¹³C NMR chemical shifts in substituted benzenes: analysis using natural perturbation orbitals and substitution effects

The natural perturbation orbital (NPO) method, combined with the gauge-including atomic orbital (GIAO), was applied to nuclear magnetic resonance (NMR) chemical shifts. The substituent effects of electron-donating (NH₂) and electron-withdrawing (NO₂) groups on the ¹³C NMR chemical shifts of monosubstituted benzenes were analysed using NPOs. ¹³C chemical shifts at the ortho-, meta- and para-positions were efficiently decomposed into contributions from two NPO pairs (${\sigma }_x-{\pi }_x^{\ast }$, ${\sigma }_y-{\pi }_y^{\ast }$). These contributions were further divided into two factors, namely electron density in the vicinity of resonance nuclei and orbital energy gaps. Trends in the electron density matched with the ortho-, meta- and para-directions of the substituent effects. Specifically, NH₂ shielded the ortho- and para-carbons, while NO₂ deshielded the same carbons. The orbital energy gaps of ortho-carbons in NH₂- and NO₂-benzenes were significantly decreased, showing that these substituents shielded the ortho-carbons.