| Abstract
| - Deviations from bond enthalpy additivity (ΔBEA) are frequently used to assess the thermodyamicstabilities of diradicals. (U)B3LYP/6-31G* calculations have been performed in order to determinehow well ΔBEA values actually do reflect the thermodynamic stabilities of the triplet states ofdiradicals in which one or both nonbonding electrons occupy a delocalized π orbital. The calculationsfind that different pathways for forming σ,π-diradicals, such as α,2- and α,4-dehydrotoluene (4and 6), give ΔBEA values that differ by ca. 1 kcal/mol. The path dependency of the ΔBEA valuesis computed to be one order of magnitude larger for non-Kekulé hydrocarbon diradicals, such asm-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 ΔBEAvalues for diradicals with one or two delocalized, nonbonding π electrons do not, in general, uniquelydefine the thermodynamic stabilities of the diradicals. Hence, ΔBEA values should not be used forthis purpose, especially for non-Kekulé hydrocarbon diradicals.
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