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81.
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
82.
A formal total synthesis of the immunosuppressant FR901483 has been accomplished. The key step in the synthesis utilizes a tandem cationic aza-Cope rearrangement/Mannich cyclization reaction for accessing the unprecedented bridging tricyclic azaspirane substructure of this compound. The tandem reaction proceeds through a bridgehead iminium ion, a functionality that has rarely been explored in the context of natural product syntheses. Improved stereoselectivity was observed in an aldol reaction when using a Boc-protected amino aldehyde and zinc chloride as an additive. A stereoselective epimerization of the aldehyde-containing stereocenter was achieved with l-phenylalanine upon completion of the Mannich cyclization. Finally, this synthesis is the only one to date that controls the stereochemistry of the oxygen-bearing stereocenters. All other synthetic routes required late stage adjustments to at least one of these stereocenters. 相似文献
83.
Genetically modified organisms (GMOs) entered the European food market in 1996. Current legislation demands the labeling of food products if they contain <1% GMO, as assessed for each ingredient of the product. To create confidence in the testing methods and to complement enforcement requirements, there is an urgent need for internationally validated methods, which could serve as reference methods. To date, several methods have been submitted to validation trials at an international level; approaches now exist that can be used in different circumstances and for different food matrixes. Moreover, the requirement for the formal validation of methods is clearly accepted; several national and international bodies are active in organizing studies. Further validation studies, especially on the quantitative polymerase chain reaction methods, need to be performed to cover the rising demand for new extraction methods and other background matrixes, as well as for novel GMO constructs. 相似文献
84.
Cryogenic CO2 formation on oxidized gold clusters synthesized via reactive layer assisted deposition
Gas-phase Au atoms deposited onto a multilayer film of molecular oxygen produce atomic oxygen bound to gold clusters. After removal of molecular O2, temperature programmed desorption and molecular beam techniques show that the atomic oxygen readily reacts with CO to produce CO2. At present, the structure and size distribution of these clusters are unknown. Nevertheless, CO2 forms on these clusters upon exposure to CO at temperatures as low as 35 K. Furthermore, above 120 K, the reaction goes to completion with initial reaction yields as high as 50%. 相似文献
85.
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
86.
87.
88.
M. Donath D. Scholl H. C. Siegmann E. Kay 《Applied Physics A: Materials Science & Processing》1991,52(3):206-209
A surface probing depth of only 2 layers for low energy cascade electrons excited with a 3.2 keV primary electron beam is clearly demonstrated by model experiments with non-magnetic overlayers of Ta on magnetic substrates of Fe/Ni80Fe20. This result establishes a short probing depth of low energy electrons in transition metals generalizing the previously observed short magnetic probing depth for spin-polarized electrons in ferromagnets. The short probing depth sheds new light on a number of spectroscopic observations on ferromagnetic transition metals, and has important implications concerning surface magnetic properties and scattering processes of hot electrons in transition metals. 相似文献
89.
90.
Preparation and Crystal Structure of the Pnictide Oxides Na2Ti2As2O and Na2Ti2Sb2O Na2Ti2As2O and Na2Ti2Sb2O were synthesized in form of very easily hydrolysed metallic-grey powders by reaction of Na2O and TiAs resp. TiSb in sealed tantalum tubes under argon. The tetrahedral bodycentered crystallizing compounds from a modified anti-K2NiF4 structure type [1] (also called Eu4As2O-type [2,3]), space group I4/mmm (no. 139), with the lattice constants for Na2Ti2As2O: a = 407.0(2) pm, c = 1528.8(4) pm and for Na2Ti2Sb2O: a = 414.4(0) pm, c = 1656.1(1) pm. Magnetic measurements of powder samples of Na2Ti2Sb2O show antiferromagnetic interaction within the Ti—O-layers. Superconductivity was not found by ac-shielding method down to 4 K. 相似文献