INTERPRETATION OF C-13 NMR CHEMICAL-SHIFTS IN HALOMETHYL CATIONS - ONTHE IMPORTANCE OF SPIN-ORBIT-COUPLING AND ELECTRON CORRELATION

The first explicit study of both spin-orbit and electron correlation e ffects on NMR chemical shifts has been carried out. It is shown by spi n-orbit corrected DFT-IGLO calculations on the halomethyl cations CX: (X = F, Cl, Br, I) that the experimentally observed decrease in the C- 13 chemical shifts with increasing atomic number of X is due to spin-o rbit coupling, Previous explanations for this example of a 'normal hal ogen dependence' via electronegativity arguments were based on fortuit ious empirical correlations. Comparison between DFT-IGLO, HF-GIAO and MP2-GIAO results for these systems also demonstrates an increasing imp ortance of electron correlation along the same series.