全文获取类型
收费全文 | 1316篇 |
免费 | 58篇 |
国内免费 | 5篇 |
专业分类
化学 | 858篇 |
晶体学 | 9篇 |
力学 | 16篇 |
数学 | 198篇 |
物理学 | 298篇 |
出版年
2023年 | 9篇 |
2022年 | 9篇 |
2021年 | 29篇 |
2020年 | 28篇 |
2019年 | 23篇 |
2018年 | 10篇 |
2017年 | 14篇 |
2016年 | 45篇 |
2015年 | 30篇 |
2014年 | 41篇 |
2013年 | 61篇 |
2012年 | 68篇 |
2011年 | 92篇 |
2010年 | 49篇 |
2009年 | 47篇 |
2008年 | 64篇 |
2007年 | 80篇 |
2006年 | 88篇 |
2005年 | 50篇 |
2004年 | 62篇 |
2003年 | 28篇 |
2002年 | 26篇 |
2001年 | 21篇 |
2000年 | 13篇 |
1999年 | 19篇 |
1998年 | 24篇 |
1997年 | 13篇 |
1996年 | 17篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 15篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 21篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 15篇 |
1977年 | 11篇 |
1975年 | 9篇 |
1974年 | 12篇 |
1972年 | 9篇 |
排序方式: 共有1379条查询结果,搜索用时 109 毫秒
41.
Determination of heavy metal complexes with humic substances by HPLC/ICP-MS coupling using on-line isotope dilution technique 总被引:3,自引:0,他引:3
An isotope dilution mass spectrometric (IDMS) method has been developed for the simultaneous determination of the complexes
of 11 heavy metals (Ag, Cd, Cu, Mo, Ni, Pb, Tl, U, W, Zn and Zr) with humic substances (HS) by coupling HPLC with ICP-MS and
applying the on-line isotope dilution technique. The HPLC separation was carried out with size exclusion chromatography. This
HPLC/ICP-IDMS method was applied to samples from a brown water, ground water, sewage and seepage water as well as for a sample
containing isolated fulvic acids. The total contents of heavy metals and of their complexes were analyzed in these samples
with detection limits in the range of 5–110 ng/L. The analysis of heavy metal/HS complexes from the different waters resulted
in characteristic fingerprints of the distribution pattern of heavy metals in the separated HS fractions. A comparison between
the total heavy metal concentrations and their portions bound to humic substances showed distinct differences for the various
metals. Simultaneous 12C detection was used for the characterization of HS complexes not identified by UV detection and for the determination of
relative DOC concentrations of chromatographic peaks.
Received: 21 February 1997 / Revised: 27 May 1997 / Accepted: 28 May 1997 相似文献
42.
43.
The cold shock protein CspB adopts its native and functional tertiary structure on the millisecond time scale. We employed transverse relaxation NMR methods, which allow a quantitative measurement of the cooperativity of this fast folding reaction on a residue basis. Thereby, chemical exchange contributions to the transverse relaxation rate (R(2)) were observed for every residue of CspB verifying the potential of this method to identify not only local dynamics but also to characterize global events. Toward this end, the homogeneity of the transition state of folding was probed by comparing Chevron plots (i.e., dependence of the apparent folding rate on the denaturant concentration) determined by stopped-flow fluorescence with Chevron plots of six residues acquired by R(2) dispersion experiments. The coinciding results obtained for probes at different locations in the three-dimensional structure of CspB indicate the ability and significance of transverse relaxation NMR to determine Chevron plots on a residue-by-residue basis providing detailed insights on the nature of the transition state of folding. 相似文献
44.
The reaction of 1,1,1-tris(diiodarsinomethyl)ethane, CH3C(CH2AsI2)3 (I), with i-C3H7NH2, n-C4H9NH2, C6H5NH2, p-CH3C6H4NH2 and [(CH3)3Si]2NH in the presence of (C2H5)3N as auxiliary base in THF gives the adamantane cage compounds CH3C(CH2AsNC3H7)3 (III), CH3C(CH2AsNC4H9)3 (IV), CH3C(CH2AsNC6H5)3 (V), CH3C(CH2AsNC6H4CH3)3 (VI) and CH3C[CH2AsNSi(CH3)3]3 (VII). VII is also obtained in the reaction of I with NaN[Si(CH3)3]2. The by-product (CH3)3SiO(CH2)4I (VIII) could be isolated in both syntheses of VII. All compounds have been characterized by mass spectrometry and infrared, Raman and 1H NMR spectroscopy. 相似文献
45.
