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51.
Jean-Jacques HelesbeuxOlivier Duval David GuiletDenis Séraphin David RondeauPascal Richomme 《Tetrahedron》2003,59(27):5091-5104
The ene reaction of singlet oxygen with prenylated dihydroxyacetophenones led to the 2-hydroperoxy-3-methylbut-3-enyl derivatives as the major product. This original regioselectivity outlined a new effect, in competition with the previously established large group non-bonding effect. The oxidation products distribution could be explained by a stabilising interaction between the phenolic hydrogen, ortho to the prenyl side chain, and the perepoxide intermediate. 相似文献
52.
Solvolysis of 2-bromo-2-(4′-fluoromethyl)phenyladamantane (4), and 2-bromo-2-(4′-trifluoromethyl)phenylpropane (5) were carried out in a number of solvents, and their rate constants measured. Significant deviation from linear logk-YBr plots was observed, and the order of reactivities, e.g., k(100M) > k(90E) > k(80A), was in the reverse order of Swain's solvent parameter B. Different extent of solvation to the localized and the delocalized cationic transition state was then likely to play an important role. Similar result was also found in the solvolysis of 1-bromo-2,2,2-trifluoro-1-(4′-methylphenyl)-1-phenylethane (6). A new Y scale, YBnBr, for the correlation of solvolytic reactivities of benzylic bromides based on k(4) was established. Intervention of solvent assistance in the solvolysis of 5 is discussed. 相似文献
53.
仪器分析实验往往需要多人合作完成。我们在高效液相色谱、红外光谱、紫外光谱、原子吸收光谱实验过程中尝试运用同伴教育方法起到了良好的教学效果。同伴教育主要体现为同伴相互帮助、相互竞争、相互学习及相互督促四个方面。同伴教育部分减轻了教师的带教负荷,促进同伴之间相互交流,使实验中小组各成员的凝聚力增强,从整体上提升了实验效果。教师在组织实验时适度采取同伴教育方法,有助于仪器分析实验的深入开展。 相似文献
54.
报道了目前所做的确定DF激光系统各种宽光谱反射膜性能特性的工作。利用制备的介质金属增强反射镜,由实验获得了DF激光波长为3.8μm时反射率大于99.4%、可见光区和8~14μm红外区平均反射率高于95%的结果。为说明这些反射镜性能,进行了反射率测量和环境试验。 相似文献
55.
56.
We report a study of charge transfer mechanisms of electrons stimulated optically from very deep traps, also known as donor traps, in α-Al2O3:C. The investigations were carried out using thermally-assisted time-resolved optical stimulation, thermoluminescence and by way of residual thermoluminescence from the main electron trap. When the charges are optically stimulated from the deep traps, they are redistributed via the conduction band to the main electron trap and the shallow trap from where they are optically or thermally released for recombination at luminescence centres. The luminescence is strongly quenched at high measurement temperature as evident by very short luminescence lifetimes at these temperatures. The main peak due to residual thermoluminescence is located at a higher temperature than the conventional main peak. 相似文献
57.
Mykyta Iazykov Moisés Canle J. Arturo Santaballa Ludmila Rublova 《Journal of Physical Organic Chemistry》2018,31(2)
We have studied the mechanism of solvolysis of arenesulfonyl chlorides by propan‐1‐ol and propan‐2‐ol at 303‐323 K. Kinetic profiles were appropriately fit by first‐order kinetics. Reactivity increases with electron‐donating substituents. Ortho‐alkyl substituted derivatives of arenesulfonyl chlorides show increased reactivity, but the origin of this “positive” ortho‐effect remains unclear. Likely, ortho‐methyl groups restrict rotation around the C‐S bond, facilitating the attack of the nucleophile. No relevant reactivity changes have been found with propan‐1‐ol and propan‐2‐ol in terms of nucleophile steric effect. The existence of isokinetic relationships for all substrates suggests a single mechanism for the series. Solvolysis reactions of all substrates in both alcohols show isokinetic temperatures (Tiso) close to the working temperature range, which is an evidence of the process being influenced by secondary reactivity factors, likely of steric nature in the TS. Solvation plays a relevant role in this reaction, modulating the reactivity. In some cases, the presence of t‐Bu instead of Me in para‐ position leads to changes in the first solvation shell, increasing the energy of the reaction (ca. 1 kJ·mol?1). The obtained results suggest the same kinetic mechanism of solvolysis of arenesulfonyl chlorides for propan‐1‐ol and propan‐2‐ol, as in MeOH and EtOH, where bimolecular nucleophilic substitution (SN2) takes place with nucleophilic solvent assistance of one alcohol molecule and the participation of the solvent network involving solvent molecules of the first solvation shell. 相似文献
58.
O. A. Luk'yanov G. A. Smirnov S. V. Nikitin A. Z. Lisitsin 《Russian Chemical Bulletin》1999,48(1):124-130
Some characteristic features of reactions ofN-(β-hydroxyalkyl)-N′-hydroxydiazeneN-oxide salts with various α- and β-functionalized alkyl halides were established. Some α-and β-functionalizedN-(β-hydroxyalkyl)-N′-alkoxydiazeneN-oxides and e ethylenebis[N-(β-hydroxyalkyl)-N′-oxydiazeneN-oxides] were synthesized for the first time.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 123–129, January, 1999. 相似文献
59.
Clia Fonseca Guerra F. Matthias Bickelhaupt 《Angewandte Chemie (International ed. in English)》1999,38(19):2942-2945
A hitherto unresolved discrepancy between theory and experiment is unraveled. Charge transfer and the influence of the environment in the crystal are vital for understanding the nature and for reproducing the structure of hydrogen bonds in DNA base pairs. The introduction of water molecules and a sodium counterion into the theoretical model (see picture) deforms the geometry of AT and GC in such a way that excellent agreement with the experimental structures is obtained. 相似文献
60.