Antibonding MO do they exist in reality?

[sudar_dhoni]

do antibonding molecular orbital exist in reality or
is it an empty space in which an electron can move about freely and not involving itself in bonding.
if they exist then how can the 2 atomic orbitals interfere both constructively as well as destructively simultaineously to give both Bonding as well as AntiBonding orbitals?

 

[wsttiger]

Within the single-particle approximation, the antibonding orbital is an excited state of the molecule. Furthermore, in the ground state this orbital should be empty.

I'm not sure what you're asking in the second question. When you bring multiple atoms together, the Hamiltonian changes and thus the eigenstates of the Hamiltonian change. One way of describing the new eigenstates is to write them as linear combinations of the atomic orbitals. Basically, you're just using the atomic orbitals as a basis to construct the molecular orbitals (or eigenstates) of the new Hamiltonian.

 

[blkqi]

The plus OR minus represents two different possibilities. Bonding and anti-bonding do not occur in the same orbital. Electrons generally take the lower energy orbital, that is the bonding orbital. Antibonding occurs when two orbitals come together out of phase.

 

[alxm]

do antibonding molecular orbital exist in reality or
is it an empty space in which an electron can move about freely and not involving itself in bonding.

Antibonding orbitals certainly exist. Orbitals are your solutions to the molecular schrödinger equation. Not all of these solutions correspond to 'bonding' patters, i.e. attraction between nuclei. The ones which are anti-bonding are generally the ones which have a node-plane (plane where the wave-function is zero, i.e. a change of sign) between the nuclei. More generally, such wavefunctions tend to be termed ungerade (German for 'odd'), whereas the 'bonding' orbitals are gerade ('even').

(The fact that they have a node is a clue to why they're usually higher in energy as well)

if they exist then how can the 2 atomic orbitals interfere both constructively as well as destructively simultaineously to give both Bonding as well as AntiBonding orbitals?

Ah! Now you get to the actual MO theory, which is to basically form the MO's by combining atomic orbitals. This is actually just an approximation, but it does qualitatively describe which MOs you end up with.

Anyway, the basic rationale for this is simple superposition. If A and B are your wave functions for individual atoms, then A + B is the wavefunction for the two of them together. (This is true if the electrons of the two atoms don't interact. Since they do interact, this becomes an approximation) But: A - B is a solution as well. That's superposition.