Discussion
For a butadiene molecule, the energy values of levels obtained by using HMO theory are :
(1) α - 1.6 β
(2) α + 1.6 β
(3) α + 0.6 β
(4) α - 0.6 β
What is the sequence of levels in the order of increasing energy ?
Answer: C
The order of increasing energy is (2), (3), (4), (1).
In Huckel theory the resonance integral β is negative, so adding a multiple of β to α lowers the energy. For butadiene the four levels are α + 1.62β, α + 0.62β, α - 0.62β and α - 1.62β, in that order from lowest to highest.
- Lowest: α + 1.6β (2), the fully bonding orbital.
- Then α + 0.6β (3), the HOMO.
- Then α - 0.6β (4), the LUMO.
- Highest: α - 1.6β (1).
Option A lists them as numbered, and option D treats β as if it were positive, which reverses the order.
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