Pauling Electronegativity On/Off Effects Assessed by ¹³ C and ²⁹ Si NMR Spectroscopic Analysis

In carbon and silicon tetrahalide compounds, the experimental ¹³ C and ²⁹ Si NMR chemical-shift values are known to increase or decrease on increasing the overall sum of the ionic radii of the bonded halides Σ(r_h ) (normal and inverse halogen dependence (NHD and IHD, respectively)). Herein, we extrapolate the main factors responsible for such NMR chemical shifts. Intriguingly, we found a characteristic value for the overall sum of the Pauling electronegativities of the bonded halides Σ(χ_h ), which works as a triggering factor to determine the transition from the NHD to IHD. Below this Σ(χ_h ) value, the chemical shift of the central atom was strictly related to only the Σ(r_h ) value, thus producing a NHD trend. Conversely, above this value, the chemical shift of the central atom was dependent on both the Σ(r_h ) and Σ(χ_h ) values, thus producing a IHD trend. A simple model, in which the effect of the Σ(χ_h ) value on ¹³ C and ²⁹ Si NMR chemical shifts is related to an apparent increase in the Σ(r_h ) value, is deduced.