Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 1 / A4 / c / ii / Solution

There are three stretching fundamentals, although four stretching-region features are listed. The low feature at $1746\mathrm{c}{\mathrm{m}}^{-1}$ is predominantly C=O stretch ($A_1$). The two features at $2783$ and $2843\mathrm{c}{\mathrm{m}}^{-1}$ are the symmetric and antisymmetric C–H stretches; their order is not determined by this mode count alone.

The $3482\mathrm{c}{\mathrm{m}}^{-1}$ feature is consistent with the first C=O vibrational overtone, close to $2(1746)=3492\mathrm{c}{\mathrm{m}}^{-1}$ and shifted downward by anharmonicity. Its symmetry is $A_1\otimes A_1=A_1$, so symmetry permits the weak anharmonic transition. It is not a fourth stretching fundamental. The combination-band evidence in the next part distinguishes the two C–H assignments.