Origin of the Captodative Effect: The Lone-Pair Shielded Radical

We have quantum chemically analyzed the origin of the captodative effect in the dimerization of para-substituted phenyl dicyanomethyl radicals RPh(CN)₂C⋅ in the gas phase and in solution. Captodative radicals are characterized by the presence of both, electron-donating and electron-withdrawing groups, and a weakening of the associated C−C bond in the dimer of these radicals. Our quantitative bonding analyses reveal that the captodative weakening of the C−C bond is the consequence of a special feature in the RPh(CN)₂C⋅ electronic structure which we designate “lone-pair shielded radical”. Solvation effects weaken the C−C bond as the radicals have a more prominent internal charge separation than the dimer and are, therefore, stabilized more than the intact dimer. Interestingly, we find that differences in solvent effects as a function of the para-substituent in the most prominent case arise from variations in the charge distribution in the dimer, not from that in the separate radicals which experience very similar solvation in those instances.