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991.
Crystal Structure of the “Supramolecular” Complex [Cs2(18-crown-6)][HgI4] with Unusually Coordinated Cs Ions The reaction of 18-crown-6, 1,4,7,10,13,16-hexaoxacyclooctadecane, with HgI2/CsI in methanol yields crystals of [Cs2(C12H24O6)][HgI4]. The compound crystallizes monoclinically, space group P21/c, Z = 4, a = 1574.8(3), b = 1067.0(3), c = 1693.2(6) pm, and β = 98.29(3)º. The structure consists of a network made up of two different types of [Cs-(18-crown-6)-Cs]2+ cations, interconnected by [HgI4]2? anions. The cations form an “anti-sandwich” structure with relatively short Cs ? Cs distances of 382 pm in the first type of cations and a longer distance of 480 pm in the second type of cations. 相似文献
992.
Sergey?P.?BabailovEmail author Lubov?D.?Nikulina 《Journal of inclusion phenomena and macrocyclic chemistry》2005,51(1-2):103-109
1H and 13C NMR measurements are reported for the CDCl3 and CD2Cl2 solutions of [La(NO3)3(18-crown-6)] (I), [Pr(NO3)3(18-crown-6)] (II) and [Ce(NO3)3(18-crown-6)] (III) complexes. Temperature dependencies of the 1H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Two types of conformational dynamic processes in II were identified (the first one with activation enthalpy ΔH
‡=26 ± 4 kJ/mol is conditioned by interconversion of complex enantiomeric form and pseudorotation of macrocycle molecule upon the C
2 symmetry axis, the second one with ΔH
‡=46 ± 5 kJ/mol is conditioned by macrocycle molecule inversion). Studies of the values of the lanthanide-induced shifts revealed that the structure of complexes in solution is similar to that reported for the complex I in the crystal state.This revised version was published online in July 2005 with a corrected issue number. 相似文献
993.
《Electroanalysis》2005,17(17):1589-1594
Simultaneous determination of bismuth and copper by anodic stripping voltammetry using H‐point standard addition method (HPSAM) with simultaneous addition of analytes is described. The effect of various parameters including acid concentration, accumulation time, accumulation potential and concentration ratio of analytes in the standard solution on the sensitivity and accuracy of method were investigated. The results of applying the H‐point standard addition method showed that Cu2+ and Bi3+ could be determined simultaneously with the concentration ratios of Cu2+ to Bi3+ varying from 1 : 15 to 16 : 1 in the mixed sample. The method was successfully applied to the simultaneous determination of copper and bismuth in some synthetic mixtures. 相似文献
994.
Anatoliy Nechepurenko 《Journal of solid state chemistry》2004,177(2):616-618
The investigation of sintering conditions for magnesiothermic amorphous boron (MgmBn) powder is presented. The results of chemical and X-ray analyses of magnesiothermic boron (MgmBn) indicate that it consists of amorphous boron MgB12 and a lesser amount of β-rhombohedral boron. The MgmBn-sintering process is determined by the conditions of amorphous boron transformation into β-boron (crystallization), such as the process of decomposition of MgB12 followed by formation of the “new” centers of active elementary boron. As a result of the experimental investigations of this process the following three stages—thermal decompositions, crystallization and MgmBn sintering—were combined into one sintering process with the sintered bodies as a result of it. 相似文献
995.
Accurate, practical prediction of 13C NMR chemical shifts has been achieved with a new system, CAST/CNMR, taking account of stereochemistry. The CAST/CNMR system has solved the critical problem of the accurate distinction of differences and similarities in stereochemical structures around a specific carbon, which has not yet been achieved by any other database-oriented system for prediction of 13C NMR chemical shifts. CAST/CNMR uses a three-dimensional structural database together with a 13C NMR spectral database. Absolute/relative configurational and conformational structural information are described by the CAST (CAnonical-representation of STereochemistry) coding method. This paper provides an overview of the CAST/CNMR system, and describes its application to two natural products as examples. 相似文献
996.
Hiroki FurutaToyohisa Takase Hisafumi HayashiRyoji Noyori Yuji Mori 《Tetrahedron》2003,59(49):9767-9777
An efficient method has been developed for the synthesis of trans-fused tetrahydropyrans with angular methyl groups adjacent to the ring oxygen. The procedure involves the coupling reaction of a sulfonyl-stabilized oxiranyl anion with an appropriate triflate followed by 6-endo cyclization, and is effective for the construction of 6/6- and 6/7/6-cyclic ether ring systems with sterically congested 1,3-diaxial methyl groups. 相似文献
997.
A stereocontrolled linear synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin, diarrhetic shellfish toxins, is described. Iterative application of a tetrahydropyran synthesis by reaction of the alkylation of a sulfonyl-stabilized oxiranyl anion followed by 6-endo cyclization of a 4,5-epoxy alcohol led to the synthesis of the trans-fused hexacyclic ether system, and the seven-membered E ring was constructed by ring expansion reaction. 相似文献
998.
Thermal analysis methods are well-established techniques in research laboratories of pharmaceutical industry. The robustness
and sensitivity of instrumentation, the introduction of automation and of reliable software according to the industrial needs
widened considerably the areas of applications in the last decade. Calibration of instruments and validation of results follow
the state of the art of cGMP as for other analytical techniques. Thermal analysis techniques are especially useful for the
study of the behavior of the poly-phasic systems drug substances and excipients and find a unique place for new delivery systems.
Since change of temperature and moisture occur by processing and storage, changes of the solid state may have a considerable
effect on activity, toxicity and stability of compounds. Current requirements of the International Conference of Harmonisation
for the characterization and the quantitation of polymorphism in new entities re-enforce the position of thermal analysis
techniques. This challenging task needs the use of complementary methods. Combined techniques and microcalorimetry demonstrate
their advantages. This article reviews the current use of thermal analysis and combined techniques in research and development
and in production. The advantage of commercially coupled techniques to thermogravimetry is emphasized with some examples.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
999.
In order to detect small variations in 13C isotopomers concentrations, high sensitivity, accuracy and precision have to be achieved. To assess such criteria, when using 13C NMR, 13C bi-labelled ethanol has been proposed as a molecular probe. Advantage has been taken of the pre-established structural relationship between the peak areas of the 13C NMR spectrum of this molecule, i.e. the ratio of signal areas is set to a fixed value. It is shown that the quality performance, required by quantitative 13C NMR spectroscopy, is not affected by a large reduction of the repetition delay using relaxation reagents. 相似文献
1000.