Weakly bound carbon-carbon bonds in acenaphthene derivatives and hexaphenylethane.

A class of acenaphthene derivatives is shown to contain weak central carbon-carbon bonds that may be easily cleaved at high temperatures or even at ambient conditions to yield persistent free diradicals. To demonstrate the weak C-C bond strength, density functional theory calculations were carried out at several levels of theory for both the parent molecules and the diradicals resulting from the C-C bond cleavage. To assess the accuracy of the calculations, hexaphenylethane was chosen as a model compound due to its similarity with the molecules studied here, its great resonance stabilization, and long-standing history within the chemistry community. The C-C bond dissociation energy of hexaphenylethane was determined to be 11.3 +/- 1.4 kcal/mol using a combination of isodesmic reactions and calculations at the M06-2X/6-31+G(d,p) level of theory. The types of molecules presented here are proposed as strong possibilities for the natural existence of free radicals in young and mature soot formed in hydrocarbon combustion.