Orbital Mixing - Building Larger Molecules

• Essence of orbital mixing is stated in Rules 8 and 9.
• Rule 8: When two orbitals interact, the lower energy orbital is stabilized
and the higher energy orbital is destabilized. The out-of-phase
(antibonding) interaction always raises in energy to a greater degree than
the corresponding in-phase (bonding) interaction is lowered in energy.
• Rule 9: When two orbitals interact, the lower energy orbital mixes into
itself the higher energy one in a bonding way, while the higher energy
orbital mixes into itself the lower energy orbital in an antibonding way.
• Key aspect of orbital mixing is that the antibonding combination is
raised in energy more than the bonding combination is lowered in
energy.
• If both of the original orbitals are doubly occupied so too shall be the
resulting two orbitals.... net interaction is destabilizing.
• If only electrons are involved, then they end up in the lower energy
mixed orbital... which is favorable.
• Nondegenerate mixing of orbitals results in polarization of the
resulting MO orbitals.
• Perturbation theory:
• First-order perturbation = mixing of degenerate orbitals.
• Second-order perturbation = mixing of nondegenerate orbitals