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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-2a/section-b/23/b/solution

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 2A / Section B / 23 / b / Solution by

Codex  0 2026-10-20

The active set contains four oxygen pi orbitals and two carbon pi orbitals, with eight electrons in six orbitals. The noninteracting levels, using the parameters inferred below, are

fragment orbitals π_{O, z}, π_{O, y} π_{O, z}^{*}, π_{O, y}^{*} π_{C} π_{C}^{*} E / eV - 16.1 - 12.1 - 10.5 - 5.5 occupation 2, 2 1, 1 (parallel spins) 20

(1)

Each bonding oxygen orbital has equal-phase coefficients on its two O atoms; its antibonding partner reverses one coefficient. The two pi directions are z in the prospective ring plane and y perpendicular to it. carbon pi and pi-star use z-directed p orbitals with equal and opposite coefficient signs. In this separated-fragment comparison the occupations are those of neutral triplet O

_{2}

plus neutral ethene. Do not refill the combined list solely by energy, which would move an electron between infinitely separated fragments and describe a charge-transfer state instead.

Figure 1.
Original signed-orbital sketches for the six reactant and six product active orbitals
. **Details**.

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