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131.
Self-assembly of amphiphilic hexapyridinium cations at the air/water interface and on HOPG surfaces.
Haofei Gong Dr. Björn Bredenkötter Dr. Christoph Meier Carola Hoffmann‐ Richter Dr. Ulrich Ziener Dr. Dirk G. Kurth Dr. Dirk Volkmer Prof. Dr. 《Chemphyschem》2007,8(16):2354-2362
Mono- and multilayers of a novel amphiphilic hexapyridinium cation with six eicosyl chains (3) are spread at the air/water interface as well as on highly ordered pyrolytic graphite (HOPG). On water, the monolayer of 3 is investigated by recording surface pressure/area and surface potential/area isotherms, and by Brewster angle microscopy (BAM). Self-organized tubular micelles with an internal edge-on orientation of molecules form at the air/water interface at low surface pressure whereas multilayers are present at high surface pressure, after a phase transition. Packing motifs suggesting a tubular arrangement of the constituting molecules were gleaned from atomic force microscopy (AFM) investigations of Langmuir-Blodgett (LB) monolayers being transferred on HOPG at different surface pressures. These LB film structures are compared to the self-assembled monolayer (SAM) of 3 formed via adsorption from a supersaturated solution, which is studied by scanning tunnelling microscopy (STM). On HOPG the SAM of 3 consists of nanorods with a highly ordered edge-on packing of the aromatic rings and an arrangement of alkyl chains which resembles the packing of molecules at the air/water interface at low surface pressure. Additional details of the molecular packing were gleaned from single-crystal X-ray structure analysis of the hexapyridinium model compound 2b, which possesses methyl instead of eicosyl residues. 相似文献
132.
Michael Ptzel Andreas Schulz Jürgen Liebscher Wolfgang Richter Monika Richter 《Journal of heterocyclic chemistry》1992,29(5):1209-1211
Cyclic N-cyanocarbonimidodithioesters 4 or N-aroylcarbonimidothioic acid esters 10 react regioselectively with arylhydrazines and methylhydrazine by a ring chain transformation reaction forming ω-functionalized 3-alkylthio-1,2,4-triazoles 8 and 11 or 5-alkylthio-1,2,4-triazoles 9 . 相似文献
133.
Dabkowska I Rak J Gutowski M Nilles JM Stokes ST Bowen KH 《The Journal of chemical physics》2004,120(13):6064-6071
The photoelectron spectrum of the uracil-alanine anionic complex (UA)(-) has been recorded with 2.540 eV photons. This spectrum reveals a broad feature with a maximum between 1.6 and 2.1 eV. The vertical electron detachment energy is too large to be attributed to an (UA)(-) anionic complex in which an intact uracil anion is solvated by alanine, or vice versa. The neutral and anionic complexes of uracil and alanine were studied at the B3LYP and second-order M?ller-Plesset level of theory with 6-31++G(*) (*) basis sets. The neutral complexes form cyclic hydrogen bonds and the three most stable neutral complexes are bound by 0.72, 0.61, and 0.57 eV. The electron hole in complexes of uracil with alanine is localized on uracil, but the formation of a complex with alanine strongly modulates the vertical ionization energy of uracil. The theoretical results indicate that the excess electron in (UA)(-) occupies a pi(*) orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of alanine to the O8 atom of uracil. As a result, the four most stable structures of the uracil-alanine anionic complex can be characterized as a neutral radical of hydrogenated uracil solvated by a deprotonated alanine. Our current results for the anionic complex of uracil with alanine are similar to our previous results for the anion of uracil with glycine, and together they indicate that the BFPT process is not very sensitive to the nature of the amino acid's hydrophobic residual group. The BFPT to the O8 atom of uracil may be relevant to the damage suffered by nucleic acid bases due to exposure to low energy electrons. 相似文献
134.
A recently characterized oviposition-deterring pheromone (ODP, structure 1) of the European cherry fruit fly was used as a test case for probing the potential of tandem mass spectrometry (MS/MS) in structure elucidation as a stand-alone technique. The glycolipid-taurinate 1 was subjected to MS/MS analyses under a variety of conditions with and without preceding chemical degradation. Acidic methanolysis of 1 and subsequent in-batch derivatization (trideuterioacetylation) yielded methyl 2,3,4,6-tetrakis-O-trideuterioacetyl-glucopyranoside (2), methyl 8,15-bis-trideuterioacetoxy-palmitate (3), and taurine (4) as suitable target compounds for direct mixture analysis.Low energy collision induced dissociation (CID) on selected precursor ions (MS/MS on [M + H – CH3OH]+ and [M + H]+ produced by fast atom bombardment (FAB)) allowed direct identification of 2 and 4, respectively, by comparison with appropriate reference ions. In the case of 3, low energy CID (desorption chemical ionization (DCI) instead of FAB, MS/MS on [M + H]+) permitted deduction of gross molecular structure, but failed to provide positional detail. In sharp contrast,high energy CID of trideuterioacetylated intact 1 (FAB-MS/MS on [M – H]– ions of la) clearly revealed a linear 8,15-hydroxylated palmitic acid backbone. Less certain was assignment of 15-O-glucosylation by this approach. 相似文献
135.
