phytochrome. Both the aforementioned compounds, which were used as chromophores, are not able to form covalent bonds with a phytochrome-type apoprotein because of their chemical structure (vinyl group at position 3 or methoxy group at position 3'). The BV adducts of both apoproteins showed phytochrome-like photochemistry (formation of red and far-red-absorbing forms of phytochrome [P(r) and P(fr) forms]). However, incubation of the oat apophytochrome with BV primarily yields a 700 nm form from which the P(r)-P(fr) photochemistry can be initiated and to which the system relaxes in the dark after illumination. The results for CphB were compared with a CphB mutant where the chromophore-binding cysteine had been introduced, which, upon incubation with PCB, shows spectral properties nearly identical with its (covalently binding) CphA homolog. A comparison of the spectral properties (P(r) and P(fr) forms) of all the PCB- and BV-containing chromoproteins reveals that the binding site of the cyanobacterial apoprotein is better suited than the plant (oat) phytochrome to noncovalently incorporate the chromophore and to regulate its photochemistry. Our findings support the proposal that the recently identified phytochrome-like prokaryotic photoreceptors, which do not contain a covalently bound chromophore, may trigger a light-induced physiological response. 相似文献
30.
Summary The addition of dienes to the system [(Cp2TiCl)2] LiAlH4toluene changes the system so that the complex [Cp2TiAlH4] is quantitatively formed instead of a titanocene hydride — aluminium hydride cluster. The complex [Cp2TiAlH4] is further converted into 3-allyltitanocene derivatives ([Cp2TiA]) if the diene structure is suitable for formation of stable [Cp2TiA] compounds and if the equilibrium [Cp2TiAlH4]+diene[Cp2TiA]+A1H3 is shifted towards the formation of [Cp2TiA] by the excess of diene. All the compounds [Cp2TiA] exhibit high-resolution e.s.r. spectra at g=1.993, showing interaction of the unpaired electron with the cyclopentadienyl and 3-allyl protons. The e.s.r. spectra clearly reveal the presence of alkyl substituents atsyn-1,3-positions of 3-allyl ligand, and show a triplet of multiplets for (3-allyl)titanocene, doublets of multiplets for (1-alkyl-3-allyl)titanocenes and single multiplets for (1,3-dialkyl-3-allyl)-titanocenes. thermal isomerization of (1,3-dimethyl-3-allyl)-titanocene and (1-methyl-3-ethyl-3-allyl)titanocene, hitherto considered as the stable Cp2TiA compounds, into (1-alkyl-3-allyl)titanocenes was confirmed by e.s.r. and electronic absorption spectroscopy as well as by chemical means. 相似文献