Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / Section B / 39 / f / Solution

The 375 meV pair should shift most fractionally, towards zero bias, because C–H stretching has strong hydrogen displacement. A reduced-mass estimate givesso the pair would move approximately from $\pm 375$ to $\pm 275$ mV. A frozen-carbon estimate gives $1/\sqrt{2}$, or about 265 mV; the actual normal mode includes coupled motion. Whole-molecule modes change much less: the molecular mass rises only from approximately 41 to 44, giving a translational estimate $\sqrt{41/44}\simeq 0.965$.