Do deviations from bond enthalpy additivity define the thermodynamic stabilities of diradicals?

Abstract

Deviations from bond enthalpy additivity (ABEA) are frequently used to assess the thermodyamic stabilities of diradicals. (U)B3LYP/6-31 G* calculations have been performed in order to determine how well ΔBEA values actually do reflect the thermodynamic stabilities of the triplet states of diradicals in which one or both nonbonding electrons occupy a delocalized π orbital. The calculations find that different pathways for forming σ,π-diradicals, such as α,2- and α,4-dehydrotoluene (4 and 6), give ΔBEA values that differ by ca. 1 kcal/mol. The path dependency of the ΔBEA values is computed to be one order of magnitude larger for non-Kekulé hydrocarbon diradicals, such as m-benzoquinodimethane (12) and 1,3-dimethylenecyclobutane-2,4-diyl (15), than for σ,π-diradicals. Since the ΔBEA values for forming 4, 6, 12, and 15 are all path dependent, we conclude that ΔBEA values for diradicals with one or two delocalized, nonbonding π electrons do not, in general, uniquely define the thermodynamic stabilities of the diradicals. Hence, ΔBEA values should not be used for this purpose, especially for non-Kekulθ hydrocarbon diradicals.