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Issue 9, 2017
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The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

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Abstract

Effects of electron-donating (R = NH2) and electron-withdrawing (R = NO2) groups on 13C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The 13C shift substituent effect in ortho, meta, and para 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 NH2 and NO2 substituents induce changes in the σ orbital framework, and the 13C 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, via unoccupied π* orbitals, due to the fact that the nuclear shielding is a response property.

Graphical abstract: The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

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Publication details

The article was received on 14 May 2017, accepted on 20 Jul 2017 and first published on 21 Jul 2017


Article type: Edge Article
DOI: 10.1039/C7SC02163A
Citation: Chem. Sci., 2017,8, 6570-6576
  • Open access: Creative Commons BY-NC license
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    The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

    R. V. Viesser, L. C. Ducati, C. F. Tormena and J. Autschbach, Chem. Sci., 2017, 8, 6570
    DOI: 10.1039/C7SC02163A

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