全文获取类型
收费全文 | 971篇 |
免费 | 33篇 |
国内免费 | 3篇 |
专业分类
化学 | 780篇 |
晶体学 | 7篇 |
力学 | 11篇 |
数学 | 91篇 |
物理学 | 118篇 |
出版年
2023年 | 10篇 |
2022年 | 22篇 |
2021年 | 23篇 |
2020年 | 18篇 |
2019年 | 24篇 |
2018年 | 21篇 |
2017年 | 13篇 |
2016年 | 24篇 |
2015年 | 34篇 |
2014年 | 32篇 |
2013年 | 58篇 |
2012年 | 64篇 |
2011年 | 84篇 |
2010年 | 38篇 |
2009年 | 35篇 |
2008年 | 55篇 |
2007年 | 66篇 |
2006年 | 61篇 |
2005年 | 40篇 |
2004年 | 50篇 |
2003年 | 38篇 |
2002年 | 32篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 9篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 13篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1940年 | 2篇 |
1926年 | 3篇 |
排序方式: 共有1007条查询结果,搜索用时 0 毫秒
51.
G. Várhegyi M. J. Antal Jr. Piroska Szabó Emma Jakab F. Till 《Journal of Thermal Analysis and Calorimetry》1996,47(2):535-542
The complexity of the phenomena which arise during the heating of the various substances seldom can be described by a single reaction kinetic equation. As a consequence, sophisticated models with several unknown parameters have to be developed. The determination of the unknown parameters and the validation of the models requires the simultaneous evaluation of whole series of experiments. We can accept a model and its parameters if, and only if we get a reasonable fit to several experiments carried out at different experimental conditions. In the field of the thermal analysis the method of least squares alone seldom can select abest model or abest set of parameter values. Nevertheless, the careful evaluation of the experiments may help in the discerning between various chemical or physical assumptions by the quality of the corresponding fit between the experimental and the simulated date. The problem is illustrated by the thermal de-composition of cellulose under various experimental conditions.This research program was funded by the National Science Foundation (grant INT 8914934), the US Hungarian Science and Technology Joint Fund (grants 90b-22 and 93b-375), the Hungarian National Research Fund (OTKA, grant 3077/91) and the Coral Industries Endowment. 相似文献
52.
Rondahl Stina Holmgren Pointurier Fabien Ahlinder Linnea Ramebäck Henrik Marie Olivier Ravat Brice Delaunay François Young Emma Blagojevic Ned Hester James R. Thorogood Gordon Nelwamondo Aubrey N. Ntsoane Tshepo P. Roberts Sarah K. Holliday Kiel S. 《Journal of Radioanalytical and Nuclear Chemistry》2018,315(2):395-408
Journal of Radioanalytical and Nuclear Chemistry - This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear... 相似文献
53.
John T. Gupton Alex Shimozono Evan Crawford Joe Ortolani Evan Clark Matt Mahoney Campbell Heese Jeffrey Noble Carlos Perez Mandry Rene Kanters Raymond N. Dominey Emma W. Goldman James A. Sikorski Daniel C. Fisher 《Tetrahedron》2018,74(21):2650-2663
Highly functionalized pyrroles with appropriate regiochemical functionality represent an important class of marine natural products and potential drug candidates. We describe herein a detailed study of the reaction of α-aminoacid esters with vinylogous amides and also β-halovinylaldehydes for the regiospecific synthesis of 2,3,4-trisubstituted and 1,2,3,4-tetrasubstituted pyrroles. Since the vinylogous amides and β-halovinylaldehydes are readily available precursors, rapid access to a wide variety of unsymmetrically substituted pyrroles is accomplished via this methodology. 相似文献
54.
Total Synthesis of Tambromycin by Combining Chemocatalytic and Biocatalytic C−H Functionalization 下载免费PDF全文
Dr. Xiao Zhang Emma King‐Smith Prof. Dr. Hans Renata 《Angewandte Chemie (International ed. in English)》2018,57(18):5037-5041
A combination of genomic and metabolomic approaches recently resulted in the identification of a nonribosomal tetrapeptide tambromycin, which possesses promising antiproliferative activity and several unusual structural features, including a densely substituted indole, a methyloxazoline ring, and an unusual pyrrolidine‐containing amino acid called tambroline. In this work, we identify a concise synthetic route to access tambromycin, which relies on the strategic use of biocatalytic and chemocatalytic C?H functionalization methods to prepare two key precursors to the natural product in an efficient and scalable manner. The success of our study highlights the benefits of applying the principles of biocatalytic retrosynthesis as well as C?H functionalization logic to the synthesis of complex molecular scaffolds. 相似文献
55.
