全文获取类型
收费全文 | 462篇 |
免费 | 12篇 |
专业分类
化学 | 334篇 |
晶体学 | 7篇 |
力学 | 2篇 |
数学 | 8篇 |
物理学 | 123篇 |
出版年
2021年 | 6篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 6篇 |
2013年 | 9篇 |
2012年 | 16篇 |
2011年 | 24篇 |
2010年 | 10篇 |
2009年 | 8篇 |
2008年 | 15篇 |
2007年 | 18篇 |
2006年 | 18篇 |
2005年 | 21篇 |
2004年 | 31篇 |
2003年 | 21篇 |
2002年 | 22篇 |
2001年 | 14篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1984年 | 3篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1973年 | 5篇 |
1969年 | 4篇 |
1941年 | 4篇 |
1940年 | 3篇 |
1935年 | 7篇 |
1934年 | 5篇 |
1933年 | 4篇 |
1930年 | 3篇 |
1926年 | 6篇 |
1925年 | 6篇 |
1904年 | 3篇 |
1899年 | 4篇 |
1898年 | 3篇 |
1885年 | 5篇 |
排序方式: 共有474条查询结果,搜索用时 15 毫秒
91.
R Glaser 《The Journal of organic chemistry》2001,66(3):771-779
The potential energy surface of acetylsalicylic acid, aspirin, has been explored at the RHF/6-31G* and B3LYP/6-31G* levels, and single-point calculations were performed at levels up to B3LYP/6-311G**//B3LYP/6-31G*. All conformational isomers have been located, the thermochemical functions have been computed, and relative energies and free enthalpies were determined. The conformational space of aspirin is spanned by three internal coordinates, and these are the carboxylic acid C-O conformation (s-trans preferred by about 7 kcal/mol), the C-COOH conformation (Z preferred unless there are H-bonding opportunities), and the ester C-O conformation (s-trans preferred by about 4 kcal/mol). There are nine aspirin isomers since one of the conformers realizes hydrogen-bonding structure isomerism as well. Neighboring group interactions are discussed with reference to the intrinsic properties of benzoic acid and phenyl acetate. The intrinsic conformational preference energies for benzoic acid and phenyl acetate are not additive. The acid s-trans preference energies differ by as much as 9 kcal/mol depending on the Ph-COOH and ester conformations. Similarly, the E-preference energies about the Ph-COOH bond vary by as much as 6 kcal/mol depending on the ester conformation. The structural discussion suggests an overall ortho repulsion between the functional groups in all aspirin isomers including the intramolecularly hydrogen-bonded isomers. The isodesmic reaction between the most stable conformers of benzoic acid and phenyl acetate to form aspirin and benzene is found to be endothermic by 2.7 kcal/mol and provides compelling evidence for and a quantitative measure of ortho repulsion. The ortho repulsion of 2.7 kcal/mol is a lower limit, and the ortho repulsion can increase to as much as 6 kcal/mol in some aspirin isomers. 相似文献
92.
The Material Force Method (MFM) and the Extended Finite Element Method (X-FEM), both have been major subjects of computational fracture mechanics in recent time. Thus combining the advantages of both concepts [1, 2] seems a promising approach to describe the behaviour of discontinuities such as cracks in otherwise continuous bodies. As the X-FEM models a crack independently of the mesh, the crack tip is in general not located at a node which is why a central question is, which nodal material forces do contribute to the resulting material force vector at the crack tip. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
93.
Christian Hundshammer Martin Grashei Alexandra Greiner Prof. Dr. Steffen J. Glaser Dr. Franz Schilling 《Chemphyschem》2019,20(6):798-802
Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin-lattice relaxation time (T1), which is on the order of a few tens of seconds. Here, we analyzed the pH-dependence of T1 for commonly used HP 13C-labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T1 of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B0=1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B0=1 T). Our data shows that increasing hydronium ion concentrations shorten the T1 of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T1 shortening. In addition, enhanced proton exchange and chemical reactions at the pKa appear to be detrimental for the HP-state. 相似文献
94.
95.
Katharina Gibson Markus Strbele Bjrn Blaschkowski Jochen Glaser Martina Weisser Radhakrishnan Srinivasan Heinz‐Jürgen Kolb Hans‐Jürgen Meyer 《无机化学与普通化学杂志》2003,629(10):1863-1870
Solid state metathesis reactions have been studied in fused silica tubes, by differential thermal analysis, and by X‐ray powder diffraction. A selection of reactions between metal (La, Nb, and Ni) chlorides and lithium nitride or lithium acetylide were investigated to get more insight into reaction pathways and intermediate reaction stages that may be adopted on course of the formation of metal nitrides or carbides. Intermediate compounds are considered to be important because they can control the reactivity of a system. Such compounds were traced by changing the molar ratios of reaction partners away from the salt‐balanced binary metal nitride or carbide target compositions. New preparative perspectives are discovered when metal chlorides were reacted with lithium nitridoborate or lithium cyanamide. Due to their reductive nature towards several d‐block metal chlorides, (BN2)3‐ and (CN2)2‐ react to form metals or metal nitrides plus X‐ray amorphous BN, and probably C3N4. With lanthanum chloride they can react to form nitridoborates and nitridocarbonates. The metathesis reaction between lithium cyanamide and cyanuric chloride (C3N3Cl3) instead of metal chloride was studied for the synthesis of C3N4. 相似文献
96.
Preparation of Arsenic Chloridetetrafluoride, AsClF4, and Spectroscopical Interpretation of the Series AsClnF5?n During the pyrolysis of [AsCl4]+[AsF6]? AsClF4 is formed in preparative amounts under suitable conditions. The I.R.-matrix and Low-Temperatur-Raman Spectrum are reported. In the series AsClnF5?n exists a linear context between the average valence frequency 1/5 Σνval and the sum of the electronegativities of the ligands. 相似文献
97.
98.
99.
100.
Marsiske H Antreasyan D Bartels HW Besset D Bieler C Bienlein JK Bizzeti A Bloom ED Brock I Brockmüller K Cabenda R Cartacci A Cavalli-Sforza M Clare R Compagnucci A Conforto G Cooper S Cowan R Coyne D Engler A Fairfield K Folger G Fridman A Gaiser D Gelphman D Glaser G Godfrey G Graaf K Heimlich FH Heinsius FH Hofstadter R Irion J Jakubowski Z Janssen H Karch K Keh S Kiel T Kilian H Kirkbride I Kloiber T Kobel M Koch W König AC Königsmann K Kraemer RW Krüger S Landi G Lee R Leffler S 《Physical review D: Particles and fields》1990,41(11):3324-3335