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61.
Andrs Strawczynski Renzo Ros Raymond Roulet Fabrizia Grepioni Dario Braga 《Helvetica chimica acta》1988,71(8):1885-1894
The two complexes [Ir4(CO)10(diarsine)] (1) and [Ir4(CO)10(1,5-cyclooctadiene)] (2) bear a bidentate ligand chelating one metal atom of the basal face of the Ir4 tetrahedron. However, they differ in fluxional behaviour as observed by 2D-exchange and variable-temperature 13C-NMR. The CO-site exchange with lowest activation energy proceeds via an unbridged intermediate in 2 , whereas that in 1 occurs via a concerted edge-bridging of CO's to an alternative face of the metal core. This difference is apparently related to different ground-state geometries: the basal CO's are symmetrically bridging in 2 , whereas two CO's are semi-bridging in 1 . The molecular structure of 2 was determined by single crystal X-ray diffractometry. The crystals are orthorhombic, space group Pbca, a = 11.651(4), b = 13.118(3), c = 28.64(1)Å. The idealized molecular symmetry is Cs. The diolefin chelates a basal Ir-atom replacing an axial and a radial CO group on the tetrahedral metal-atom framework. 相似文献
62.
N. Omenetto B.W. Smith L.P. Hart P. Cavalli G. Rossi 《Spectrochimica Acta Part B: Atomic Spectroscopy》1985,40(10-12)
Two pulsed dye lasers pumped by an excimer laser are simultaneously directed into the analytical zone of an inductively coupled argon plasma. When the two beams are tuned to the appropriate ionic transitions, highly excited ionic levels can be efficiently populated in saturated conditions, the resulting fluorescence being then spectrally isolated with a monochromator and measured. A theoretical outline of this technique, variously called double-resonance fluorescence or two-step fluorescence, is given. The experimental results obtained with the alkaline-earth metals Ca, Sr, Ba and Mg show that the technique does provide excellent sensitivity, freedom from scattering problems and unprecedented spectral selectivity. The laser characteristics, the time overlap between the pulses and the spectral characteristics of the transitions used are discussed. Finally, ionic fluorescence in the plasma is the most suitable analytical application of such double-resonance technique since its use in flame atomic fluorescence suffers from the strong depletion of the excited levels due to collisionally assisted ionization. 相似文献
63.
Roberta Cavalli Francesco Trotta Wander Tumiatti 《Journal of inclusion phenomena and macrocyclic chemistry》2006,56(1-2):209-213
Nanosponges were prepared from β-cyclodextrins as nanoporous materials for possible use as carriers for drug delivery. The structure of β-cyclodextrin-based nanosponges was principally investigated by FT-IR, DSC and RX analyses. Sizes, morphology and toxicity were also examined. The capacity of the nanosponges to incorporate molecules within their structure was evaluated using drugs with different structures and solubilities. The nanosponges were found capable of carrying both lipophilic and hydrophilic drugs and of improving the solubility of poorly water-soluble molecules. 相似文献
64.
Ion chromatographic determination of sulfide and cyanide in real matrices by using pulsed amperometric detection on a silver electrode 总被引:1,自引:0,他引:1
The determination of free sulfide and cyanide by pulsed amperometric detection (PAD) at a silver-working electrode was improved through a deep de-oxygenation (at least 10 min) of both standard and real solutions containing the two analytes and adopting a two-potential waveform able to eliminate Ag working electrode fouling. The waveform stepped around the oxidation of Ag in the presence of 0.1-0.4 M hydroxyl ion, from -0.1 to 0.1 V versus saturated calomel electrode (SCE). The eluent composition (0.4 M NaOH plus 7.5 mM oxalate solution) allowed a very good column efficiency and selectivity. The presence of a polysulfide species was hypothesized in sulfide solutions that had not been de-oxygenated and aged. The polysulfide eluted just before sulfide and was confirmed by a chemical test with SO3(2-) producing the elimination of the polysulfide peak. Detection limits, according to the Hubaux-Vos method, were 1.0 and 2.0 microg/l for S2- and CN , respectively. We demonstrated good performance of the optimized method by repeatedly injecting standard solutions and by analyzing different real matrices. The method exhibited very good accuracy and repeatability (10 microg/l and a 500 microl injection loop, had a repeatability better than 3% for sulfide and 100 microg/l had a repeatability better than 1% for cyanide). The two-potential waveform ensured long-term stability of the electrode surface that required no manual polishing procedure for at least 1 month (20 analysis per day). 相似文献
65.
