The halogen effect on the ¹³C NMR chemical shift in substituted benzenes

Recent research [Chem. Sci., 2017, 8, 6570-6576] showed for R-substituted benzenes with R = NH₂, NO₂ that the substitution effects on the ¹³C NMR chemical shifts are correlated with changes in the σ-bonding framework and do not follow directly the electron-donating or -withdrawing effects on the π orbitals. In the present work we extend the study to halogen (X = F, Cl, Br or I) substituted R-benzenes. The effect of X and R groups on ¹³C NMR chemical shifts in X-R-benzenes are investigated by density functional calculations and localized molecular orbital analyses. Deshielding effects caused by the X atom on the directly bonded carbon nucleus are observed for F and Cl derivatives due to a paramagnetic coupling between occupied π orbitals and unoccupied.