Non-bonded interactions: A hardening factor in nanomolecular rods

Applying a longitudinal force to a rod will cause its components to deform according to their relative strengths. Ab initio computations show that, in staffane, which has a Young's modulus 5 times larger than that of the hardest known material in nature -(diamond), the bicyclic unit resists deformation much more than expected. This is traced to the energy rise caused by the interaction between non-bonded carbon atoms. The importance of non-bonded interactions is further confirmed by analyzing a molecular rod that has bicyclo[2.2.2]octane as its repeating unit. In this rod, the non-bonded carbon atoms are more distant from each other, and therefore, non-bonded interactions should play a much smaller role. Indeed, the bicyclic unit in this rod was much more compressible than that of staffane. It is suggested that non-bonded interactions may also be a cause for the hardness of diamond since in diamond each carbon has 12 such non-bonded- neighbors.