Paper: CSIR-NET June 2011 · Subject: Physical Chemistry · Chapter: Quantum Chemistry · Topic: Quantum Chemistry – General · Marks: 2 · Difficulty: Medium
The energy levels for cyclobutadiene are $\alpha+2 \beta, \alpha, \alpha$ and $\alpha-2 \beta$. The delocalization energy in this molecule is:
(a)0
(b)$-4\beta$
(c)$-8 \beta$
(d)$4 \alpha$
Answer
Answer: A ✓ checked by 4AB · confidence high
The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
Cyclobutadiene: 4 π electrons fill α + 2β (2 e) and one electron each in the two α NBMOs: E = 4α + 4β. Two isolated ethylenes: 2(2α + 2β) = 4α + 4β. Delocalisation energy = 0 — the classic anti-aromatic result.