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

The C=O length is invariant under every operation, so its stretch is $A_1$. If $s_L,s_R$ are the two C–H bond-length changes, their sum and difference giveHence $\boxed{{\Gamma }_{\mathrm{stretch}}=2A_1+B_1}$. These are stretching symmetry coordinates; the two $A_1$ motions can mix in actual normal modes, and mass-weighted atomic motions need not have the schematic arrow lengths below.