全文获取类型
收费全文 | 460篇 |
免费 | 37篇 |
国内免费 | 1篇 |
专业分类
化学 | 359篇 |
晶体学 | 25篇 |
力学 | 4篇 |
数学 | 52篇 |
物理学 | 58篇 |
出版年
2023年 | 6篇 |
2022年 | 2篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 14篇 |
2018年 | 16篇 |
2017年 | 16篇 |
2016年 | 15篇 |
2015年 | 23篇 |
2014年 | 20篇 |
2013年 | 36篇 |
2012年 | 44篇 |
2011年 | 43篇 |
2010年 | 20篇 |
2009年 | 19篇 |
2008年 | 33篇 |
2007年 | 29篇 |
2006年 | 25篇 |
2005年 | 22篇 |
2004年 | 19篇 |
2003年 | 7篇 |
2002年 | 8篇 |
2001年 | 4篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1961年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有498条查询结果,搜索用时 296 毫秒
1.
2.
3.
The hybridization in several cyclic polyacetylene compounds has been calculated by the maximum overlap method, assuming planar and non-planar geometries of the molecules. In the planar configuration the hybrids describing the molecular skeleton deviate from the corresponding bond directions. We have a few “bent” bonds, but in contrast to the situation in small rings, here the deviation angles are negative, i.e., the hybrids point toward the inside of the ring. Non-planar structures in which acetylene groups are kept in a plane and CCH2 or CH2 groups are displaced out of the plane show less deviation from the bond directions of bent bonds. Furthermore, the deviation angles decrease with an increase in the out-of-plane displacement of methylene groups. Finally, when the angle of bending of the molecules approaches 50°, the deviation vanishes, predicting a puckered conformation for the molecules. Correlation between CC stretching vibration frequencies and the corresponding CC bond overlap is discussed. 相似文献
4.
García-Orozco I López-Cortés JG Ortega-Alfaro MC Toscano RA Penieres-Carrillo G Alvarez-Toledano C 《Inorganic chemistry》2004,43(26):8572-8576
The paper describes the synthesis and structural characterization of four novel copper(I) complexes [CuL(PPh(3))(2)] (L = 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate). In addition, a tautomeric equilibrium in solution was found and Hammett correlations with (13)C NMR parameters were studied. The structure of one complex was fully established by X-ray diffraction analysis. 相似文献
5.
Fernández-G Juan M. Ruíz-Ramírez Oscar L. Toscano Rubén A. Macías-Ruvalcaba Norma Aguilar-Martínez Martha 《Transition Metal Chemistry》2000,25(5):511-517
Three Schiff base copper(II) complexes have been prepared and characterized by elemental analysis, mass spectra, i.r., electronic spectra, eff and X-ray crystal structures. Cyclic voltammetry studies on the complexes indicate a dependence of the cathodic potentials upon electronic effects, but independence on the solid state structure. 相似文献
6.
S. Yariv M. Müller-Vonmoos G. Kahr A. Rub 《Journal of Thermal Analysis and Calorimetry》1989,35(6):1941-1952
The adsorption of crystal violet on laponite was investigated by X-ray diffraction and thermal analysis. DTA, TG and DTG curves were recorded in air. The evolved H2O, CO2, NO2, H2 and C2H6 were simultaneously determined by mass spectrometry. The thermal analysis curves were compared on one hand with the thermal analysis curves of laponite and on the other hand with thermal analysis curves of non-adsorbed crystal violet and of crystal violet adsorbed on montmorillonite. The thermal analysis curves of crystal violet adsorbed on laponite show similarities to the curves of the non-adsorbed crystal violet, but differ from the curves of crystal violet adsorbed on montmorillonite. The differences in the thermal behaviour were attributed toπ interactions which do not occur between crystal violet and laponite but do occur between this dye and montmorillonite. 相似文献
7.
Ariza-Castolo A Montalvo-González JA Montalvo-González R 《Magnetic resonance in chemistry : MRC》2005,43(12):975-978
The configurational properties of a series of cyclohexylidene imines are discussed on the basis of their 1H, 13C and 15N NMR spectral data. 相似文献
8.
Michelle Hernández-Arganis Rubén A. Toscano Mónica Moya-Cabrera Verónica García-Montalvo Raymundo Cea-Olivares Dr. 《无机化学与普通化学杂志》2004,630(11):1627-1631
The alkaline 1-phenyl-1H-1, 2, 3, 4-tetrazole-5-thiolate salts, M[C6H5N4CS] (M = Li ( 1 ), Na ( 2 ), K ( 3 ), Rb ( 4 ) and Cs ( 5 )) were obtained and characterized by means of mass spectrometry (FAB+) and NMR (1H; 13C) spectroscopy. The structures of Na ( 2 ), K ( 3 ), Rb ( 4 ) and Cs ( 5 ) compounds were determined by X-ray diffraction methods. The ligand shows a rich variety of coordination patterns with the alkaline cations. The formation of a four-membered ring MSCN in the compounds with heavier alkali cations (K, Rb and Cs) is shown. In all the cations the coordination number around it increases with the ionic radius. Compounds with Cs+ and Rb+ exhibited the formation of Cs-C and Rb-C interactions with the phenyl group. 相似文献
9.
The ring-closing metathesis (RCM) of two types of unsaturated chiral allylamines III, easily available from enantiomerically enriched epoxy alcohols, has been studied. Fully protected allylamines IIIa [(1)R = CH(2)-(CH(2))(n)()-CH=CH(2); (2)R = Boc; (3)R = PMB] have been prepared from unsaturated epoxy alcohols, whereas bis-allylamines IIIb ((1)R = Ph, (2)R = allyl,(3)R = Boc or PMB) have been prepared from 2,3-epoxy-3-phenylpropanol. Both types have been subjected to RCM to provide either cyclic allylamine I or II. The synthetic potential of these intermediates has been demonstrated by the enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline. 相似文献
10.
Antonio Fernández-Ramos Benjamin A. Ellingson Rubén Meana-Pañeda Jorge M. C. Marques Donald G. Truhlar 《Theoretical chemistry accounts》2007,118(4):813-826
This article shows how to evaluate rotational symmetry numbers for different molecular configurations and how to apply them
to transition state theory. In general, the symmetry number is given by the ratio of the reactant and transition state rotational
symmetry numbers. However, special care is advised in the evaluation of symmetry numbers in the following situations: (i)
if the reaction is symmetric, (ii) if reactants and/or transition states are chiral, (iii) if the reaction has multiple conformers
for reactants and/or transition states and, (iv) if there is an internal rotation of part of the molecular system. All these
four situations are treated systematically and analyzed in detail in the present article. We also include a large number of
examples to clarify some complicated situations, and in the last section we discuss an example involving an achiral diasteroisomer. 相似文献