全文获取类型
收费全文 | 271篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
化学 | 247篇 |
数学 | 6篇 |
物理学 | 31篇 |
出版年
2023年 | 4篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 3篇 |
2017年 | 4篇 |
2016年 | 4篇 |
2014年 | 3篇 |
2013年 | 19篇 |
2012年 | 17篇 |
2011年 | 11篇 |
2010年 | 14篇 |
2009年 | 10篇 |
2008年 | 10篇 |
2007年 | 12篇 |
2006年 | 6篇 |
2005年 | 11篇 |
2004年 | 4篇 |
2003年 | 7篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 3篇 |
1957年 | 2篇 |
1942年 | 1篇 |
排序方式: 共有284条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
The mass spectral behaviour of α,ω-disubstituted alkanes and, especially, that of different N-substituted α,ω-diaminoalkanes has been investigated. It was found that the two amino groups which are separated by CH2-groups can fragment only to a small extent indepently from each other. Yet those fragmentation reactions are predominant in which both functional groups participate. The main reactions of this type are:
- 1 Loss of the N-substituent (R) from the molecular ion, leading to the [M+—R]-ions.
- 2 Loss of NH3, primary or secondary amines from the [M+—R]-ion in the case of monodi-, tri- and tetra-substituted diamino compounds respectively.
- 3 α-Cleavage to the non charged nitrogen atom by forming the ions
- 4 SNi-type fragmentation.
5.
H. J. Veith 《Journal of mass spectrometry : JMS》1976,11(6):629-633
The field desorption mass spectra of salts such as quaternary ammonium and carbenium salts with organic cations in addition to high cation intensities show signals for cluster ions composed of the salt cation + salt molecule, i.e. [C + nM]+, n = 1–5, thus allowing determination of the molecular weights of salts. In some cases cluster ions of the type [nM – 1]+ are detected. Conditions for the formation of cluster ions are discussed. 相似文献
6.
Reactions of 2- and 3-Phenyl Substituted Alkylalkylidene Iminium Ions in the Gas Phase The collision-induced fragmentations of 2- and 3-phenyl substituted alkylalkylidene iminium ions are reported. Besides the homolytic cleavage of the azaallyl bond a nucleophilic attack of the unsubstituted phenyl group at the iminium function is observed in the gas phase, yielding carbonium ions such as cyclopropanspirobenzenium ( 3 ), indanylium ( 10 ) and indenylium ions ( 11 ). 相似文献
7.
1,3-Di-t-butyl-2,2-dimethyl-1,3,2,4λ2-diazasilastannetidine (II) reacts with 3,4,5-trimethoxybenzaldehyde to yield 3,4,5-trimethoxybenzaldehyde-t-butylimine (IV) and the title compound 2,2-dimethyl-3-t-butyl-1,2,3,4λ2-oxasilazastannetidine (III). III combines four different Main Group elements in one ring and is tetrameric in solvents as well as in the tetragonal crystalline state. An X-ray structure determination (R-value: 0.037) reveals the tetramer to have crystallographic S4-symmetry, and that the molecules of III interact via oxygen-tin bonds, which are perpendicular to the plane of the ring. The oxygen atoms thus adopt trigonal-planar coordination, while the tin atoms have grigonal-pyramidal environments. The tin atoms are substituted by three different ligands: one nitrogen and two chemically non-equivalent oxygen atoms (SnN: 209.5(9), SnO: 218.8(6), SnO′: 213.7(6) pm). Each tin atom thus represents a center of chirality, which due to symmetry must be divided into two R- and two S-enantiomers. As a consequence the whole tetrameric molecule belongs to the optically inactive “meso”-form. 相似文献
8.
The mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field-free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C = $ \mathop {\rm N}\limits^{\rm +} $+R1R2: R1 = R2 = n-C3H7; R1 = R2 = i-C3H7; R1 = n -C3H7; R2 = C2H5; R1 = n-C3H7; R2 = CH3; R1 = n-C3H7; R2 = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion-neutral complex intermediates. For the di-n-propyl species rate-determining and H/D distribution-determining steps are clearly distinct Whereas the former corresponds to a 1,2-hydride shift in a 1-propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2-propyl cation generated by the previous step. For the diisopropyl-substituted ions which directly form the 2-propyl cation-containing complex, the rate-determining hydride shift vanishes. The 2-propyl cation-containing complex can decompose directly or via an intermediate proton-bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α- and β-methylene of the initial n-propyl chain as the source of the hydrogen transferred is detected for n-propylimmonium ions containing a second alkyl chain R2. This preference shows a clear dependence on the steric influence of R2. During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ-H transfer is observed. 相似文献
9.
Veith SR Hughes E Vuataz G Pratsinis SE 《Journal of colloid and interface science》2004,274(1):216-228
The restricted diffusion coefficient of water through porous silica is measured by pulsed field gradient (PFG) NMR as a function of loading in order to develop a model for self-diffusion at full pore filling in sol-gel-made porous silica particles. This model describes the pore or intraparticle diffusion coefficient as a function of particle porosity, tortuosity, and the steric hindrance applied on the molecules by the pore space. The particle morphology is characterized by nitrogen adsorption and an appropriate tortuosity model is chosen in comparison with literature data. To characterize the material, NMR relaxation and diffusion studies at different degrees of pore filling were carried out in relation to the silica/water adsorption isotherm. 相似文献