An Intrinsic Radical Stability Scale from the Perspective of Bond Dissociation Enthalpies: A Companion to Radical Electrophilicities

F. De Vleeschouwer, V. Van Speybroeck, M. Waroquier, P. Geerlings, F. De Proft
Journal of Organic Chemistry
73 (22) 9109-9120


Bond dissociation enthalpies (BDEs) of a large series of molecules of the type A−B, where a series of radicals A ranging from strongly electrophilic to strongly nucleophilic are coupled with a series of 8 radicals (CH2OH, CH3, NF2, H, OCH3, OH, SH, and F) also ranging from electrophilic to nucleophilic, are computed and analyzed using chemical concepts emerging from density functional theory, more specifically the electrophilicities of the individual radical fragments A and B. It is shown that, when introducing the concept of relative radical electrophilicity, an (approximately) intrinsic radical stability scale can be developed, which is in good agreement with previously proposed stability scales. For 47 radicals, the intrinsic stability was estimated from computed BDEs of their combinations with the strongly nucleophilic hydroxymethyl radical, the neutral hydrogen atom, and the strongly electrophilic fluorine atom. Finally, the introduction of an extra term containing enhanced Pauling electronegativities in the model improves the agreement between the computed BDEs and the ones estimated from the model, resulting in a mean absolute deviation of 16.4 kJ mol−1. This final model was also tested against 82 experimental values. In this case, a mean absolute deviation of 15.3 kJ mol−1 was found. The obtained sequences for the radical stabilities are rationalized using computed spin densities for the radical systems.