Past exam of the chemistry A course of the University of Cambridge / 2022 / Paper 1 / A5 / b / ii / Solution

From the complete normal mode result, the vibrational infrared activity belongs toThe $A_{2u}$ fundamental couples to the out-of-plane dipole component and each $E_{1u}$ fundamental to the two in-plane components. Counting one frequency for each degenerate pair, the expected number is $1+3=4$ fundamental bands, containing seven vibrational coordinates.

This is the symmetry count for the full molecule, including hydrogen displacements. The experimental compilation likewise lists the out-of-plane $A_{2u}$ fundamental and three $E_{1u}$ fundamentals; anharmonic resonances and weak intensity borrowing can produce additional observed features beyond the ideal fundamental count.^[ref]