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211.
A mathematical method for the calculation of the dead time (tm) in HPLC was evaluated using a computer simulation approach, in which artificial perturbations were introduced to Simulated homolog retention times. The calculation was based on a modified and extended Grobler and Bálisz (GB) method. Investigated wav how the precision of the calculated tM is affected by: statistical fluctuations in retention times and which, and how many homolog retention times are used. Based on these simulations a two-step procedure for the tM calculation is proposed: In the first step the linearity of log tR, n vs carbon number n is checked using as many homolog retention times as possible. The slope value bo of the first linear regression in the GB method is used for the selection of homolog retention times in the final tM calculation. In the second step the optimal selection of homologs is made and the final tM calculation is carried out. Guidelines for homolog selection are given. 相似文献
212.
B. F. Myasoedov L. A. Gribov A. I. Pavlyuchko I. V. Rybalchenko G. I. Sigeikin A. F. Kireev V. N. Suvorkin 《Journal of Structural Chemistry》2006,47(3):434-441
For highly toxic organophosphorus compounds on the limiting lists of the International Chemical Weapons Ban Treaty, fragment methods may be used for calculating their IR vibrational spectra; this is shown for O-alkyl alkylfluorophosphonates used as examples. The geometrical parameters and the parameters of the potential and electrooptic functions are found for the major fragments of these compounds. Due to this, fast predictive computation of IR spectra of O-alkyl alkylfluorophosphonates is possible, the accuracy of calculation being sufficient for spectral identification of these compounds; a database of the calculated IR spectra may be created. 相似文献
213.
Electrochemical properties of tetra(3,5-di-tert-butyl-4-hydroxyphenyl)dodecachlorophthalocyanine complexes of cobalt(II) and lutetium(III) in DMF solutions have been studied.
It was suggested that in the presence ofn-butyl bromide, the electrochemically generated monoanionic complexes undergo alkylation at the metal atom. The geometries
of the neutral and anionic complexes were calculated by the ZINDO/I semiempirical method.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 289–293, February, 1998. 相似文献
214.
Although a number of computational studies have examined the relative stability of icosahedral and decahedral gold clusters from 1 to 3 nm in size, few studies have focussed on the variety of face-centered cubic (fcc) nanoparticles in this size regime. In most cases small fcc gold particles are assumed to adopt the truncated octahedral shape, but in light of the fact that the shape and structure of gold nanoparticles is known to vary, the relative stability of fcc polyhedra may change with size. Presented here are results of first-principles calculations investigating the preferred shape of gold particles less than 3 nm in size. Our results indicate that the equilibrium shape of fcc gold nanoparticles less than 1 nm is the cuboctahedron, but this shape rapidly becomes energetically unstable with respect to the truncated octahedron, octahedron and truncated cube shapes as the size increases. 相似文献
215.
Massimo Ragazzi Paola Carbone Dino R. Ferro 《International journal of quantum chemistry》2002,88(5):663-669
Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force‐field computations allowed us to define the main conformational characteristics of ethylene–norbornene (E–N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and 13C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio 13C chemical shifts computations [gauge‐invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6‐311+G(2d,p) basis set on properly energy‐minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E–N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distortions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows the great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献
216.
Ruifeng Liu Paula R. Moody Alex S. Vanburen Jeffrey A. Clark Joel A. Krauser Dennis R. Tate 《Vibrational Spectroscopy》1996,10(2):325-329
Results of ab initio and density functional theory calculations on the structure and vibrational frequencies of hypophosphite anion indicate earlier experimental assignments of the fundamental vibrational modes are correct while the recent reassignments of several modes proposed by Bickley et al. are inconsistent with the calculated results. 相似文献
217.
Tricyclo[2.2.0.0]hexane: a new hypothetical molecule which should have only one inverted carbon atom
HF and MP2 calculations with the 6-31G** and 6-311G** basis sets and those at MP2/cc-pVTZ level were carried out for the hypothetical tricyclo[2.2.0.01,3]hexane. The results indicate that the molecule under study should have one carbon atom with highly unusual inverted configuration. The analysis of the vibrational frequencies of this molecule as well as the analysis of its plausible decomposition routes performed at the DFT level indicate that this unique molecule could be a plausible synthetic target. 相似文献
218.
Ali Jabalameli Andrzej Nowek Richard H. Sullivan Jerzy Leszczyński 《Structural chemistry》1997,8(4):245-256
Ab initio studies (LCAO-MO method) on conformers of three seleno derivatives of the biuret molecules diselenobiuret [I], selenobiuret [II], and selenothiobiuret [III] were carried out at the Hartree-Fock (HF) and MP2 levels. The molecular geometries of these species were fully optimized at the HF level and characterized by analysis of the harmonic vibrational frequencies using a split-valence triple-zeta basis set augmented by a set ofd polarization functions on heavy atoms andp polarization functions on hydrogen atoms [TZP(d, p)]. The total energies of the HF-optimized structures were calculated at the MP2 (frozen core) level using a larger TZP (2df, 2pd) basis set. The potential energy searches revealed a total of 11 minimum-energy conformers (assigned astrans-trans, trans-cis, cis-trans, andcis-cis) and seven transition-state species for the title molecules. The two predicted conformers for diselenobiuret (Ia=trans-trans andIc=cis-cis) are characterized byC
2 and the third byC
s symmetry. For selenothiobiuret two forms (IIIa=trans-trans andIIId=cis-cis) possessC
1 and two (IIIb=trans-cis andIIIc=cis-trans) possessC
s symmetries, respectively. For selenobiuret, four formsIIa=trans-trans (C1),IIb=trans-cis (C
s),IIc=cis-trans (C
1), andIId=cis-cis (C1), were obtained as a result of gradient optimization. Comparison of the relative energies for the considered species indicated that thecis-trans forms are the most stable conformations for all three systems at both the HF and MP2 levels of theory. 相似文献
219.
Ab initio calculations using the MP2/cc-pVTZ basis set do an excellent job of predicting the inversion barrier (247 vs. 232 cm−1) and dihedral angle (26°) of cyclopentene. DFT calculations also do an excellent job of predicting the vibrational frequencies of the d0, d1, d4, and d8 isotopomers. They have also allowed the reassignments of several of the vibrational frequencies. 相似文献
220.
Torrent-Sucarrat M Luis JM Solà M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(20):6024-6031
The vibrational motions that disobey the minimum polarizability principle (MPP) in pi-conjugated molecules are distortions of the equilibrium geometry that produce a reduction in the polarizability due to the localization of pi electrons. For aromatic species, this electronic localization is responsible for the subsequent reduction in the aromaticity of the system. In the present work, we diagonalize the Hessian matrix of the polarizability with respect to the vibrational nontotally symmetric normal coordinates, to calculate the nontotally symmetric distortions that produce the maximum breakdown of the MPP in a series of twenty polycyclic aromatic hydrocarbons. It is shown that the nuclear displacements that break the MPP have larger components in those rings that possess the highest local aromaticity. Thus, these vibrational motions can be used as an indicator of local aromaticity. 相似文献