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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.

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.

SUBMITTER: Viesser RV

PROVIDER: S-EPMC5627350 | biostudies-literature | 2017 Sep

REPOSITORIES: biostudies-literature

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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.

Viesser Renan V RV Ducati Lucas C LC Tormena Cláudio F CF Autschbach Jochen J

Chemical science 20170721 9

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 ...[more]

PMID: 28989684

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