全文获取类型
收费全文 | 1421篇 |
免费 | 31篇 |
国内免费 | 5篇 |
专业分类
化学 | 1053篇 |
晶体学 | 13篇 |
力学 | 18篇 |
数学 | 215篇 |
物理学 | 158篇 |
出版年
2022年 | 15篇 |
2021年 | 16篇 |
2020年 | 25篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 25篇 |
2014年 | 34篇 |
2013年 | 57篇 |
2012年 | 80篇 |
2011年 | 87篇 |
2010年 | 55篇 |
2009年 | 38篇 |
2008年 | 67篇 |
2007年 | 61篇 |
2006年 | 72篇 |
2005年 | 82篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 55篇 |
2001年 | 14篇 |
2000年 | 15篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 12篇 |
1994年 | 17篇 |
1993年 | 26篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 17篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1984年 | 12篇 |
1983年 | 17篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 15篇 |
1973年 | 16篇 |
1970年 | 9篇 |
1884年 | 6篇 |
排序方式: 共有1457条查询结果,搜索用时 0 毫秒
11.
The photoelectron spectrum of tetrakis (methylidene)cyclobutane ( 1 , ‘[4]radialene’) is reported. The electronic states of 1 + are assigned on the basis of model calculations and with reference to related systems. Jahn-Teller activity in the degenerate states is discussed. A failure of the simple LCBO-model for the π(eg)-orbital of 1 is noted and traced to the fact that this orbital, though having a symmetry-equivalent π*-counterpart, does not interact with it. This feature is confined to [4n]radialenes; their total π-energies are therefore higher than those of the other members. It is shown that radialenes, in principle, do not constitute a class analogous to that of the linear polyenes as inferred earlier. 相似文献
12.
UV/Vis-spectra of Cyclosilanes (SiX
2)
n
withX=Cl, Br, I, OMe andn=4, 5, 6 are recorded and interpreted qualitatively. The spectra exhibit increasing influence of the substituents on the Si-Si-bond system within the rings in the order Cl OMe Br I. At the low energy side of the spectra weak absorption bands, possibly due to intramolecular charge transfer transitions, appear. The strong inductive effect of the electronegative substituents is documented by a marked increase of the first IP's compared to permethylcyclosilanes. 相似文献
13.
A simple procedure for the determination of manganese in different sections of human brain samples by graphite furnace atomic absorption spectrometry has been developed. Brain sections included cerebellum, hypothalamus, frontal cortex, vermix and encephalic trunk. Two sample preparation procedures were evaluated, namely, slurry sampling and microwave-assisted acid digestion. Brain slurries (2% w/v) could be prepared in distilled, de-ionized water, with good stability for up to 30 min. Brain samples were also digested in a domestic microwave oven using 5 ml of concentrated HNO3. A mixed palladium+magnesium nitrate chemical modifier was used for thermal stabilization of the analyte in the electrothermal atomizer up to pyrolysis temperatures of 1300 °C, irrespective of the matrix. Quantitation of manganese was conducted in both cases by means of aqueous standards calibration. The detection limits were 0.3 and 0.4 ng ml−1 for the slurry and the digested samples, respectively. The accuracy of the procedure was checked by comparing the results obtained in the analysis of slurries and digested brain samples, and by analysis of the NIST Bovine Liver standard reference material (SRM 1577a). The ease of slurry preparation, together with the conventional set of analytical and instrumental conditions selected for the determination of manganese make such methodology suitable for routine clinical applications. 相似文献
14.
Reinhard Haase Dorit S. Meinhold Berthold Thomas Edwin Weber Gerd Rheinwald 《Structural chemistry》2002,13(5-6):471-477
The crystal structures of inclusion compounds of 3,3-bis(9-hydroxy-9-fluorenyl)-2,2-binaphthyl host (1) and its chloro (2) or bromo (3) derivatives substituted in 2,7-positions of the fluorene units with acetone guests (1A–3A) were determined by X-ray studies as well as by 1H-CRAMPS solid-state NMR. Using this NMR technique allows identification of differently bound guest molecules due to their different chemical shifts caused by the influence of the ring current effects of the host aryl units. 相似文献
15.
Homoallylic phosphine boranes undergo intramolecular hydroboration upon activation by triflic acid. The reaction occurs via an intermediate B-trifluorosulfonyloxyborane complex such as 15, followed by S(N)1-like or S(N)2-like displacement of the triflate leaving group, apparently leading to the formation of a four-center transition state. In the case of trisubstituted double bonds, as in the substrates 29 and 32, ionic hydrogenation of the alkene competes with internal hydroboration. 相似文献
16.
