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

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.