Reactions that occur within or between molecular crystals, in particular those reactions that are activated by mechanical methods, are reviewed. The focus is on processes (whether intrasolid or intersolid) that are controlled primarily by supramolecular bonding, such as template cycloadditions, formation of inclusion compounds, reactions between molecular crystals by the reassembling of noncovalent bonds, and the formation of complexes and coordination compounds. It is proposed that solvent-free mechanochemical methods, for example, cogrinding, milling, and kneading, represent viable "green" routes for the preparation of novel molecular and supramolecular solids. 相似文献
66.
Federica Cavalli Fabian R. Bloesser Prof. Dr. Christopher Barner-Kowollik Prof. Dr. Leonie Barner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(43):10049-10053
We introduce a protecting-group-free concept for the powerful para-fluoro-thiol reaction (PFTR) employing a source of fluoride ions as base. The reaction is shown to be self-propagating, with under-stoichiometric amounts of base, thus effectively foregoing the need for high base concentrations. Careful tuning of the solvent–thiol combination allows for quantitative conversion, in some cases within a short timeframe, when only minimal amounts of base are used, allowing the PFTR reaction to essentially proceed base-free. 相似文献
67.
A. Agnesi S. Dell’Acqua A. Guandalini G. Reali A. Toncelli M. Tonelli E. Cavalli 《Applied physics. B, Lasers and optics》2000,71(2):153-156
CW diode-pumped laser operation of Nd3+ in Ca3Sc2Ge3O12 (CaSGG) single garnet crystal has been demonstrated for the first time. We measured an 18% slope efficiency and 2.7-nm tunability
with two different laser cavities. A spectroscopic characterization of our sample has also been performed including the absorption
cross section in the 800-nm region and the emission cross section and radiative lifetime of the upper laser level.
Received: 7 December 1999 / Revised version: 6 February 2000 / Published online: 27 April 2000 相似文献
68.
Michele R. Chierotti Dr. Luca Ferrero Dr. Nadia Garino Dr. Roberto Gobetto Prof. Luca Pellegrino Dr. Dario Braga Prof. Fabrizia Grepioni Prof. Lucia Maini Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(14):4347-4358
Five new polymorphs and one hydrated form of 2‐thiobarbituric acid have been isolated and characterised by solid‐state methods. In both the crystalline form II and in the hydrate form, the 2‐thiobarbituric molecules are present in the enol form, whereas only the keto isomer is present in crystalline forms I (reported in 1967 by Calas and Martinex), III , V and VI . In form IV , on the other hand, a 50:50 ordered mixture of enol/keto molecules is present. All new forms have been characterised by single‐crystal X‐ray diffraction, 1D and 2D (1H, 13C, and 15N) solid‐state NMR spectroscopy, Raman spectroscopy and X‐ray powder diffraction at variable temperature. It has been possible to induce keto–enol conversion between the forms by mechanical methods. The role of hydrogen‐bond interactions in determining the relative stability of the polymorphs and as a driving force in the conversions has been ascertained. To the best of the authors’ knowledge, the 2‐thiobarbituric family of crystal forms represents the richest collection of examples of tautomeric polymorphism so far reported in the literature. 相似文献
69.
Visible optical properties of two crystalline fibers (KY3F10 and LiLuF4) doped with Dy3+ have been investigated. In particular, the room-temperature absorption spectra have been analyzed using the Judd–Ofelt theory and the intensity parameters have been obtained. A criterion suitable for determining the proper choice of the dopant composition to tailor the luminescence properties of the crystals is proposed. 相似文献
70.
Radosaw Lisiecki Witold Ryba-Romanowski Enrico Cavalli Marco Bettinelli 《Journal of luminescence》2010,130(1):131-136
Transparent single crystals of erbium-doped LaVO4 in form of platelets having an average size 0.5×2×2 mm3 were obtained by the flux method using Pb2V2O7 as the solvent. Inductively coupled plasma (ICP) measurement revealed that the actual Er/La=0.3% molar ratio in the crystals is lower than Er/La=1% nominal molar ratio due to significant difference of ionic radii of La and Er. Luminescence spectra and luminescence decay curves for Er3+ transitions in the visible and near infrared region were recorded. Unpolarized Raman spectrum of undoped LaVO4 crystal was acquired, too. Up-converted green emission following excitation at 808 and 970 nm was observed and dependence of its intensity on incident excitation power was determined. Based on experimental data gathered the relaxation dynamics of excited states of Er3+ was analyzed and mechanisms involved in the up-conversion phenomena were discussed. 相似文献