After looking at the MO diagram for conjugated pi systems , it seems that electr

Question

After looking at the MO diagram for conjugated pi systems , it seems that electrons are never in the antibonding orbital in the ground state. Are there big enough systems where electrons are in the antibonding orbital in the ground state?Even for UV-VIS it is called a pi to pi* transition , why can't there be a pi* to pi transition?

molecular orbitals T antibonding benzene T4 atomic orbitals bonding t the six pi molecular orbitals in benzene, formed from six atomic p orbitals, occupy four separate

Explanation / Answer

In any aromatic compound or in any cyclic system which has alternate double bonds will always have electrons in the bonding orbitals for aromatic compounds and annulenes and for the anti aromatic it may occupy the non-bonding orbitals. It would never occupy the anti-bonding orbitals because regardless of the size of the ring, the number of bonding orbitals will be enough to hold all the electrons for aromatic compounds.

A frost diagram would make this observation very clear.

Foe UV-VIS, the transition is always from pi-pi* because, pi-is bonding orbital and at lower energy than the pi*(anti-bonding). Electron transition happens from lower to higher energy level when it abosorbs energy and move form ground state to excited state.

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