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171.
The rotational potential around the interannular bond in 2,2-bipyrrole has been calculated making use of standard minimal STO-3G and split valence 4-31G basis sets. Geometrical optimization concerning the most significant interannular internal parameters has been performed with both basis sets. The trans conformer is predicted to be more stable than the cis. The minimal basis set predicts the existence of a cisoid-gauche minimum which after limited optimization becomes very shallow and it seems to be an artifact of the rigid rotor approximation. At 4-31G level, both the trans and cis conformers represent maxima in the potential curve and two gauche minima appear at =46.0° and =147.6°, the latter being the absolute minimum. The absolute maximum of the potential curve corresponds to the cis conformer. 相似文献
172.
Driess M Monsé C Merz K van Wüllen C 《Angewandte Chemie (International ed. in English)》2000,39(20):3515
The cover picture shows the structure, determined crystallographically, of the tetrakis(trimethylstannyl)phosphonium cation that is formed with surprising ease from the reaction of P(SnMe(3))(3) with Me(3)SnOTf (OTf=OSO(2)CF(3)) and is isolated as the OTf salt. It is the first completely substituted main group organometallic phosphonium derivative, and, in contrast to the more common tetraorganic-substituted phosphonium cations is only stable in the solid state; in solution it functions as a masked Me(3)Sn(+) reagent. More about this chameleonlike ion and the N(SnMe(3))(4) cation homologue, which is equally dynamic in solution and has unusual long Sn-N bond lengths, is reported by M. Driess et al. on p. 3684 ff. 相似文献
173.
The feasibility of comprehensive two-dimensional gas chromatography with electron-capture detection (GC x GC-ECD) for the enantioseparation of chiral PCBs from other possible interfering compounds has been evaluated. Three commercially available enantioselective beta-cyclodextrin-based capillary columns (Chirasil-Dex, BGB-172 and BGB-176SE) have been tested as first-dimension columns. Three non-enantioselective stationary phases (HT-8, BPX-50 and Supelcowax-10) were combined with the enantioselective columns to allow the unambiguous determination of the enantiomers of the target chiral PCBs. Each enantioselective first-dimension column tested was able to separate into enantiomers different PCB congeners, but in all cases, the use of Supelcowax-10 as second-dimension column provided the most satisfactory results. The Chirasil-Dex x Supelcowax-10 column combination allowed the determination of the enantiomeric fraction (EF) of PCBs 84, 91, 95, 132, 136, 149, 174 and 176 in the working standard solution, while that of congener 135 was hindered. The BGB-172 x Supelcowax-10 column set allowed a proper EF determination of congeners 45, 84, 131, 132, 135, 171, 174 and 183, while that of PCB 91 was interfered with co-elutants. The column combination BGB-176SE x Supelcowax-10 allowed the determination of all congeners that this enantioselective stationary phase was able to separate into enantiomers, i.e. PCBs 45, 91, 95, 136, 149 and 176. These column combinations have also been evaluated for the simultaneous determination of the 12 congeners with a toxic equivalency factor assigned by the WHO (PCBs 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, 189) and the seven indicator congeners (PCBs 28, 52, 101, 118, 138, 153 and 180), and evaluated for the analysis of food samples. 相似文献
174.
Jos Luis Lpez Marcos Mandado Ana M. Graa Ricardo A. Mosquera 《International journal of quantum chemistry》2002,86(2):190-198
The atomic and bond properties of a series of alkanenitriles were calculated in order to analyze the transferability of the CN, methyl, and methylene groups. The calculations were carried out using the atoms in molecules (AIM) theory on RHF/6‐31++G**// RHF/6‐31G** wave functions obtained for compounds CN–R (R ranging from H to C11H23). Linear correlations between L(Ω) and N(Ω) were used to establish N(CH2) and N(CH3) nearly transferable values. Average values and maximum differences to the mean value of several properties were used for classifying the CN group. It shows a transferable behavior along the CN–R series for R>Et. The methyl group presents specific properties when R<Pr. The methylene groups can be classified considering both their position with respect to the end of the chain and the position with respect to the CN group. The atomic energy displays a dependence on the molecular size. Although this behavior does not allow to consider this property as transferable, both the ab initio total electronic molecular energies and the experimental heats of formation can be fitted, by linear regression analysis, as a function of the number of methylene groups. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 相似文献
175.