Rainer Telle Ai Momozawa Jochen M. Schneider 《Journal of solid state chemistry》2006,179(9):2850-2857
MAX-phases being usually composed of transition metals, group A elements and carbon/nitrogen are considered interesting materials for many applications because of their tremendous bulk modulus, “reversible” plasticity, and machinability. This is mainly due to their unique kind of bonding comprising covalent, ionic as well as metallic bonds providing “easy” planes of rupture and deformability due to the layered crystal structures.In transition metal boride systems, similar types of bonding are available. In particular the W2B5-structure type and its stacking variations allow the synthesis of strongly layered crystal structures exhibiting unique delamination phenomena.The paper presents ab initio calculations showing the similarities of bonding between the ternary carbides and the corresponding ternary or quaternary borides. Formation of boride-based nano-laminates from auxiliary liquid phases, from the melt as well as during sintering and precipitation from supersaturated solid solutions will be discussed by means of SEM and TEM studies. The role of impurities weakening the interlayer bonding will be addressed in particular. The pronounced cleavage parallel to the basal plane gives rise for crack deflection and pull-out mechanisms if the laminates are dispersed in brittle matrices such as boron carbide, silicon carbide or other transition metal borides. 相似文献
46.
Andreas?HeintzEmail author Dagmar?Klasen Jochen?K.?Lehmann Christiane?Wertz 《Journal of solution chemistry》2005,34(10):1135-1144
Experimental densities were measured for the system 1-methyl-3-octyl-imidazolium tetrafluoroborate [OMIM][BF4] + butan-1-ol, + pentan-1-ol at 298.15 K and ambient pressure using a vibrating tube densimeter, taking into account the
influence of the viscosity correction. Excess molar volumes VE have been determined. VE is quite small and negative in the alcohol-rich range of the mixture composition and positive in the alcohol-poor range.
LLE data of [OMIM][BF4] + pentan-1-ol have been measured using a laser light scattering cell for detecting cloud points at different compositions
in the temperature range of 282–292 K. A miscibility gap with an upper critical solution temperature (UCST) of 292 K has been
found. 相似文献
47.
48.
One-bond Pt-Pt nuclear spin-spin coupling constants J(Pt-Pt) for closely related dinuclear Pt complexes can differ by an order of magnitude without any obvious correlation with Pt-Pt distances. As representative examples, the spin-spin couplings of the dinuclear Pt(I) complexes [Pt(2)(CO)(6)](2+) (1) and [Pt(2)(CO)(2)Cl(4)](2-) (2) have been computationally studied with a recently developed relativistic density functional method. The experimental values are (1)J((195)Pt-(195)Pt) = 5250 Hz for 2 but 551 Hz for 1. Many other examples are known in the literature. The experimental trends are well reproduced by the computations and can be explained based on the nature of the ligands that are coordinated to the Pt-Pt fragment. The difference for J(Pt-Pt) of an order of magnitude is caused by a sensitive interplay between the influence of different ligands on the Pt-Pt bond, and relativistic effects on metal-metal and metal-ligand bonds as well as on "atomic orbital contributions" to the nuclear spin-spin coupling constants. The results can be intuitively rationalized with the help of a simple qualitative molecular orbital diagram. 相似文献
49.
Studies on nitrogen iodine compounds. VII. The IR spectrum of nitrogen triiodide-1 ammonia in the range of N—I fundamental vibrations and the valence force constants of the N—I bonds New infrared spectra in the region 33—600 cm?1 of 14NI3 · 14NH3, 15NI3 · 15NH3 and 14NI3 · pyridine, respectively, have been obtained. In addition, the infrared spectrum of 14NI3 · 14ND3, which has been prepared for the first time, was obtained. All absorption frequencies can be coordinated on the ground of the molecule model for the NI3 scaffold with 5 atoms Z2XY2 of the symmetry C2v which has been proved by X ray examination. A set of force constants has been calculated by approximation. The various nitrogeniodine valence force constants are discussed. 相似文献
50.
Le Guennic B Hieringer W Görling A Autschbach J 《The journal of physical chemistry. A》2005,109(21):4836-4846
The circular dichroism spectra of the tris-bidentate metal complexes Lambda-[M(phen)3]2+, with M = Fe, Ru, Os and phen = 1,10-tris-phenanthroline, are investigated computationally, employing time-dependent density functional theory. Good agreement with experimental spectra is obtained for Ru and Os. The Lambda-[Os(phen)3]2+ spectrum is analyzed in detail. It is shown how relativistic effects red shift CD bands where the Os 5d-orbital participates to a large extent in the excitations. Further, the participation of the metal in the ligand pi --> pi exciton CD is determined to be of the order of 10%. Though solvent effects can have a noticeable effect on individual transitions and rotatory strengths, they are demonstrated to have only a very small overall effect on the resulting simulated CD spectra. For Lambda-[Fe(phen)3]2+, the results are shown to be rather sensitive to the choice of the applied hybrid and nonhybrid density functionals, and the optimized geometries based thereupon. In particular, the sign pattern of the lower-energy part (up to 33 x 10(3) cm(-1)) of the Lambda-[Fe(phen)3]2+ CD spectrum is difficult to reproduce. Some combinations of functionals and geometries yield good agreement with experiment, but no "best" approach can be devised based on the available results. Possible sources of errors in the spectrum of Lambda-[Fe(phen)3]2+ due to deficiencies in the functionals and the exchange-correlation kernels are investigated. 相似文献