Theoretical Study of Extremely Long yet Stable Carbon-Carbon Bonds: Effect of Attractive C•••H Interactions and Small Radical Stabilization of Diamondoids

The origin of the stability of diamondoid dimers containing very long carbon carbon bonds was examined using density functional theory (DFT) calculations with local response dispersion correction. It has been suggested that noncovalent CH center dot center dot center dot HC contacts are the probable source of their extraordinary stability as evidenced by dispersion-corrected DFT calculations. In this work, we numerically proved that the small radical stabilization energy, which was achieved through the geometric relaxation of cleaved radicals, led to the high stability of diamondoid dimers compared to other hydrocarbons. The bond energy density analysis showed that the CH center dot center dot center dot HC contacts are repulsive though the dispersion force somewhat stabilizes the dimer. We further decomposed CH center dot center dot center dot HC interaction energies to discover strong attractive interaction between C delta-center dot center dot center dot H delta+ intermonomer contacts.