Inverse halogen dependence in anion 13C NMR

Abstract

Halogens cause pronounced and systematic effects on the ¹³C NMR chemical shift (δ¹³C) of an adjacent carbon nucleus, usually leading to a decrease in the values across the halogen series. Although this normal halogen dependence (NHD) is known in organic and inorganic compounds containing the carbon atom in its neutral and cationic forms, information about carbanions is scarce. To understand how δ¹³C changes in molecules with different charges, the shielding mechanisms of CHX₃, CX₃⁺, and CX₃⁻ (X = Cl, Br, or I) systems are investigated via density functional theory calculations and further analyzed by decomposition into contributions of natural localized molecular orbitals. An inverse halogen dependence (IHD) is determined for the anion series as a result of the negative spin–orbit contribution instead of scalar paramagnetic effects. The presence of a carbon nonbonding orbital in anions allows magnetic couplings that generate a deshielding effect on the nucleus and contradicts the classical association between δ¹³C and atomic charge.