An approach based on relief of molecular strain in the parent hydrocarbon, extended conjugation in the radical, and the driving force toward aromaticity is used to design molecules with ultraweak C-H bonds. The molecular strain is generated by two fused rings containing (5,5)-, (5,6)-, or (6,6)-membered ring structures. Homodesmotic reactions are used to calculate the molecular strain enthalpy (MSE) of the parent hydrocarbons and the corresponding radicals, and to analyze how it changes through these reactions. B3LYP calculations are used to obtain the bond dissociation enthalpies (BDEs) for breaking one or more C-H bonds as well as the C-O bond formed after oxygen addition to the radical. Loss of a second H-atom can lead to very low R-H BDE values, especially when the ultimate product is aromatic. Molecular structures based on these ideas may be of interest as novel antioxidants based on carbon-centered radicals.

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Journal Journal of Chemical Theory and Computation
Shadnia, H. (Hooman), & Wright, J.S. (2009). Using molecular strain and aromaticity to create ultraweak C-H bonds and stabilized carbon-centered radicals. Journal of Chemical Theory and Computation, 5(4), 1129–1136. doi:10.1021/ct900049j