 Solution

ID: past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2022/paper-1/a4/c/i/2/solution

For

β_{L} = 0

, four isolated short-bond dimers each contribute

2 (α + β_{S})

E_{π}^{dimers} = 8 α + 8 β_{S} .

These are different coupling limits. Keeping

β_{S} = β

while simply removing long-bond coupling does not represent a small distortion at fixed mean bond strength.

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