The experimental 1 H NMR-observed substitution products (Fig. ) confirm the validity of this approach and can be successfully predicted using Fukui indices -a natural bond orbital (NBO)based metric that describes localisation of extra electron density at each carbon centre in the aromatic ring, predicting the stability of the radical aryl intermediate. The higher the Fukui index at a carbon site, the better its ability to accommodate an extra electron, making the site more susceptible to be attacked by the alkyl radical. An exception is Boscalid (47, Fig. ), where significant steric hindrance shields the position alpha to the carbonyl group, limiting the predictive power of the solely electronic Fukui index.