Proton NMR spectroscopy provides evidence for the restricted rotation of a peptide bond. For example, N,N‑dimethylformamide exhibits three signals in its proton NMR spectrum at room temperature. Two of those signals are observed upfield, at 2.9 and 3.0 ppm. As the temperature is raised, the two signals begin to merge together. Over 180ºC, these two signals combine into one signal. Explain how these results are evidence for the restricted rotation of peptide bonds.