全文获取类型
收费全文 | 9262篇 |
免费 | 332篇 |
国内免费 | 41篇 |
专业分类
化学 | 6636篇 |
晶体学 | 137篇 |
力学 | 161篇 |
综合类 | 1篇 |
数学 | 946篇 |
物理学 | 1754篇 |
出版年
2023年 | 62篇 |
2022年 | 85篇 |
2021年 | 111篇 |
2020年 | 200篇 |
2019年 | 169篇 |
2018年 | 105篇 |
2017年 | 105篇 |
2016年 | 225篇 |
2015年 | 243篇 |
2014年 | 250篇 |
2013年 | 359篇 |
2012年 | 469篇 |
2011年 | 535篇 |
2010年 | 307篇 |
2009年 | 325篇 |
2008年 | 478篇 |
2007年 | 491篇 |
2006年 | 487篇 |
2005年 | 447篇 |
2004年 | 382篇 |
2003年 | 276篇 |
2002年 | 306篇 |
2001年 | 199篇 |
2000年 | 188篇 |
1999年 | 117篇 |
1998年 | 135篇 |
1997年 | 115篇 |
1996年 | 152篇 |
1995年 | 127篇 |
1994年 | 107篇 |
1993年 | 93篇 |
1992年 | 109篇 |
1991年 | 92篇 |
1990年 | 100篇 |
1989年 | 76篇 |
1988年 | 74篇 |
1987年 | 84篇 |
1986年 | 72篇 |
1985年 | 93篇 |
1984年 | 74篇 |
1983年 | 72篇 |
1982年 | 71篇 |
1981年 | 76篇 |
1980年 | 70篇 |
1979年 | 50篇 |
1978年 | 59篇 |
1977年 | 53篇 |
1976年 | 53篇 |
1973年 | 63篇 |
1972年 | 44篇 |
排序方式: 共有9635条查询结果,搜索用时 18 毫秒
141.
From a molecular orbital study of model systems we derive the electronic requirements for the Grob fragmentation. The necessary condition for an allowed fragmentation in an X-C1-C2-C3-N system, or the amino cation +C1-C2-C3-N is the level ordering A below S . This in turn is set by maximal through-bond coupling of the empty cation orbital and the nitrogen lone pair. The conformational dependence of through-bond coupling is exactly that derived by Grob, namely parallel orientation of the cation orbital (or the C-X bond), the C2-C3-σ-bond, and the N-lone-pair. When the C1-C2-C3 and C2-C3-N angles are small, the through-space interaction dominates, reversing the level ordering to S below A , and consequently makes the fragmentation forbidden even though the conformational requirements for it are met. Ring closure becomes allowed. Some examples exploiting this result are presented, as well as procedures for enhancing through-bond coupling and thus fragmentation. The through-bond-effect has also kinetic consequences, allowing the definition of a new type of remote neighbouring group participation operative through bonds and not by direct overlap. The position of equilibria in nitrogen inversion processes should also be influenced by remote substituents which are π-acceptors or donors. 相似文献
142.
Monochrome coloured glass beads of the Merovingians (5th–7th cent. AD) have been examined by different analytical methods. The elemental composition of a large number of mostly unprepared
beads have been measured non-destructively by X-ray fluorescence analysis. After subtracting the content of the colouring
oxides of the glass beads and normalising the residual values to 100% an identical soda-lime-glass matrix was obtained. X-ray
diffraction was used for the identification of the crystalline colouring and opacifying pigments (SnO2, Cu, Cu2O, PbSnO3), and scanning electron microscopy as well as electron probe microanalysis were applied to study the microstructure and the
composition of white, brown, green, orange and yellow coloured glass beads. Oxidised metals, alloys (lead, copper, bronze,
brass and mixtures of them) and iron smelting slag have been identified as raw materials to colour the soda-lime-glass. 相似文献
143.
Different synthetic routes for the preparation of diiodo(methylthio)gallane are given. Some reactions of this compound with Lewis bases, such as O(CH3)2, S(CH3)2, S2(CH3)2, N(CH3)3, P(CH3)3, and other compounds, such as CH3I, CH3OH, C2H5OH, C2H5SH, i-C3H7SH, and C2H5SeH are investigated. Spectra and some physical and chemical properties of the new compounds are reported. The structure of diiodo(methylthio)gallane is discussed in view of some interesting differences of this molecule in solution and in crystal form. 相似文献
144.
A high-performance liquid chromatographic procedure was developed for the isolation and quantitation of coumarin from vanilla-based liquid flavorings of Mexican origin. Forty products representing fourteen different Mexican brands were assayed for coumarin, vanillin, and ethyl vanillin by the proposed method. The procedure has been adapted to the analysis of other products including domestic vanilla extracts and imitation vanilla flavorings for vanillin, ethyl vanillin, 4-hydroxybenzaldehyde and piperonal. Chromatographic retention data for thirty-seven compounds associated with vanillin and vanilla products employing two mobile phase systems are presented. 相似文献
145.
