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

In carbon and silicon tetrahalide compounds, the experimental C-13 and Si-29 NMR chemical-shift values are known to increase or decrease on increasing the overall sum of the ionic radii of the bonded halides Sigma(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 Sigma(chi(h)), which works as a triggering factor to determine the transition from the NHD to IHD. Below this Sigma(chi(h)) value, the chemical shift of the central atom was strictly related to only the Sigma(chi(h)) value, thus producing a NHD trend. Conversely, above this value, the chemical shift of the central atom was dependent on both the Sigma(r(h)) and Sigma(chi(h)) values, thus producing a IHD trend. A simple model, in which the effect of the Sigma(chi(h)) value on C-13 and Si-29 NMR chemical shifts is related to an apparent increase in the Sigma(r(h)) value, is deduced.