The kinetics of base hydrolysis of the trans-[Cr(NH3)2(NCS)4]– anion follows the rate law: -d[complex]/dt = k
0 + k
1[OH–] (50–70 °C, [OH–] = 0.1–1.9 M and = 2.0 M). The specific salt effect has been investigated for eight aqueous media: NaCl, NaBr, NaI, NaClO4, KCl, KBr, CsCl and CsBr. The alkali-independent path (k
0) does not show any specific effect of inert electrolyte ions, the activation parameters: H
= 113.5 ± 0.4 kJ mol–1 and S
= 24.1 ± 1.3 J mol–1 K–1 are interpreted in the frame of a dissociative interchange mechanism (I
d). For the alkali-dependent path (k
1) the specific salt effect is observed for cations of the inert electrolyte, showing an important role for ion-pair formation between the cations and reagent complex anion in the activation process. A linear correlation between lnk
1 and lnK
0 (K
0 – ion-pair formation constant) has been found for the cations studied. The dissociative, via conjugate base, mechanism (D
CB) has been proposed for the alkali-dependent path. 相似文献
136.
H. J. Cortes C. D. Pfeiffer B. E. Richter T. S. Stevens 《Journal of separation science》1987,10(8):446-448
Porous ceramic bed supports for fused silica packed capillary columns utilized in liquid chromatography were prepared by polymerizing solutions containing potassium silicate in-situ within a column to create a mechanically stable, rugged, and easily constructed termination. The effect of the bed support length on efficiency, and comparisons to glass wool bed supports, were considered in terms of column efficiencies and hydrodynamic variables. Results obtained indicate better performance for the ceramic bed support. 相似文献
137.
K. H. Richter W. Güttler M. Schwoerer 《Applied Physics A: Materials Science & Processing》1983,32(1):1-11
Holographic gratings with grating periods between 0.37 m and 4.4 m, respectively, were recorded on the surface of TS-diacetylene single crystals by two interfering 257 nm laser beams. Diffraction efficiencies of up to 35 % were obtained for readout with red light of = 633 nm. The main modulation mechanism was phase modulation. The spatial resolution was better than 1600 lines/mm. The upper limit of the polymer chain length was 0.6 m. The holographic sensitivity was 4.5 cm2/J for immediate read-out at 633 nm without processing after exposure. — A weakly exposed latent hologram may be developed simply by gentle annealing the crystal which increases the sensitivity by one order of magnitude. 相似文献
138.
Danuta Slawinska Janusz Slawinski Krzysztof Polewski Wojciech Pukacki 《Photochemistry and photobiology》1979,30(1):71-80
Abstract— Peroxidation of tannins with alkaline H2 O2 is accompanied by weak chemiluminescence in the spectral region 480–800 nm. o-Di and tri-hydroxy groups of polyphenols undergo oxidation by a free-radical mechanism and a green intermediate anion-radical with absorption Δmax = 600 nm is formed. The radical mechanism is supported by the low activation energy 14–20 kJ/mol and the quenching effect of radical scavengers. The reaction of the green intermediate with peroxy anions is the chemiluminescence rate limiting step. In the presence of a-hydroxy-methylperoxide formed from H2 O2 and formaldehyde, the alkaline peroxidation of tannins is accompanied by strong red luminescence (420–800 nm). The base catalyzed decomposition of peroxides gives only a weak red emission (460–800 nm). Light intensity is enhanced in D2 O by a factor 6.5. Quenchers of O2 (1 Δg ) and 1,3-di-phenylisobenzofurane diminish light intensity in non-aqueous solutions. The data suggest 1 O2 participation in the observed chemiluminescence. Thermo-chemical calculations give —ΔH values from 250–1000 kJ/mol for one elementary reaction step which limits the mechanism of chemi-enereization. Chemiexcitation of tannins is relevant to biochemical mechanisms of aerobic degradation of aromatic compounds, energy utilization as well as to defense and resistance processes in plants. 相似文献
139.
Janusz Moskal 《Tetrahedron》1984,40(21):4447-4453
Highly substituted 1,Δ-diazabutadienes react with aroyl isothiocyanates in a 1,3-dipolar cycloaddition node yielding five-membered thiohydantoin-type heterocycles. The cycloaddition is accompanied by 1,4 shift of hydrogen from a methyl group attached to C2 of the 1,C-diazabutadiene moiety. The mechanism of this reaction is discussed in comparison with similar cycloadditions with aryl isocyanates. 相似文献
140.
The Na[Cr(PDA)2] · 2H2O complex (PDA1 = dipicolinic acid anion) and its aquation product, [Cr(PDA)(H2O)3]+, were prepared and characterized. The electronic spectra demonstrate that the bis(dipicolinato) complex undergoes very fast partial dechelation during dissolution. In acidic media, pH controlled, rapid protolytic and ring opening processes lead to coexistence of complexes with one tridentate (PDA) and the other bi- or mono-dentate (PDA). The kinetics of PDA ligand liberation were followed spectrophotometrically within the 0.1–2.0 M HClO4 range at I = 2.0 M. The observed first-order rate constant depends on [H+] according to the equation: k
obs = A[H+]/(1 + B[H+] + C[H+]2). A reaction course via the uncharged [Cr(PDA)(HPDA)(H2O)2]0 complex is proposed. The observed rate increase, followed by rate retardation with [H+] increase, is attributed to the unreactive [Cr(PDA)(H2PDA)(H2O)2]+ complex. In terms of the proposed mechanism, A, B, C parameters have been defined as: A = k
1
Q
1, B = Q
1, C = Q
1
Q
2 where k
1 is the rate constant of the CrIII-carboxylato oxygen bond-breaking in the monodentate HPDA ligand, Q
1 is a composite value describing protolytic and dechelation processes and Q
2 is the protonation constant of the uncharged [Cr(PDA)(HPDA)(H2O)2]0 complex. 相似文献