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[[ampi]]pound;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[[ampi]]pound;HC contacts are repulsive though the dispersion force somewhat stabilizes the dimer. We further decomposed CH[[ampi]]pound;HC interaction energies to discover strong attractive interaction between Ca[[ampi]]die; [[ampi]]pound;Ha[[ampi]]die;+ intermonomer contacts.
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