Interpretation of 13C NMR chemical shifts in halomethyl cations. On the importance of spin-orbit coupling and electron correlation

The first explicit study of both spin-orbit and electron correlation effects on NMR chemical shifts has been carried out. It is shown by spin-orbit corrected DFT-IGLO calculations on the halomethyl cations CX3+ (X  F, Cl, Br, I) that the experimentally observed decrease in the 13C chemical shifts with increasing atomic number of X is due to spin-orbit coupling. Previous explanations for this example of a ‘normal halogen dependence’ via electronegativity arguments were based on fortuitious empirical correlations. Comparison between DFT-IGLO, HF-GIAO and MP2-GIAO results for these systems also demonstrates an increasing importance of electron correlation along the same series.