Q91 · CSIR-NET Chemistry, June 2013
Paper: CSIR-NET June 2013 · Subject: Physical Chemistry · Chapter: Quantum Chemistry · Topic: Huckel MO Theory · Marks: 2 · Difficulty: Medium
Identify the Huckel determinant for cyclobutadiene
(a)$\left|\begin{array}{cccc}\alpha-\mathrm{E} & \beta & 0 & 0\\ \beta & \alpha-\mathrm{E} & \beta & 0\\ 0 & \beta & \alpha-\mathrm{E} & \beta\\ 0 & 0 & \beta & \alpha-\mathrm{E}\end{array}\right|$
(b)$\left|\begin{array}{cccc}\alpha-\mathrm{E} & \beta & 0 & \beta\\ \beta & \alpha-\mathrm{E} & \beta & 0\\ 0 & \beta & \alpha-\mathrm{E} & \beta\\ \beta & \beta & 0 & \alpha-\mathrm{E}\end{array}\right|$
(c)$\left|\begin{array}{cccc}\alpha-\mathrm{E} & \beta & 0 & \beta\\ \beta & \alpha-\mathrm{E} & \beta & 0\\ 0 & \beta & \alpha-\mathrm{E} & \beta\\ \beta & 0 & \beta & \alpha-\mathrm{E}\end{array}\right|$
(d)$\left|\begin{array}{cccc}\alpha-\mathrm{E} & \beta & 0 & \beta\\ \beta & \alpha-\mathrm{E} & \beta & 0\\ 0 & \beta & \alpha-\mathrm{E} & \beta\\ 0 & 0 & \beta & \alpha-\mathrm{E}\end{array}\right|$
Answer
Answer: C ✓ checked by 4AB · confidence high
The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
In cyclobutadiene each carbon is bonded to its two ring neighbours (1–2, 2–3, 3–4, 4–1) (c).
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