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61.
Joanna Szpunar-Łobińska Michiel Ceulemans Wilfried Dirkx Claudia Witte Ryszard Łobiński Freddy C. Adams 《Mikrochimica acta》1994,113(3-6):287-298
Interferences in trace and ultratrace speciation analysis of organotin and organolead compounds in various samples by gas chromatography coupled with atomic absorption spectrometry (AAS) and/or microwave induced plasma atomic emission spectrometry (MIP AES) are investigated. Particular attention is given to the effects of matrix co-extractives and reagents impurities introduced during sample preparation. Their influence on the detection limits is discussed in terms of baseline noise level, blank value, formation of artefacts and signal suppression. Loss of column resolution during the analysis of some matrices is observed. 相似文献
62.
Wilfried Heller 《Journal of Polymer Science.Polymer Physics》1966,4(2):209-226
The principles of a method are outlined whereby one can determine the partial specific volume of a solute, V?, by means of measurements of dn/dc using a suitable pair of equations and a simple graphical interpolation procedure. The method yields V? data which compare well with densitometrically obtained V? data if the polarizability of both solvent and solute molecules is unaffected by the solution process. It is tested successfully on solutions of polystyrene in various solvents. The method appears to be particularly attractive for measurements of the change of V? with temperature and also for conveniently following the time rate of changes in - such as during the coil → helix transition. 相似文献
63.
Intercalation of trioxatriangulenium ion in DNA: binding,electron transfer,x-ray crystallography,and electronic structure 总被引:1,自引:0,他引:1
Reynisson J Schuster GB Howerton SB Williams LD Barnett RN Cleveland CL Landman U Harrit N Chaires JB 《Journal of the American Chemical Society》2003,125(8):2072-2083
Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+). 相似文献
64.
Schuster DI Cheng P Jarowski PD Guldi DM Luo C Echegoyen L Pyo S Holzwarth AR Braslavsky SE Williams RM Klihm G 《Journal of the American Chemical Society》2004,126(23):7257-7270
As part of a continuing investigation of the topological control of intramolecular electron transfer (ET) in donor-acceptor systems, a symmetrical parachute-shaped octaethylporphyrin-fullerene dyad has been synthesized. A symmetrical strap, attached to ortho positions of phenyl groups at opposing meso positions of the porphyrin, was linked to [60]-fullerene in the final step of the synthesis. The dyad structures were confirmed by (1)H, (13)C, and (3)He NMR, and MALDI-TOF mass spectra. The free-base and Zn-containing dyads were subjected to extensive spectroscopic, electrochemical and photophysical studies. UV-vis spectra of the dyads are superimposable on the sum of the spectra of appropriate model systems, indicating that there is no significant ground-state electronic interaction between the component chromophores. Molecular modeling studies reveal that the lowest energy conformation of the dyad is not the C(2)(v)() symmetrical structure, but rather one in which the porphyrin moves over to the side of the fullerene sphere, bringing the two pi-systems into close proximity, which enhances van der Waals attractive forces. To account for the NMR data, it is proposed that the dyad is conformationally mobile at room temperature, with the porphyrin swinging back and forth from one side of the fullerene to the other. The extensive fluorescence quenching in both the free base and Zn dyads is associated with an extremely rapid photoinduced electron-transfer process, k(ET) approximately 10(11) s(-)(1), generating porphyrin radical cations and C(60) radical anions, detected by transient absorption spectroscopy. Back electron transfer (BET) is slower than charge separation by up to 2 orders of magnitude in these systems. The BET rate is slower in nonpolar than in polar solvents, indicating that BET occurs in the Marcus inverted region, where the rate decreases as the thermodynamic driving force for BET increases. Transient absorption and singlet molecular oxygen sensitization data show that fullerene triplets are formed only with the free base dyad in toluene, where triplet formation from the charge-separated state is competitive with decay to the ground state. The photophysical properties of the P-C(60) dyads with parachute topology are very similar to those of structurally related rigid pi-stacked P-C(60) dyads, with the exception that there is no detectable charge-transfer absorption in the parachute systems, attributed to their conformational flexibility. It is concluded that charge separation in these hybrid systems occurs through space in unsymmetrical conformations, where the center-to-center distance between the component pi-systems is minimized. Analysis of the BET data using Marcus theory gives reorganization energies for these systems between 0.6 and 0.8 eV and electronic coupling matrix elements between 4.8 and 5.6 cm(-)(1). 相似文献
65.