56.
Rubn A. Toscano Emma Maldonado Alfredo Ortega Jorge Crdenas 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(7):846-847
The absolute configuration of the neoclerodane glycoside amarisolide, presented here as the monohydrate, C26H36O9·H2O, has been determined by association with the known configuration of the glucose moiety. Its structure was established as 2β‐(O‐β‐d ‐glucopyranosyl)neocleroda‐3,13(16),14‐trien‐15,16‐epoxy‐18,19‐olide. Extensive hydrogen bonding among the hydroxyl groups of the sugar moiety forms layers which are interconnected by water molecules. 相似文献
57.
Jixin Qiao Petra Lagerkvist Ilia Rodushkin Susanna Salminen-Paatero Per Roos Syverin Lierhagen Karl Andreas Jensen Emma Engstrom Yann Lahaye Lindis Skipperud 《Journal of Radioanalytical and Nuclear Chemistry》2018,315(3):565-580
Inductively coupled plasma mass spectrometry (ICP-MS) techniques are widely used for determination of long-lived radionuclides and their isotopic ratios in the nuclear fields. Uranium (U) and Plutonium (Pu) isotopes have been determined by many researchers with ICP-MS due to its relatively high sensitivity and short measurement time. In this work, an inter-laboratory comparison exercise among the Nordic countries was performed, focusing on the measurement of U and Pu isotopes in certified reference materials by ICP-MS. The performance and characters of different ICP-MS instruments are evaluated and discussed in this paper. 相似文献
58.
59.
The regioselectivity of the nitrous acid mediated dealkylation of 4-substituted-N-ethyl-N-methylanilines is a function of the acidity of the reaction mixture. At high acidity deethylation predominates, whereas demethylation is the predominant reaction in nitrosamine formation at pH 2 and above. In some cases the regioselectivity of nitrosative dealkylation changes as the run proceeds. Through the use of the corresponding 4-nitroaniline as the primary substrate, CIDNP, kinetics, kinetic deuterium isotope effects and other transformations involving nitrosations with NO2 or NOBF4 in aprotic solvents, a new mechanism of tertiary amine nitrosation has been deduced and proposed to explain regioselective deethylation. The mechanism involves the oxidation of the substrate to the amine radical cation by NO+. This is followed by the abstraction of a hydrogen atom from the carbon adjacent to the amine nitrogen by NO2 to produce an iminium ion which reacts further to produce the corresponding aldehyde and the nitrosamine. Depending upon the acidity, this process competes with three other mechanistic pathways, two of which give the nitrosamine through the iminium ion, and one leads to the formation of C-nitro compounds. The competing pathways to nitrosamine formation involve NOH elimination from a nitrosammonium ion and deprotonation of the radical cation to give an alpha-amino radical which rapidly oxidized to the iminium ion. Predominant, but not highly regioselective demethylation occurs by these pathways. Nitro compound formation principally arises from the reaction of NO2 with the radical cation followed by deprotonation, but also occurs by para C-nitrosation followed by oxidation. 相似文献
60.
Brown EJ Whitwood AC Walton PH Duhme-Klair AK 《Dalton transactions (Cambridge, England : 2003)》2004,(16):2458-2462
The butoxy- and methoxy-derivatives (HLc, and HLb, respectively) of 1-[(ethoxycarbonyl)methyl]-3-hydroxy-2-(1H)-pyridinone, HLa, were synthesized by transesterification and the mononuclear complexes [MoO2(L(a-c))2] were obtained by the reaction of the ligands with molybdenyl acetylacetonate. The three complexes were fully characterized by (1)H and (13)C NMR spectroscopy, mass spectrometry and elemental analyses. A variable temperature NMR study showed that the N-substituents do not have a significant influence on the racemisation process of the complexes. The comparison of the crystal structure of [MoO2(Lb)2] with [MoO2(La)2] confirmed that an additional methylene group does not affect the immediate coordination environment of the molybdenum centre. The methoxy-derivative HLb was also found to produce a dinuclear complex of composition [(MoO2(Lb)O)2], the structure of which was determined by X-ray diffraction. 相似文献