Peter L. Simon Stephen K. Scott Serafim Kalliadasis John H. Merkin 《Journal of mathematical chemistry》2004,35(4):309-328
The stability of a premixed laminar flame supported by a general combustion reaction system is considered using the Evans function method. The spectrum of the linearised second-order differential operator is investigated in detail. The special structure of the differential equations due to an Arrhenius temperature dependence is exploited. It is shown that, for certain combustion systems, the limit of the Jacobian of the reaction terms as the travelling wave coordinate approaches the front and rear of the flame is a lower triangular matrix. For this type of system a simple geometrical method is shown for the study of the essential spectrum of the linearised operator, and for determining the domain of the Evans function. The results are applied to some representative combustion reactions. 相似文献
146.
N-(2-Nitro)benzoylamino acids were prepared by 2-nitrobenzoylation of amino acids via the mixed ethylcarbonic anhydride procedure. They were reduced catalytically to N-(2-amino)benzoylamino acids which underwent cyclization to the corresponding lactams under a variety of conditions. The use of this reaction sequence for stepwise degradation of peptides seems possible. 相似文献
147.
Alistair J Usher Nigel T Lucas Simon Petrie Mark G Humphrey Anthony C Willis 《Journal of organometallic chemistry》2004,689(1):50-57
Reaction of WH(CO)3(η-C5Me5) with IrCl(CO)2(4-H2NC6H4Me) affords WIr3(μ-CO)3(CO)8(η-C5Me5) in low yield. A structural study reveals a WIr2-centred plane of bridging carbonyls, in contrast to the crystal structure of WIr3(CO)11(η-C5H5) (all-terminal carbonyl distribution). DFT calculations reveal an increasing proclivity to adopt an all-terminal CO disposition for clusters MIr3(CO)11(η-C5H5) in the gas phase on proceeding from M=Cr to Mo and then W, consistent with structural studies in the solid state for which the tungsten-containing cluster is the only all-terminal example. Increasing electron donation from the ligands in the tungsten system (either from phosphine substitution or cyclopentadienyl permethylation) suffices to impose a plane of bridging carbonyls in the ground state structure. 13C NMR fluxionality studies reveal that CO exchange mechanism(s) for WIr3(CO)11(η-C5H5) and the related tetrahedral cluster W2Ir2(CO)10(η-C5H5)2 are very fast and involve all carbonyls on the clusters. DFT calculations on MIr3(CO)11(η-C5H5) (M=Cr, Mo) substantiate a ‘merry-go-round’ mechanism for carbonyl scrambling in these systems, a result which is consistent with the scrambling behaviour seen in the NMR fluxionality studies on the W-containing congener. 相似文献
148.
Bailey S Davies SG Smith AD Withey JM 《Chemical communications (Cambridge, England)》2002,(23):2910-2911
The asymmetric synthesis of (1R,2S,3R)-3-methyl-2-amino-cyclopentane carboxylic acid has been achieved via kinetic resolution of racemic tert-butyl 3-methyl-cyclopentene-1-carboxylate with homochiral lithium (S)-N-benzyl-N-alpha-methylbenzylamide. 相似文献
149.
Lucas NT Blitz JP Petrie S Stranger R Humphrey MG Heath GA Otieno-Alego V 《Journal of the American Chemical Society》2002,124(18):5139-5153
A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr(3)(CO)(11-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (5), C(5)HMe(4) (6), C(5)Me(5) (7); M = W, Cp = C(5)H(4)Me, x = 1 (13), x = 2 (14)] and M(2)Ir(2)(CO)(10-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (8), C(5)HMe(4) (9), C(5)Me(5) (10); M = W, Cp = C(5)H(4)Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr(3)(CO)(11)(eta(5)-C(5)H(4)Me) (11), and W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(4)Me)(2) (12), together with the spectrum of Ir(4)(CO)(12), have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A(1) frequencies and cluster core bond strength, frequencies for the A(1) breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr(3)(CO)(11)(eta(5)-C(5)H(5)) (1), and Mo(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (3) have been collected. The [MIr(3)] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M(2)Ir(2)] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12(+) has a low-energy band at 8000 cm(-1), a spectrally transparent region for 12, and the latter reveals that 12(+) exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12(+) arises from a sigma(W-W) --> sigma*(W-W) transition. 相似文献
150.
M. Mallouki F. Tran-Van C. Sarrazin P. Simon B. Daffos A. De C. Chevrot J. F. Fauvarque 《Journal of Solid State Electrochemistry》2007,11(3):398-406
We report on the synthesis and electrochemical characterization of nanohybrid polypyrrole (PPy) (PPy/Fe2O3) materials for electrochemical storage applications. We have shown that the incorporation of nanoparticles inside the PPy
notably increases the charge storage capability in comparison to the “pure” conducting polymer. Incorporation of large anions,
i.e., paratoluenesulfonate, allows a further improvement in the capacity. These charge storage modifications have been attributed
to the morphology of the composite in which the particle sizes and the specific surface area are modified with the incorporation
of nanoparticles. High capacity and stability have been obtained in PC/NEt4BF4 (at 20 mV/s), i.e., 47 mAh/g, with only a 3% charge loss after one thousand cyles. The kinetics of charge–discharge is also
improved by the hybrid nanocomposite morphology modifications, which increase the rate of insertion–expulsion of counter anions
in the bulk of the film. A room temperature ionic liquid such as imidazolium trifluoromethanesulfonimide seems to be a promising
electrolyte because it further increases the capacity up to 53 mAh/g with a high stability during charge–discharge processes. 相似文献