首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   194篇
  免费   7篇
化学   165篇
晶体学   3篇
力学   2篇
数学   19篇
物理学   12篇
  2023年   2篇
  2022年   1篇
  2021年   2篇
  2020年   4篇
  2019年   2篇
  2018年   4篇
  2017年   1篇
  2016年   2篇
  2015年   4篇
  2014年   3篇
  2013年   10篇
  2012年   3篇
  2011年   5篇
  2010年   4篇
  2009年   5篇
  2008年   8篇
  2007年   17篇
  2006年   4篇
  2005年   14篇
  2004年   9篇
  2003年   9篇
  2002年   17篇
  2001年   13篇
  2000年   6篇
  1999年   6篇
  1998年   5篇
  1997年   13篇
  1996年   4篇
  1995年   4篇
  1994年   1篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1989年   2篇
  1988年   2篇
  1987年   1篇
  1984年   1篇
  1977年   2篇
  1976年   3篇
  1972年   2篇
  1968年   1篇
  1882年   1篇
排序方式: 共有201条查询结果,搜索用时 15 毫秒
51.
The mass spectra of the catenanes functionalized in one ring by β-phenylethylaza-, aza-, acetylaza-, trideuteroacetylaza- and trifluoroacetylaza groups respectively, are compared with the spectra of correspondingly substituted macrocyclic and long chain compounds. The mass spectra of most of the catenanes investigated are distinguished by a dominant fragmentation process, which isinitiated by the interannular transfer of ahydrogen atom to the functional group. Fragmentations caused by initial rupture of one of the macrocycles are much less favoured. Apart from H-transfer, no interannular reactions could be detected. In the presence of a β-phenylethylaza substituent, the loss of the benzyl radical is so fast, however, that no fragmentation initiated by H-transfer can be detected. The comparison of the spectra of the open chain compounds with the macrocyclic and catenane compounds suggests that significant solvation of these functional groups by long aliphatic chains does not exist.  相似文献   
52.
A principle for creating a new generation of nonionic superbases is presented. It is based on attachment of tetraalkylguanidino, 1,3-dimethylimidazolidine-2-imino, or bis(tetraalkylguanidino)carbimino groups to the phosphorus atom of the iminophosphorane group using tetramethylguanidine or easily available 1,3-dimethylimidazolidine-2-imine. Seven new nonionic superbasic phosphazene bases, tetramethylguanidino-substituted at the P atom, have been synthesized. Their base strengths are established in tetrahydrofuran (THF) solution by means of spectrophotometric titration and compared with those of eight reference superbases designed specially for this study, P2- and P4-iminophosphoranes. The gas-phase basicities of several guanidino- and N',N',N',N'-tetramethylguanidino (tmg)-substituted phosphazenes and their cyclic analogues are calculated, and the crystal structures of (tmg)3P=N-t-Bu and (tmg)3P=N-t-Bu x HBF4 are determined. The enormous basicity-increasing effect of this principle is experimentally verified for the tetramethylguanidino groups in the THF medium: the basicity increase when moving from (dma)3P=N-t-Bu (pKalpha = 18.9) to (tmg)3P=N-t-Bu (pKalpha = 29.1) is 10 orders of magnitude. A significantly larger basicity increase (up to 20 powers of 10) is expected (based on the high-level density functional theory calculations) to accompany the similar gas-phase transfer between the (dma)3P=NH and (tmg)3P=NH bases. Far stronger basicities still are expected when, in the latter two compounds, all three dimethylamino (or tetramethylguanidino) fragments are replaced by methylated triguanide fragments, (tmg)2C=N-. The gas-phase basicity (around 300-310 kcal/mol) of the resulting base, [(tmg)2C=N-]3P=NH, having only one phosphorus atom, is predicted to exceed the basicity of (dma)3P=NH by more than 40 powers of 10 and to surpass also the basicity of the widely used commercial [(dma)3P=N]3P=N-t-Bu (t-BuP4) superbase.  相似文献   
53.
Cross sections for the production of O 2 ? in charge transfer collisions of fast molecular hydrogen ions (H 2 + , D 2 + , H 3 + , and D 3 + of 10 to 140 keV kinetic energy) with O2 molecules have been determined by means of a time-of-flight mass spectrometer analysing the slow negative product ions from the collisions. Within the measuring accuracy equivelocity H 2 + and D 2 + ions have the same cross sections for the generation of O 2 ? . The projectile velocity dependence curve of the cross section passes through a broad maximum with a peak value of about 6.5×10?18 cm2 around the Bohr velocity (25 keV/u) before showing an asymptotic decrease still within the limited energy range under investigation that is in inverse proportion to the square of velocity. Throughout the examined energy range H 3 + ions yield a cross section which is about 1.4 times larger than that of H 2 + ions of the same velocity. The fragment ion O? has been found to appear with cross sections between 10?19 and 10?18 cm2 upon collisional excitation in the energy range under investigation, with ever decreasing intensity when the energy of the positive hydrogen ions, the proton included, was increased.  相似文献   
54.
55.
