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

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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(rh ) (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(χh ), which works as a triggering factor to determine the transition from the NHD to IHD. Below this Σ(χh ) value, the chemical shift of the central atom was strictly related to only the Σ(rh ) value, thus producing a NHD trend. Conversely, above this value, the chemical shift of the central atom was dependent on both the Σ(rh ) and Σ(χh ) values, thus producing a IHD trend. A simple model, in which the effect of the Σ(χh ) value on 13 C and 29 Si NMR chemical shifts is related to an apparent increase in the Σ(rh ) value, is deduced.