全文获取类型
收费全文 | 270篇 |
免费 | 1篇 |
国内免费 | 2篇 |
专业分类
化学 | 218篇 |
晶体学 | 3篇 |
物理学 | 52篇 |
出版年
2023年 | 2篇 |
2020年 | 2篇 |
2018年 | 3篇 |
2016年 | 1篇 |
2015年 | 1篇 |
2014年 | 3篇 |
2013年 | 8篇 |
2012年 | 10篇 |
2011年 | 15篇 |
2010年 | 2篇 |
2009年 | 11篇 |
2008年 | 14篇 |
2007年 | 5篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 21篇 |
2003年 | 13篇 |
2002年 | 15篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1990年 | 1篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 2篇 |
1965年 | 1篇 |
排序方式: 共有273条查询结果,搜索用时 0 毫秒
61.
二(2,6-二甲基-4-甲氧基)苯基卡宾的产生和反应 总被引:4,自引:1,他引:3
为了考察卡宾稳定性的电子效应,合成了对位具有强推电性甲氧基的二(2, 6-二甲基-4-甲氧基)苯基重氮甲烷(1c),通过光分解而产生了二(2,6-二甲基- 4-甲氧基)苯基卡宾(2c)。分别用电子顺磁共振(EPR)光谱,紫外可见(UV/vis) 光谱及激光闪光光分解法对二(2,6-二甲基-4-甲氧基)苯基卡宾(2c)的产生和衰 减过程进行了详细的观察。由于甲氧基的强推电子作用使二苯基卡宾的稳定性大大 降低,其二次动力学衰减速度常数为130s~(-1),是对位非取代二苯基卡宾(2b) (14s~(-1))的约9倍,且室温苯溶液中的寿命为20ms,仅为2b(180ms)的1/9。且通过 产物分析对二(2,6-二甲基-4-甲氧基)苯基卡宾(2c)的反应及甲氧基的取代基效 应进行了详细探讨。 相似文献
62.
通过对二苯基重氮甲烷的光照射产生了一系列具有对称对位取代基的三线态二(2,6-二甲苯基)卡宾.用电子顺磁共振波谱对其进行了研究.通过对不同粘度的基质(matrix)中零磁场分裂参数D和E的测定,依据电子自旋离域取代基常数σr对三线态二苯基卡宾的分子结构的取代基效应进行了分析.并通过对卡宾的热消失温度及其室温脱气苯溶液中寿命的测定,对三线态二(2,6-二甲苯基)卡宾的稳定性进行了定量考察.结果表明,对卡宾中心的自旋电子具有离域效应的取代基使三线态二(2,6-二甲苯基)卡宾采取低能稳定的直线型结构,且显示了更好的热稳定性和更长的寿命. 相似文献
63.
A three-step methodology involving an external chiral ligand-mediated asymmetric addition of phenyllithium to an anisidine imine, oxidative removal of N-PMP group, and finally oxidative conversion of the phenyl group to a carboxyl group provides a facile synthesis of optically pure α-amino acid derivatives bearing a bulky α-substituent. 相似文献
64.
Highly diastereoselective tandem alkylation process of acyclic α,β-unsaturated esters was developed based on the novel use of dithioacetal as a stereocontrolling unit. 相似文献
65.
Organic dye particles of micrometer and submicrometer diameters were prepared by a wetting/dewetting procedure on a hydrophilic glass surface and a self-organized one- or two-dimensional registration was observed. To analyze the molecular assembly in these particles the near-field-excited near-field fluorescence from single particles were detected, while the majority of particles with diameters around 2 μm or less did not show fluorescence. Far-field fluorescence, in contrast, was observed for every particle, and the intensity depended on the excitation polarization when a polarized evanescent field was used for excitation, indicating that the molecules’ transition moment within dye particles orient parallel to the substrate surface. These two observations suggest that the near-field at the tip of the probe was polarized parallel to the probe axis. Another observation, that neighboring particles show similar molecular orientations, suggests that the dewetting process contributed to the alignment of the molecular directions in adjacent particles, which further proves that the dye particles were formed by a self-organizing mechanism. 相似文献
66.
Hideo Tomioka Norihiro Hayashi Yasuji Izawa Michael T.H. Liu 《Tetrahedron letters》1984,25(39):4413-4416
Benzylchlorocarbenes generated photolytically from diazirines in EtOH afford chlorostyrenes and acetals, whose distributions are sensitive to the reaction temperature. 相似文献
67.
68.
69.
Itoh T Matsuno M Ozaki S Hirai K Tomioka H 《The journal of physical chemistry. B》2005,109(44):20763-20772
To show that persistent high-spin polycarbenes can be realized by utilizing hetero spin systems, two diphenyldiazomethanes having pyridyl groups, i.e., bis{4-(4-pyridyl)-2,6-dimethylphenyl}diazomethane (4,4'-DPy-1-N(2)) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2,4-DPy-1-N(2)), were prepared. Triplet carbenes, 4,4'-DPy-1 and 2,4-DPy-1, generated by photolysis of the corresponding diazomethanes were characterized by spectroscopic means (ESR and UV/vis in matrix at low temperatures and laser flash photolysis in solution at room temperature). The results showed that they were fairly persistent. Magnetic properties of the photoproducts from a 1:1 complex between DPy-1-N(2) and Cu(hfac)(2) (hfac = hexafluoroacetylacetonate) were characterized by ESR and a superconducting quantum interference device (SQUID) magneto/susceptometer. The field dependences of magnetization for the complexes, expressed by using M versus H/T plots, were analyzed in terms of the Brillouin function to be S = 6.80 (F = 0.60) for the 1:1 complex of 4,4'-DPy-1 and Cu(hfac)(2) and S = 3.71 (F = 0.73) for the 1:1 complex of 2,4-DPy-1 and Cu(hfac)(2) at 2.0 K. Thus, it has been demonstrated that a high-spin species is actually generated in the photoproducts and that the complexed carbenes showed significant stability. 相似文献
70.
[reaction: see text] A catalytic asymmetric conjugate arylation of racemic 5-(trimethylsilyl)cyclohex-2-enone with arylboronic acids was catalyzed by 3 mol % chiral amidophosphane- or BINAP-Rh(I) in dioxane-water (10:1) to afford trans- and cis-3-aryl-5-(trimethylsilyl)cyclohexanones in high enantioselectivity. Dehydrosilylation of the product mixture with cupric chloride in DMF gave 5-arylcyclohex-2-enones with up to 93% ee in good yield. Enantiofacial selectivity with chiral phosphane-Rh(I) exceeds the trans-diastereoselectivity that is maintained in the achiral or racemic phosphane-Rh(I)-catalyzed conjugate arylation of 5-(trimethylsilyl)cyclohexenone. 相似文献