Viesser Renan V, Ducati Lucas C, Tormena Cláudio F, Autschbach Jochen
Institute of Chemistry , University of Campinas - UNICAMP , P. O. Box 6154 , 13083-970 , Campinas , SP , Brazil . Email:
Department of Fundamental Chemistry , Institute of Chemistry , University of São Paulo , Av. Prof. Lineu Prestes, 748 , 05508-000 , São Paulo , SP , Brazil . Email:
Chem Sci. 2017 Sep 1;8(9):6570-6576. doi: 10.1039/c7sc02163a. Epub 2017 Jul 21.
Effects of electron-donating (R = NH) and electron-withdrawing (R = NO) groups on C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The C shift substituent effect in , , and position is determined by the σ bonding orbitals in the aryl ring. The π orbitals do not explain the substituent effects in the NMR spectrum as conventionally suggested in textbooks. The familiar electron donating and withdrawing effects on the π system by NH and NO substituents induce changes in the σ orbital framework, and the C chemical shifts follow the trends induced in the σ orbitals. There is an implicit dependence of the σ orbital NMR shift contributions on the π framework, unoccupied π* orbitals, due to the fact that the nuclear shielding is a response property.
通过分子轨道分析研究了给电子基团（R = NH）和吸电子基团（R = NO）对R-取代苯中碳核磁共振化学位移的影响。芳环中σ键轨道决定了邻位、间位和对位的碳位移取代基效应。π轨道并不能像教科书中传统认为的那样解释核磁共振谱中的取代基效应。NH和NO取代基对π体系常见的给电子和吸电子效应会引起σ轨道框架的变化，碳化学位移遵循σ轨道中诱导的趋势。由于核屏蔽是一种响应特性，所以σ轨道核磁共振位移贡献对π框架、未占据的π*轨道存在隐含的依赖性。
