Why is the sigma antibonding molecular orbital of higher energy than the pi antibonding molecular orbital?
Answers
Answer:
Since orbitals are formed by the head on collision of atomic orbitals they are stronger (so having magnitude of their potential energy (both stabilizing bonding, σ and destabilizing antibonding, σ* energy) is higher) than the interaction between the or orbitals, which meets edge-on.
Answer:
When two atoms form a bond, if two atomic orbitals mix, then two molecular orbitals are created, one of lower and one of higher energy than that of the atomic orbitals.
Now, just how much the energy changes is dictated by how well the atomic orbitals overlap. Overlapping depends on three things:
Orbital energy
Distance between atoms
Orbital symmetry (they must be "aligned")
Now, assuming that the atomic orbitals are similar in energy and the symmetries are correct, which is the case for C and O.
The sigma bond is a result of two orbitals very close together, directly facing each other frontally. The overlap is great. The pi bond emerges from two orbitals overlapping side-by-side, which isn't as efficient because it happens at a greater distance.
Explanation:
Considering P orbital to form Molecular orbitals of second energy level. If we arbitrary define Z axis of coordinate system as an axis along which bond forms for any molecule, then pz orbitals of atom will form σ bonding and σ* antibonding molecular orbitals by head on collision with adjacent atom, so electron density is along the bond axis. On the other hand π bonds concentrate electrons either above or below the axis. For the relative energies of the six molecular orbitals formed by mixing of pair of atomic orbitals, we need to understand the strength of interaction between pair of orbitals and the relative energy molecular orbitals they form. Since pz orbitals are formed by the head on collision of atomic orbitals they are stronger (so having magnitude of their potential energy (both stabilizing bonding, σ and destabilizing antibonding, σ* energy) is higher) than the interaction between the px or pyorbitals, which meets edge-on.