Overcoming lability of extremely long alkane carbon–carbon bonds through dispersion forces

The synthesis of alkane hydrocarbons containing extremely long carbon–carbon (C–C) bonds, the longest observed in alkanes to date, is reported. Long C–C bonds are usually weaker than short ones, but these new alkanes are surprisingly stable, with decomposition occurring only above 200 °C. Quantum mechanical calculations show that, remarkably, the compounds are stabilized by attractive interactions between bulky groups at either end of the long C–C bonds. The stabilizing interactions observed in these compounds could prove useful in the development of new materials that utilize attractive dispersion interactions. Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes1,2 and the possibility of constructing hydrocarbons with extraordinarily long C–C single bonds through steric crowding3. Although empirical bond distance/bond strength relationships have been established for C–C bonds4 (longer C–C bonds have smaller bond dissociation energies), these have no present theoretical basis5. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry6,7, as summarized by Pauling8 as early as 1960 and confirmed more recently4. Here we report the preparation of hydrocarbons with extremely long C–C bonds (up to 1.704 A), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids9. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with10 and without inclusion of dispersion corrections.