Recent research has demonstrated that in a simulated flue gas stream containing NO(2) and SO(2) elemental mercury is initially captured on a carbon or manganese oxide sorbent. After approximately an hour, however, mercury breaks through relatively rapidly, and the volatile form of mercury emitted is an oxidized species. The volatile mercury species emitted from a granular MnO(2) sorbent was trapped in an impinger containing cold acetonitrile. Subsequent evaporation of 95% of the acetonitrile in a Kuderna-Danish apparatus and gas chromatography (GC) of the concentrate resulted in a single mercury-containing GC peak at 5.5 min; the retention time and mass spectrum of this compound matched exactly those of a standard mercury(II) nitrate hydrate, Hg(NO(3))(2).H(2)O dissolved in acetonitrile. The volatile mercury component analyzed from injection of this standard solution was shown to be a form of methylmercury that is produced in the GC column by reaction of the highly reactive mercury nitrate with the methylsiloxane GC phase. Because the on-column derivatization reaction seems to be unique to mercury nitrate, the GC-MS (gas chromatography-mass spectroscopic) analysis provides strong evidence for identification of the trapped oxidized mercury species as mercury nitrate although, because the nitrate becomes detached from the mercury atom in the on-column reaction, the identity is not proven. 相似文献
17.
Edwin Haselbach Urs Klemm Rudolf Gschwind Thomas Bally Laurant Chassot Stephan Nitsche 《Helvetica chimica acta》1982,65(8):2464-2471
The radical cations of indeno [2, 1-a]indene ( 1 ), stilbene ( 2 ) and 3, 5, 3', 5'-tetramethylstilbene ( 3 ) were prepared by γ-irradiation of the neutral precursors in an electron-scavenging matrix at 77 K . Their electronic spectra were recorded and compared to the photoelectron spectra ( PE .) of the neutral precursors. The results show that either the fourth or the fifth excited doublet state of the cations is of «Non-Koopmans» type, with specific doublet energy (D) D (2Bg)=2.74 eV ( 1 +), =2.59 eV ( 2 +), =2.49 eV ( 3 +). Remarkably, 1 + possesses two electronic states in the 2.7-2.8 eV energy range: 2Au («Koopmans»-type) and 2Bg («Non -Koopmans»-type). The «SDT»-equation \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm D} = \sqrt {{\rm S} \cdot {\rm T}} $\end{document} approximately connecting excited singlet (S) and triplet (T) states of a neutral alternant system with the excited doublet (D) states of its radical cation - provided e-promotion occurs For all three excited states between the same (paired) orbitals-is satisfyingly exemplified by 1 : S1 = 3.92 eV and T1= 2.06 eV for 1 , D4 or 5=2.74 eV for 1 +. 相似文献
18.
N. Thornton Lipscomb Edwin C. Weber 《Journal of polymer science. Part A, Polymer chemistry》1967,5(4):779-784
Studies have been made of the γ-radiation-induced polymerization of methyl methacrylate in bulk, in the solid state at a temperature of ?65°C. and a radiation intensity of 346,000 rad/hr. The reaction was found to have an extremely long induction period (~50 hr.) when pure monomer was used, and to be first-order with respect to polymer concentration. This first-order dependency was confirmed by a series of irradiations in which 0.6% poly(methyl methacrylate) was dissolved in the monomer before irradiation. These irradiations showed no induction period. Nuclear magnetic resonance spectroscopy indicated a much more heterotactic polymer than that obtained in the liquid state at ?49°C. 相似文献
19.
Conventional proteomics makes use of two-dimensional gel electrophoresis followed by mass spectrometric analysis of tryptic fragments derived from in-gel digestion of proteins. Although being a very strong technique capable of separating and visualizing hundreds of proteins, 2D-gel electrophoresis has some well-documented disadvantages as well. More recently, liquid chromatographic-(tandem) mass spectrometric techniques have been developed to overcome some of the shortcomings of 2D-gel electrophoresis. In this review we have described several recent applications of liquid chromatography-(tandem) mass spectrometry in the field of proteomics and especially in the field of membrane proteomics, quantitative proteomics and in the analysis of post-translational modifications. 相似文献
20.
Gabelica V Galic N Rosu F Houssier C De Pauw E 《Journal of mass spectrometry : JMS》2003,38(5):491-501
A method for determining the equilibrium association constant of a complexation reaction A + B left harpoon over right harpoon AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A-B solutions at different concentrations C(0). The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant K(a) and a factor R, defined by I(AB)/I(A) = R x [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with alpha,omega-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes. 相似文献