On the direction and magnitude of radical substituent effects: The role of polar interaction on thermodynamic stabilities of benzylic C-H bonds and related carbon radicals

The C-H bond dissociation enthalpies (BDEs) of polarized benzylic molecules, i.e., para-substituted phenylacetonitriles (PANs), and the spin variations of the radicals of the general type p-GC(6)H(4)-CH.-Y were investigated using density functional theory (DFT) calculations. In contrast to the commonly observed S-type substituent effect (see text), the present work shows that there should be three (rather than one) primary patterns (i.e., S, O, and counter-O) for remote G to affect spin and radical stability, depending upon the polarity of the alpha -Y group. Correlation analyses reveal that both the direction and magnitude of spin/radical effects are quantitatively related to the intensity of polar interaction in radical system, as registered by either the calculated group charges of the phenyl ring (C-Ph) or by the polar constant sigma (+)s (Figures 1-4). A unified platform (Scheme 1) to rationalize the apparent differences of radical substituent effect is proposed.