Recently, room-temperature crystal structures of SO(2)F(-) in its K(+) and Rb(+) salts were published in Z. Anorg. Allg. Chem. 1999, 625, 385 and claimed to represent the first reliable geometries for SO(2)F(-). However, their almost identical S-O and S-F bond lengths and O-S-O and O-S-F bond angles are in sharp contrast to the results from theoretical calculations. To clarify this discrepancy, the new [(CH(3))(2)N](3)SO(+) and the known [N(CH(3))(4)(+)], [(CH(3))(2)N](3)S(+), and K(+) salts of SO(2)F(-) were prepared and their crystal structures studied at low temperatures. Furthermore, the results from previous RHF and MP2 calculations were confirmed at the RHF, B3LYP, and CCSD(T) levels of theory using different basis sets. It is shown that all the SO(2)F(-) salts studied so far exhibit varying degrees of oxygen/fluorine and, in some cases, oxygen-site disorders, with [(CH(3))(2)N](3)SO(+)SO(2)F(-) at 113 K showing the least disorder with r(S-F) - r(S-O) = 17 pm and angle(O-S-O) - angle(F-S-O) = 6 degrees. Refinement of the disorder occupancy factors and extrapolation of the observed bond distances for zero disorder resulted in a geometry very close to that predicted by theory. The correctness of the theoretical predictions for SO(2)F(-) is further supported by the good agreement between the calculated and the experimentally observed vibrational frequencies and their comparison with those of isoelectronic ClO(2)F. A normal coordinate analysis of SO(2)F(-) confirms the weakness of the S-F bond with a stretching force constant of only 1.63 mdyn/A and shows that there is no highly characteristic S-F stretching mode. The S-F stretch strongly couples with the SO(2) deformation modes and is concentrated in the two lowest a' frequencies.  相似文献   
56.
Phenyl triflate reacts with CF3SiMe3/Q+F- (Q+ = [K(18-crown-6)]+, Me4N+) and (Me2N)3S+Me3SiF2- to afford the first noncyclic [10-S-5] sulfuranide dioxide salts, [(CF3)3SO2]-Q+, with three S-C bonds, whose molecular structure was determined by X-ray crystallography.  相似文献   
57.
We consider L 2-critical focusing nonlinear Schrödinger equations with Hartree type nonlinearity $i \partial_{t} u = - \Delta u - \left(\Phi \ast |u|^2 \right) u \quad {\rm in}\, \mathbb {R}^4,$ where Φ(x) is a perturbation of the convolution kernel |x|?2. Despite the lack of pseudo-conformal invariance for this equation, we prove the existence of critical mass finite-time blowup solutions u(t, x) that exhibit the pseudo-conformal blowup rate $\| \nabla u(t) \|_{L^2_x}\sim \frac{1}{|t|} \quad {\rm as}\, t \nearrow 0.$ Furthermore, we prove the finite-codimensional stability of this conformal blow up, by extending the nonlinear wave operator construction by Bourgain and Wang (see Bourgain and Wang in Ann. Scuola Norm Sup Pisa Cl Sci (4) 25(1–2), 197–215, 1997/1998) to L 2-critical Hartree NLS.  相似文献   
58.
We prove finite-time blowup for spherically symmetric and negative energy solutions of Hartree–Fock and Hartree–Fock–Bogoliubov-type equations, which describe the evolution of attractive fermionic systems (e.g. white dwarfs). Our main results are twofold: first, we extend the recent blowup result of Hainzl and Schlein (Comm. Math. Phys. 287:705–714, 2009) to Hartree–Fock equations with infinite rank solutions and a general class of Newtonian type interactions. Second, we show the existence of finite-time blowup for spherically symmetric solutions of a Hartree–Fock–Bogoliubov model, where an angular momentum cutoff is introduced. We also explain the key difficulties encountered in the full Hartree–Fock–Bogoliubov theory.  相似文献   
59.
Selected N-alkyl ketoimines having at least one methyl group at the imino carbon were reacted in a Stork enamine reaction with 2,2-difluoro-2-fluorosulfonylacetyl fluoride to yield enaminones carrying one fluorosulfonyldifluoromethylene function. In the case of N-tertbutyl methyl aldimine two of these groups were present in a triketone derivative. Taking a ethylene bis(imine) also a ring closure reaction was observed, proven by X-ray structure analysis.  相似文献   
60.
The Donor Properties of Bis(pyrazolyl)‐Sulfur Derivatives From the reactions of bis(pyrazolyl)sulfane S(pz)2 ( 1 ) with the fluoro Lewis acids BF3 and AsF5 in liquid SO2 the 1:2‐adducts S(pz·BF3)2 ( 2 ) and S(pz·AsF5)2 ( 3 ) are obtained. 1 reacts with [Co(SO2)4(FAsF5)2] to give the doubly bridged FAsF4F dimeric complex [Co{S(pz)2}(FAsF5)(SO2)(μ‐FAsF4F)]2 ( 5 ). From F2S(pz)2 and [Ni(SO2)6](AsF6)2, the fluorocubane [Ni4F4{S(pz)2}4(μ‐FAsF4F)2](AsF6)2·4SO2 ( 8 ) is isolated. The X‐ray structures of the compounds 2 , 3 , 5 and 8 are reported.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号