全文获取类型
收费全文 | 2513篇 |
免费 | 75篇 |
国内免费 | 2篇 |
专业分类
化学 | 1789篇 |
晶体学 | 5篇 |
力学 | 33篇 |
数学 | 249篇 |
物理学 | 514篇 |
出版年
2020年 | 26篇 |
2019年 | 32篇 |
2016年 | 52篇 |
2015年 | 43篇 |
2014年 | 38篇 |
2013年 | 88篇 |
2012年 | 82篇 |
2011年 | 111篇 |
2010年 | 58篇 |
2009年 | 64篇 |
2008年 | 95篇 |
2007年 | 81篇 |
2006年 | 84篇 |
2005年 | 78篇 |
2004年 | 70篇 |
2003年 | 65篇 |
2002年 | 54篇 |
2001年 | 53篇 |
2000年 | 57篇 |
1999年 | 28篇 |
1998年 | 30篇 |
1996年 | 42篇 |
1995年 | 56篇 |
1994年 | 32篇 |
1993年 | 47篇 |
1992年 | 43篇 |
1991年 | 30篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 25篇 |
1987年 | 26篇 |
1986年 | 28篇 |
1984年 | 33篇 |
1983年 | 24篇 |
1982年 | 37篇 |
1981年 | 41篇 |
1980年 | 26篇 |
1979年 | 35篇 |
1978年 | 24篇 |
1977年 | 32篇 |
1976年 | 31篇 |
1975年 | 26篇 |
1974年 | 41篇 |
1973年 | 30篇 |
1968年 | 30篇 |
1967年 | 35篇 |
1966年 | 42篇 |
1965年 | 28篇 |
1964年 | 40篇 |
1963年 | 36篇 |
排序方式: 共有2590条查询结果,搜索用时 31 毫秒
31.
Pd/P(t-Bu)(3) serves as an unusually reactive catalyst for Stille reactions of aryl chlorides and bromides, providing solutions to a number of long-standing challenges. An unprecedented array of aryl chlorides can be cross-coupled with a range of organotin reagents, including SnBu(4). Very hindered biaryls (e.g., tetra-ortho-substituted) can be synthesized, and aryl chlorides can be coupled in the presence of aryl triflates. The method is user-friendly, since a commercially available complex, Pd(P(t-Bu)(3))(2), is effective. Pd/P(t-Bu)(3) also functions as an active catalyst for Stille reactions of aryl bromides, furnishing the first general method for room-temperature cross-couplings. 相似文献
32.
Barbara A. Bierl-Leonhardt Daniel S. Moreno Meyer Schwarz JoAn Fargerlund Jack R. Plimmer 《Tetrahedron letters》1981,22(5):389-392
The sex attractant pheromone of the citrus mealybug, (Risso), has been Identified as (1-)-(+)-2,2-dimethyl-3-(1-methylethenyl)cyclobutanemethanol acetate (VI). 相似文献
33.
Gas-phase derivatization of [C3H5]+ ions using tandem mass spectrometry methods A13C labelling study
Nguyen Kieu My Marita Schilling Helmut Schwarz 《Journal of mass spectrometry : JMS》1987,22(5):254-258
The study of specifically 13C-labelled precursors sheds further light on the gas-phase chemistry of allyl and 2-propenyl cations. It is demonstrated that both species are formed from allyl and 2-propenyl bromide upon 70 eV electron impact ionization without skeletal reorganization. Gas-phase derivatization of the [C3 H5]+ ions with benzene facilitates, as suggested and observed earlier, the distinction of the two isomers using collision-induced dissociation of the Wheland complexes (or isomers thereof). The 13C labelling data clearly demonstrate that 64% of allyl cations survive the derivatization while 36% isomerize to 2-phenylpropyl cations; the latter are also formed via the reaction of 2-propenyl cation with benzene, protonation of α-methylstyrene and water loss from protonated 2-phenyl-2-propanol, respectively. Unimolecular loss of C2H4 from protonated allylbenzene proceeds via two competing reaction channels: one involves heterolysis of 1-phenylpropyl cations (~30%); the major pathway (~70%), however, involves decomposition via propylene benzenium ions. 相似文献
34.
The electron-impact fragmentation of S-p-tolylester of nitrothiobenzoic acid leads to a previously unknown rearrangement in the case of the ortho compound. This consists of an intra molecular redoxreaction with oxygen transfer from the o-nitro group onto the sulphur of the thiol ester group. The position of the nitro group is not only an essential influence on the fragmentation pattern, but also of dramatic importance in determining the lifetime of the molecular ion. 相似文献
35.
Cooperative conformational transitions of proteins and nucleic acids are of decisive importance to many processes of molecular biology, and particularly to their regulation. They proceed via numerous interdependent elementary processes, and their kinetics are therefore often complicated. They are frequently also very fast. However, kinetic analyses can be carried out by chemical relaxation methods. The theoretical interpretation is comparatively simple in the case of linear biopolymers. When the linear Ising model extended for kinetics was applied to model peptides and polynucleotides, it provided an insight into the fundamental principles of cooperative transformations. 相似文献
36.
The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes. 相似文献
37.
Mass spectroscopic investigations of 2H- and 13C-labelled derivatives of e.g. 1-, 2-chloro-methyl-naphthalene and 1-chloro-phenyl(5)-pentene-(2)-in-(4) show that the [C11H9]+ ion, which gives [C9H7]+ by acetylene elimination, has the structure of a benztropylium cation. Model considerations show that the formation of this cation [C11H9]+ through a common transition state with a three-membered ring is very probable. 相似文献
38.
P. Palade G. Principi T. Spataru P. Blaha K. Schwarz V. Kuncser S. Lo Russo S. Dal Toé V. A. Yartys 《Journal of Radioanalytical and Nuclear Chemistry》2005,266(3):553-556
Summary LaNiSn and NdNiSn compounds and their deuterides have been studied by variable temperature 119Sn M?ssbauer spectroscopy. The hyperfine parameters obtained experimentally are in good agreement with those derived from
first principle calculations. The enlargement of quadrupole splitting observed for LaNiSn after deuteration confirms the lower
symmetry of electron density around tin atoms indicated by the calculation of partial Sn-p density of states (DOS). Magnetic
ordering is observed at low temperature in deuterided NdNiSn. 相似文献
39.
H. Schwarz 《Fresenius' Journal of Analytical Chemistry》1976,279(2):136-137
Ohne Zusammenfassung
Analytical importance of electron-impact induced retro-diels-alder reactions for stereochemical investigations相似文献
40.
Winfried Wagner-Redeker Karsten Levsen Helmut Schwarz Waltraud Zummack 《Journal of mass spectrometry : JMS》1981,16(8):361-371
Using the relative abundance of metastable ions, collisional activation spectra, field ionization kinetic measurements, isotopic labelling, appearance energy and kinetic energy release data, it is shown that linear alkyne radical cations with more than six carbon atoms do not isomerize to equilibrating structures prior to decomposition. At the shortest ion lifetimes the molecular ions of linear alkynes decompose mainly by simple β-bond fission which allows an unequivocal localization of the triple bond. At medium ion lifetimes fragmentation occurs predominantly via a McLafferty rearrangement, while at long ion lifetimes competing alkyl losses prevail. These alkyl losses occur via cyclic intermediates leading to thermochemically stable cycloalkenyl ions. All these reactions occur with a high specificity with respect to the carbon and hydrogen atoms involved and are preceded by little or no hydrogen exchange reactions. 相似文献