66.
Zusammenfassung Es wird eine qualitative und eine quantitative Methode zum Nachweis und zur Bestimmung der Haupt- und einiger Nebenprodukte in den Laugen des technischen Dicyandiamid/Melamin-Prozesses vorgeschlagen. Der in bezug auf ihre Zusammensetzung komplexe Charakter dieser Laugen erfordert insbesondere bei der quantitativen Analyse (Ammelin, Melamin, Rhodanid, Thioharnstoff, Eisenverbindungen) die Beachtung einer Reihe von Störmöglichkeiten. Die Verfahren wurden mit synthetischen Gemischen getestet und in der betrieblichen Praxis überprüft.
Mitteilung aus dem Institut für Organische Chemie der E.-M.-A.-Universität Greifswald (Direktor: Prof. Dr. H. Beyer). Die Arbeit wurde im Rahmen der Vertragsforschung mit dem VEB Stickstoffwerk Piesteritz durchgeführt.
Wir danken den Herren Dr. H. Brandner und Dr. H. Lemke vom VEB Stickstoffwerk Piesteritz für die erwiesene Unterstützung sowie der Werkleitung für die Genehmigung zur Veröffentlichung dieser Arbeit. 相似文献
Summary A qualitative and a quantitative procedure is described for the analysis of the main and by-products in the liquors of the industrial dicyanodiamide/melamine process. The complex composition of the liquors requires consideration of a number of interferences, especially in quantitative analysis (ammeline, melamine, thiocyanate, thiourea, iron compounds). The procedures have been tested by means of synthetic mixtures and controlled in commercial-scale production.
Mitteilung aus dem Institut für Organische Chemie der E.-M.-A.-Universität Greifswald (Direktor: Prof. Dr. H. Beyer). Die Arbeit wurde im Rahmen der Vertragsforschung mit dem VEB Stickstoffwerk Piesteritz durchgeführt.
Wir danken den Herren Dr. H. Brandner und Dr. H. Lemke vom VEB Stickstoffwerk Piesteritz für die erwiesene Unterstützung sowie der Werkleitung für die Genehmigung zur Veröffentlichung dieser Arbeit. 相似文献
67.
P. Schuster A. Stephen O. E. Polansky F. Wessely 《Monatshefte für Chemie / Chemical Monthly》1968,99(3):1246-1276
Zusammenfassung Die Acidität von insgesamt 43 verschiedenartigen organischenLewissäuren wurde in 99 Vol.-proz. Methanol durch potentiometrische Titration bestimmt. Einige ausgewählte Vertreter wurden auch in anderen Methanol/Wasser-Mischungen und in absol. Methanol mit potentiometrischen und optischen Methoden untersucht; in zwei Fällen wird der pK
L
-Wert auf Wasser extrapoliert. Bei allen Messungen in wasserhaltigen Lösungsmitteln muß die Hydrolyse derLewissäuren als Fehlerquelle der pK
L
-Bestimmung untersucht und ausgeschaltet werden.
Mit 10 Abbildungen
Herrn Prof. Dr.J. W. Breitenbach zum 60. Geburtstag gewidmet.
Teile der DissertationenP. Schuster (Univ. Wien, 1966) undA. Stephen (Univ. Wien, 1966).
20. Mitt.:H. Peham, O. E. Polansky undF. Wessely, Mh. Chem.98, 1665 (1967).
8. Mitt.:P. Schuster undO. E. Polansky, Mh. Chem.99, 1234 (1968). 相似文献
The acidity of 43 different organicLewis acids in 99% (v/v) methanol was determined by potentiometric titration. A few compounds were investigated at different methanol/water ratios or in anhydrous methanol using potentiometric and optical methods; in two cases the pK L -values were extrapolated to water. For all measurements in water containing solvents hydrolysis of theLewis acids has to be taken into consideration and eliminated as a source of error for the determination of pK L -values.
Mit 10 Abbildungen
Herrn Prof. Dr.J. W. Breitenbach zum 60. Geburtstag gewidmet.
Teile der DissertationenP. Schuster (Univ. Wien, 1966) undA. Stephen (Univ. Wien, 1966).
20. Mitt.:H. Peham, O. E. Polansky undF. Wessely, Mh. Chem.98, 1665 (1967).
8. Mitt.:P. Schuster undO. E. Polansky, Mh. Chem.99, 1234 (1968). 相似文献
68.
69.
70.