Analysis of carbon-carbon bonding in small hydrocarbons and dicarbon using dynamic orbital forces: Bond energies and sigma/pi partition. Comparison with sila compounds

The CC bonding is analyzed using dynamic orbital forces (DOF) in the series cyclopropane-ethane- benzene-ethylene-acetylene. The sum Σ(DOF)_t of the DOF over occupied molecular orbitals (MOs) is found linearly correlated to bond energies and thus can be used as a tool for determination of CC bond strength. A partition of bonding into σ and π components indicates a weakening of the σ bonding along the series, mainly due to the decrease of the bonding character of the highest σ MO. For C₂ molecule, Σ(DOF) _t was computed taking into account the four dominant configurations. On the basis of the preceding correlation, the C₂ bond was found about 15 kcal/mol weaker than that of acetylene, with a 25% σ participation; the bond order of C₂ can be evaluated at about 2.8 if we assume bond orders of 3 for acetylene and 2 for ethylene. Some sila homologs of the preceding carbon compounds have been studied. They exhibit characteristics generally close to the carbon compounds. A quite good correlation between Σ(DOF)_t and bond energies is also observed.