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 <i>ortho</i>-, <i>meta</i>- and <i>para</i>-positions were efficiently decomposed into contributions from two NPO pairs (σx−πx∗, σy−πy∗). 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 <i>ortho</i>-, <i>meta</i>- and <i>para</i>-directions of the substituent effects. Specifically, NH₂ shielded the <i>ortho</i>- and <i>para</i>-carbons, while NO₂ deshielded the same carbons. The orbital energy gaps of <i>ortho</i>-carbons in NH₂- and NO₂-benzenes were significantly decreased, showing that these substituents shielded the <i>ortho</i>-carbons.