Ribeiro da Silva MD Gomes JR Gonçalves JM Sousa EA Pandey S Acree WE 《The Journal of organic chemistry》2004,69(8):2785-2792
The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported. 相似文献
176.
José A. Rodriguez 《Theoretical chemistry accounts》2002,107(3):117-129
The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device
fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with
the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides,
systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of
different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing
molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed
and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities
have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces.
Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary
a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital
interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain
trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework
for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect,
theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and
reducing the waste of time, energy and materials characteristic of an empirical design.
Received: 21 June 2001 / Accepted: 8 October 2001 / Published online: 1 February 2002 相似文献
177.
A sensitive LC-MS-MS method for the direct determination and quantification of 15 fluorobenzoic acids (FBAs) was developed. FBAs are used as conservative tracers for hydrological modelling of water flow and in studies of pesticides and other xenobiotic compounds. The use of FBAs is discussed in relation to other tracers (bromide, chloride, uranine). The method covers mono-substituted fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid, and tetrafluorobenzoic acid. The general detection limit in ground water was 1 microg/l using electrospray ionisation and 20 microg/l using atmospheric pressure chemical ionisation. Analysis time was less than 10 min, small sample volumes were needed and no clean-up was required. 相似文献
178.
A highly enantioselective (up to 97.5% ee) and diastereoselective (95:5 dr trans/cis) Cu(I)-catalyzed cyclopropanation of alkenes using phenyliodonium ylide generated in situ from iodosobenzene and methyl nitroacetate is reported. The cyclopropanation took place with high enantioselectivity for a wide range of alkenes, and the reaction was performed at room temperature. 1-Nitrocyclopropyl esters are versatile building blocks to access the corresponding cyclopropane amino esters and aminocyclopropanes in two and three steps, respectively, from commercially available products. 相似文献
179.
del Río JC Gutiérrez A Martínez AT 《Rapid communications in mass spectrometry : RCM》2004,18(11):1181-1185
A series of non-wood plant fibers, namely kenaf, jute, sisal and abaca, have been analyzed upon pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) of the whole material. The pyrolysis products mainly arise from the carbohydrate and lignin moieties of the fibers. The lignin-derived phenols belonged to the p-hydroxyphenylpropanoid (H), guaiacylpropanoid (G) and syringylpropanoid (S) structures, and showed a high S/G ratio of between 2.0 and 5.4, the highest corresponding to kenaf. Among the lignin-derived phenols released, small amounts of sinapyl and coniferyl acetates (in both cis- and trans-forms) were identified for the first time upon Py-GC/MS of lignocellulosic materials. Acetylation of the sinapyl and coniferyl alcohols was at the gamma-position of the side chain. The release of these alcohols derived from intact acetylated lignin units upon pyrolysis seems to indicate that the native lignin in the fibers selected for this study is at least partially acetylated. Sinapyl (and coniferyl) acetates have recently been suggested to be authentic lignin precursors involved in the polymerization of lignin along with the normal sinapyl and coniferyl alcohols. Py-GC/MS will offer a convenient and rapid tool for analyzing naturally acetylated lignins, as well as to screen plant materials for the presence of acetylated units in lignin. 相似文献
180.
Emilio San-Fabián José Pérez-Jordá Federico Moscardó 《Theoretical chemistry accounts》1990,77(3):207-212
Summary The correlation energy of two- and four-isoelectronic series, a representative example for which the existing spin-density functionals fails, is calculated using the Colle and Salvetti method, considering mono- and multideterminantal wave functions. The results are in agreement with experimental data, and show the potentiality of this method when it is applied to wave functions including the most relevant configurational features. Also, results for the ionization energies and electron affinities of first- and second-row atoms are reported. 相似文献